Is it possible to mistakenly see twins on an ultrasound, when will the result be most accurate and what are the differences? Pregnancy with twins on ultrasound: distinctive signs Ultrasound can see twins

Multiple pregnancy is something for which it is impossible to prepare, since it is impossible to predict it, or, conversely, to calculate its formation. It is worth noting that carrying such a pregnancy is much more difficult and you will have to try a lot not only to save the babies, but also to promote their uniform development.

Signs of multiple pregnancy in the early stages

Mostly signs of pregnancy are early stages are the same in any case, both when carrying twins and one baby, however, in the first case there may be significant deterioration in the condition and the manifestation of new signs.

Twin pregnancy has primary symptoms such as:

1. Severe toxicosis, and there is no way to avoid it, unlike carrying one fetus.
2. The presence of a bright second line on a test to detect pregnancy, as the level of a hormone called hCG increases significantly.
3. A rapid increase in the volume of a pregnant woman’s abdomen, which can be recognized literally at 12 weeks, but depending on physiology, this is possible much earlier.
4. Earlier manifestation of baby movements, since they have much less space to move, accordingly, the tremors will be brighter and earlier.
5. Rapid weight gain, especially during the first trimester, which in a singleton pregnancy occurs by the middle of the second trimester.
6. Having an increased appetite.
7. The presence of sudden and serious mood swings.

In addition, a woman has a need to eat exotic fruits and unusual foods, since an increased amount of vitamins and minerals is required, and, as we know, the body begins to look for them in food for which there is a strong interest.

The main signs of twins in early pregnancy

A diagnosis such as multiple pregnancy can be determined independently, but it is better to undergo an examination that will show real result and confirm your guesses. An examination by a doctor allows you to note such signs as an enlarged uterus, which, for example, at the 5th week of pregnancy may correspond to the 9th week of development interesting situation. Blood is donated for a hormone such as hCG, and if the level is too high, then this becomes another sign.

The blood must be examined for the presence of iron levels, as well as alpha-fetoprotein. If there are twins in the uterus, then the iron level will be reduced, and the ACE protein, on the contrary, will be increased.

Naturally, you can find out that there is not one fertilized egg inside, but two, by listening to two heartbeats, as well as by palpating the two heads, but this is done in later stages of pregnancy. To exclude medical error, you should definitely undergo an ultrasound. Ultrasound is an option that will not allow mistakes and will make it possible to identify the exact number of fetuses in the uterus, and also with its help, the gestational age is established, and pathologies or their absence are identified.

At what time can twins be detected?

The first signs that a woman is pregnant are quite clear, however, when will it be possible to see the presence of an embryo inside the belly? As a rule, it is possible to recognize how many fertilized eggs the uterus contains by ultrasound as early as 8 weeks of pregnancy.

If there are several fetuses in the uterus, then several heartbeats can be detected.

It is worth noting that experts do not recommend trusting this result, and to be more precise, a repeat study is required at 12 weeks, since the probability of death of one fetus is quite high and, according to statistics, only 5 out of 10 women can survive a second child, and this does not depend on the physiology or health of the body.

Identical twins: when seen on ultrasound

How is pregnancy determined if identical twins are present?

In this case, it is also carried out:

• Examination by a doctor;
• Pregnancy test;

If the pregnancy contains identical twins, then they can only be detected at 12 weeks and not earlier. After identifying the babies, a repeat examination is required after 3 weeks to confirm the viability of the fetuses. Frequent examinations using an ultrasound machine will not harm babies, since only modern technologies are now used, but you can clearly monitor their condition and how they develop.

What are the signs of multiple pregnancy

An ultrasound does not show whether twins are in the womb or twins, but their number can be determined. It is worth noting that a singleton pregnancy differs significantly from a multiple pregnancy and the latter can be diagnosed in various ways.

Diagnostic procedure:

1. Naturally, the analysis of complaints about the condition of the fair sex is first taken into account. Such signs include: delay, nausea, vomiting, malaise, weakness, drowsiness, as well as swelling of the mammary glands.
2. Anamnesis is examined menstrual function, that is, when was your last period, is the cycle regular and how exactly does it go.
3. The results of a pregnancy test done at home must be taken into account.
4. Blood testing is carried out for the hCG hormone, which in case of multiple pregnancy should go off scale several times. There are special tests that you can carry out yourself, and by how brightly they are colored, the increase or normal level of the hCG hormone will be determined. The test should be done in the morning so that the concentration of the hormone in the urine is maximum.
5. A gynecological examination is performed, during which an increase in the volume of the uterus should be detected.

In addition, an ultrasound of the pelvic organs is performed. As a rule, an ultrasound scan before 11 weeks is prescribed only if there have previously been problems such as miscarriage, fetal death, ectopic pregnancy or even suspicion of the presence of two eggs. In other cases, a routine examination of the body is sufficient.

Twin pregnancy and possible complications

Even geneticists cannot understand the reasons why a double pregnancy occurs, but one thing they are sure of is that it depends on heredity, and may also be the case if the insemination was artificial. The determination of a multiple pregnancy does not occur immediately, and the most important thing is that the formation of disorders and even complications often occurs, and the types of such consequences can be very different. The way the belly looks does not at all indicate whether there are any abnormalities inside the body, and therefore it is necessary to undergo regular examinations and report to the doctor about all manifestations of pregnancy. There can be many complications, and if there is a multiple pregnancy, then hospitalization in a clinic is advisable, which will reduce the risk of harm to the fetus and, especially, the death of one of them.

Toxicosis is both a complication and a kind of normal course of pregnancy. Ideally, it should occur during the first trimester and no more, however, if very clear signs appear in subsequent periods, then this is a signal of a deterioration in the condition.

Preeclampsia is a complication of pregnancy, which can be characterized as a dysfunction of organs important for the life of the fetus. As a rule, such a diagnosis is made after 20 weeks, as soon as the second ultrasound is performed and tests are taken. Preeclampsia can be of three forms, from mild to severe, and it manifests itself in the form of high blood pressure, swelling, strong discharge, including bright brown urine.

The occurrence of preeclampsia and eclampsia is quite possible during multiple pregnancies, and this occurs against the background of severe gestosis. Symptoms may include high blood pressure, dizziness, and pins and needles in the eyes. In this case, hospitalization in a clinic under 24-hour medical supervision is advisable.

During multiple pregnancy, the following problems are possible:

• Miscarriage;
• Premature birth;
• Threat of freezing of both fruits, or one of them.

It is worth noting that in order to exclude such consequences, you need to constantly take tests, undergo examinations and, most importantly, follow all the doctor’s recommendations regarding treatment, lifestyle and nutrition. A woman in this position needs a double dose of vitamins, peace of mind and, most importantly, fresh air and the help of loved ones.

To eliminate the threat premature birth you need to ensure that there is no tone of the uterus, which will avoid contractions, expulsion of the placenta and similar problems. A premature pregnancy is considered to be one that ends at 20-37 weeks, and to prevent it, if there is a threat, it is necessary to take special medications. There are also post-term pregnancies, and this is one that ends at more than 42 weeks. Intrauterine fetal death at 28 weeks occurs due to the mother’s own negligence, as well as medical error, for example, if defects were previously overlooked, and therefore it is worth choosing only qualified specialists and visiting the passionate community of expectant mothers.

Problems of multiple pregnancy (video)

Both pregnancy itself and its absence can cause a lot of problems, and therefore you need to treat it with full responsibility in order to avoid the death of the baby, and even more so his underdevelopment. Now there are many drugs that help maintain pregnancy and technologies that allow you to carefully monitor it, and they should not be neglected in any case.

At what period are twins determined by ultrasound....(here’s an article - but I’m interested in your opinion)...

During ultrasound diagnostics, twins are usually clearly visible in the early stages.

Gynecologists can, of course, determine twins without an ultrasound; they do this based on the size of the uterus, which is much larger at the same period in a woman pregnant with one child. But ultrasound still remains the main examination during pregnancy with twins, just as it is done every month during multiple pregnancies.

At what date are twins diagnosed?

After visiting the antenatal clinic, during examination of a pregnant woman, the doctor discovers a rapidly increasing uterus in size and sends her for an ultrasound scan; in this study, twins are detected by ultrasound as early as 5 weeks. And when examined manually, the doctor can determine twins from 9 to 10 weeks.

Of course, there are cases when ultrasound diagnostics may not recognize twins at 5 weeks. Therefore, during multiple pregnancies, doctors prescribe ultrasound diagnostics more often in order to detect complications in time, if any.

If ultrasound diagnostics at a short time did not show you twins, but it turned out much later, there is no need to be offended and scold the doctors. I would advise you to do an ultrasound in modern ultrasound centers, as old ultrasound machines have short length ultrasonic wave and not everyone can see it well, but modern ultrasound machines see everything well and the doctor will tell you everything in detail about your babies or toddler. In a modern ultrasound diagnostic center, you can even ask a specialist to burn you a disc with images of your future little ones.

Determine twins without ultrasound

There are several methods by which you can determine twins, all of them are not as informative as ultrasound examinations, but they are also often used by doctors or women who want to know about the number of babies in their tummy.

There are other ways to determine twins that are popularly used; they are not accurate, but sometimes they really allow a woman to find out about twins. In the presence of multiple pregnancy, the woman’s body experiences a greater load, her internal organs work in an intensive mode, and this may affect the well-being of the expectant mother.

All of the above signs of pregnancy are unable to accurately determine twins, so an ultrasound examination should be performed, which will help to accurately determine not only a multiple pregnancy, but also other important parameters in the development of the fetus, the absence or presence of disorders or abnormalities.

In accordance with the pregnancy management program, a woman undergoes screening, including ultrasound and laboratory blood tests, once in each trimester. Complicated or multi-embryonic pregnancies are monitored more often. An ultrasound can determine twins with an almost 100% guarantee. People can make mistakes only under the influence of unfavorable factors, which include:

• faulty or outdated diagnostic device;
• insufficient qualifications of medical specialists;
• unnatural arrangement of embryos, when one fetus “hides” behind another.

The last factor occurs only in the early stages. Even if twins were not seen at the first ultrasound, subsequent screening will certainly show the presence of two embryos.

Causes of twin pregnancy

Twin pregnancy has a genetic (hereditary) basis. This feature is transmitted more often through the female line. Other reasons may be: in vitro (unnatural) fertilization, abnormal development of the uterine structure. In addition, the likelihood of becoming pregnant with two children at once increases with age. In women aged 35+, this phenomenon occurs several times more often.

Determination of twins without ultrasound

A woman can notice the primary signs of multiple pregnancy on her own:

• pathological toxicosis in the first trimester;
• accelerated weight gain and increased abdominal volume.

In the second half of the perinatal period, the doctor determines twins based on the following characteristics:

• abnormal increased level hormone human chorionic gonadotropin human (hCG) and ACE protein;
• decreased iron level in the blood;
• size of the uterus increased inappropriately for the period;
• clear heartbeat of two hearts during auscultation (listening with a stethoscope);
• early double movement of the fetuses in the uterus (children begin to push earlier because they become cramped).

Accurate detection of twins in the early stages is only possible through ultrasound.

Determination of timing and types of multiple pregnancies

Women who have a hereditary predisposition to having two children at once are always concerned about how long it takes to determine twins. Despite the fact that in obstetric and gynecological practice there are certain deadlines, much depends on the type of pregnancy, and therefore is an individual indicator.

The placement of twins can have several variations

The presence of twins on an ultrasound is determined by listening (characteristic gurgling) to the beating of two hearts, two hollow membranes (amniotic sacs) with amniotic fluid and two provisional organs (placentas). The types of possible development of babies in the womb are determined according to the placenta-forming organ - the chorion and the amniotic sac - the amnion.

There are three options for twin pregnancy:

• Dichorionic diamniotic. The location of each embryo is determined by a personal “baby place” (placenta) and amniotic sac. With this development, babies are usually heterogeneous. Twins are visible already during the first screening, starting from 5–6 weeks of gestation.
• Monochorilic diamniotic. Embryos are located in different amniotic membranes, united by one “baby place”. Such twins can be seen around 9–11 weeks.
• Monochorionic monoamniotic. As a rule, such identical twins are visible on ultrasound only during the second screening. Future children are connected by a common amniotic sac and a common placenta. This is the most problematic type of double-embryonic pregnancy, which is difficult to endure without complications.

Nature of complications

Carrying two fruits is accompanied by a double load on the body expectant mother. The main types of possible complications are: relapse of chronic diseases (if any), severe toxic damage (toxicosis), anemia (low level of hemoglobin in the blood), abnormal development of embryos, placental insufficiency (changes in the interaction between the mother and children).

For these and other reasons, ultrasound diagnostics is prescribed to women pregnant with two children more often than once per trimester and mandatory carried out before delivery. It would be useful for expectant mothers to know about the existence of such a situation as interruption of the development of one of the babies. During the initial examination, two children were noticed, but a repeat ultrasound showed the presence of only one embryo.

This situation is not abnormal when the fetus does not freeze, but dissolves in the placenta. You shouldn't be afraid of this.

Ultrasound imaging

Objective term, in which week two fetuses are visualized is considered the fifth week from the moment of conception. On the monitor screen, the ultrasound shows two dark spots in the uterine cavity. As the babies develop, the picture will change. When there are two children, they tend to be slightly behind in phenometry (size determination via ultrasound).

• At the eleventh week, the baby's size is 43–48 mm.
• A week later, children grow to 60 mm, weighing about 8 g.
• In the middle of pregnancy (20 weeks), the weight of each fetus is close to 350 g, and the weight does not necessarily have to be the same. One child may well be larger than another. The kids no longer look like dark formations. Their limbs, other parts of the body and internal organs are well visualized.
• By the time of the third screening, children gain body weight up to 2 kg.

After the procedure, the ultrasound specialist can print out a snapshot of the image, and parents, if desired, will receive the first photo of their babies. The delivery of twins is often premature. Natural childbirth are not always possible due to the size of the children, the mother’s weak labor, and the characteristics of fetal presentation. To avoid complications, doctors prefer to resort to cesarean section.

Twin pregnancy at 15 weeks. Photo from ultrasound scan

Twins or twins

The definition of “twins” is not typical for medicine. Twins can be fraternal (dizygotic), when a woman develops not one, but two eggs and their fertilization is carried out by different sperm. These children are called twins. Fraternal babies can be different sexes, completely different from each other, have different group blood and rhesus. These are different children whose biological clocks, tastes and habits may not coincide.

In the case of dividing an already fertilized egg in half, identical (monozygotic) embryos or twins born from the same egg are obtained. Such children are always same-sex, and the pregnancy, most often, will be monochorionic diamniotic. Twins have the same appearance, similar characters and habits, and often suffer from the same diseases. Often, such children (later adults) have the ability to feel each other’s experiences and pain.

Girls and boys

When carrying one child, its gender can be determined by ultrasound during the second screening. The optimal period is considered to be the twenty-second week. With twins or twins, gender differentiation can be difficult. The fact is that children often have an incorrect presentation.

Gender will not be visible if both infants turn away or “hide” behind each other, hiding their genitals during the examination. There is currently no more informative diagnostic method than ultrasound during the perinatal period. It is possible to see twins on an ultrasound with almost a 100% guarantee.

The determination of twins and twins is carried out by such a productive research method as ultrasound. This type of examination provides information about the location of the child, whether there is a multiple pregnancy, etc.

As a rule, two embryos are clearly identified on ultrasound from the earliest weeks - 5-7. Later, the doctor is able to determine this without the use of ultrasound diagnostics. Twins can be identified by such a sign as an increased size of the uterus from an early stage, and medical diagnostics using ultrasound will show two children only at 1 screening - at 10-14 weeks.

Multiple pregnancy is the development of two or more fetuses in a woman’s uterus, which can be detected using ultrasound.

At what stage of pregnancy can twins be detected?

Twins will show up on ultrasound at 5 weeks

If, during an examination in the initial weeks, the obstetrician-gynecologist notes a larger size of the uterus than expected, the doctor will prescribe an ultrasound examination. In this case, an ultrasound of the embryos will show the presence of twins already at 5 weeks, especially with the use of highly sensitive 3D scanning sensors. The examination makes it possible to identify twins only at 9-11 weeks.

Can an ultrasound doctor make a mistake and not see twins? Yes, this happens. Common reasons for error are: the use of an ultrasound machine that is too old, a late examination (in the last weeks) or when the first ultrasound scan was missed. Old equipment is not capable of high-quality visualization, since the length of the emitted ultrasound signal is not sufficient for scanning in the last months.

Consequently, the presence of twins in the womb is detected by ultrasound a month after conception; in the picture they look like 2 dark-colored formations inside the uterine cavity (see photo of an ultrasound examination of twins below).

• Ultrasound diagnosis of twins after 11 weeks shows slightly reduced sizes of the first and second fetus for their developmental period - approximately 4.2-4.8 cm.
• When examined at 12 weeks, the embryos are 6 cm long and weigh approximately 8 grams.
• At week 20, babies weigh 350 grams. You may notice that one child is larger than the second. This occurs due to a blood shunt, which constantly drains blood to one of the two fetuses.
• After 32-34 weeks, babies gain weight up to 2 kg.

You can see twins on an ultrasound during pregnancy as early as a month after conception. In this case, the specialist will mark two black spots on the monitor screen, which are clearly visible with ultrasound.

Characteristics of multiple pregnancy

Reasons that indirectly affect probable multiple births:

• a pregnant woman over 35 years old;
• heredity through the female line;
• abnormalities in the structure of the uterus;
• pregnancy as a result of IVF;
• pregnancy after stopping oral contraceptives;
• unnatural (artificial) stimulation of ovulation.

Types of multiple births:

• by the number of embryos: twins, triplets, quadruplets, etc.;
• by the number of fertilized eggs;
• by the number of placentas and amniotic membranes formed.

Types according to the number of fertilized eggs:

• Divozygotic: born when two eggs are fertilized. The appearance of two children of the same or opposite sexes is typical.
• Identical (presence of twins). Identical twins occur when one egg is fertilized. Babies are born only of the same gender.

Types by number of placentas and amniotic membranes:

• Bichorionic biamniotic twins - both embryos are enclosed by a separate placenta and amniotic membrane. The development of fraternal and identical twins may occur if the fragmentation of one egg occurs three days after conception.
• Monochorionic biamniotic - the embryos are separated by two amniotic membranes, but are located in the same placenta, the septum between the fetuses has two layers. This type of twins will be exactly identical when the egg is crushed between 3 and 8 days after conception.
• Monochorionic monoamniotic - one placenta and one amniotic membrane for two fetuses, no interfetal membrane is present. These twins are only identical and develop from the fragmentation of one egg from 8 to 13 days after conception.

If a doctor sees twins (regardless of the type they are), it is always a big surprise for future parents

Determination of twins without the use of ultrasound

It is possible to determine the presence of two (or more) children in the womb without the use of ultrasound examination in the later stages or at the time of birth with the following signs:

• increased size of the uterus in comparison with the expected period of pregnancy;
• palpation yielded a double number of small and large parts of the torso during an external gynecological examination;
• listening to heart sounds in different parts belly.

There are several ways to identify two babies without an ultrasound:

1. Blood test for hCG. The result of the analysis will determine the indicators to be somewhat inflated. This means that the hormone level is several times higher than when carrying one baby. Since the boundaries of this hormone are very large, only a doctor can draw a conclusion about possible pregnancy two kids.
2. The doctor palpates the abdomen and listens to heart sounds. The heart is beating already at 6 weeks, but it can only be heard during an ultrasound examination. You can listen to it through your stomach from 22 weeks. If experts said that in different parts of the uterus there are different heart tones that have a different frequency, and in the gap there is an area where the heartbeat is inaudible, this is a direct sign of a multiple pregnancy in the early stages, even without the use of ultrasound. When palpating the abdomen, you can feel 2 large parts - the head and pelvis. If the doctors said that two heads and two pelvises are clearly palpable, then the conclusion is obvious.
3. There is a sign among women that supposedly identifies twins. It is believed that severe toxicosis is a clear confirmation of this. In some cases, this may coincide, since toxin levels increase when a woman is expecting two babies.

How to independently identify twins? If it is visually clear that the belly is growing rapidly and no longer corresponds to the size indicators of its term, then this may be evidence of two babies.

It is possible to detect multiple pregnancies without resorting to ultrasound. One proven method is listening to your heart rate.

Features of carrying twins

The specifics of observation include the following points, which many experts talk about:

1. Frequent visits to the gynecologist: 2 times a month before 30 weeks, 1 time every 7-10 days after 31 weeks.
2. Monitoring weight gain. During multiple pregnancy, the body's need for vitamins, minerals, and calories increases significantly. Weight gain should not exceed 20-22 kg.
3. It is necessary to take medications containing iron to prevent anemia. Multivitamins are also prescribed for use throughout the entire 9 months. Restriction of unnecessary physical activity, rash, hour-long break from activity in the middle of the day
4. Monitoring by a gynecologist for changes in the cervix.
5. Regular ultrasound examination for early detection of retardation intrauterine development.
6. Periodic monitoring of the structure and condition of the placenta, the volume of amniotic fluid.
7. Preventive use of medications against fetoplacental insufficiency (chimes, methionine), preeclampsia (magne B6), miscarriage (ginipral, magne B6).

Possible complications

Multiple pregnancy is a big burden for the female body. At the same time, the pressure on the circulatory system, digestion and breathing increases excessively, and the risk of extragenital diseases increases (6 times). Main types of complications:

• increase in chronic diseases;
• gestosis and general deterioration of a woman’s health (occurs in the early stages and resolves with severe complications);
• a sharp decrease in hemoglobin, likely the appearance of anemia;
• intrauterine retardation in the development of one or both fetuses.

Ultrasound diagnostics during multiple pregnancies is carried out more often than when carrying a single fetus. This is associated with an increased risk of complications. An additional ultrasound examination before birth is required; it makes it possible to determine prenatal preparations (since it is often possible to see the incorrect position of twins in the womb). If a woman is told during an ultrasound that she has twins, she needs to undergo ultrasound diagnostics at least once in a well-equipped medical facility.

Hello!
Why suddenly multiple pregnancy? Perhaps you were already waiting for an article about first trimester screening at 11-13 weeks? The fact is that at 6-9 weeks, ultrasound can answer very important question about chorionicity of fruits. It is chorionicity significantly determines the course of pregnancy and its outcomes. But more on that a little later.

The largest number of children born to one mother, according to official data, is 69. The wife of the Russian peasant Fyodor Vasiliev, thanks to her fertility, was included in the Guinness Book of Records. Little is known about her: she was a peasant woman, lived in Shuya in the 18th century and gave birth to 69 children during her life! Such incredible offspring were the result of 27 births that occurred between 1725 and 1765. 16 times twins were born, 7 times triplets, and 4 times there were four children. Even more incredible is that almost all of the children born to Vasilyeva survived (only two died as infants). It is worth adding that this was the first wife of Fyodor Vasiliev, and his second wife gave birth to 18 children: 6 times twins and 2 times triplets.
27-year-old Ghazali Ibrahim Omar gave birth to seven twins in a hospital in the Egyptian city of Alexandria on the Mediterranean Sea. This is truly a miracle, since the mother did not resort to artificial insemination.
On January 26, 2009, 33-year-old Nadya Suleman, after an IVF procedure, gave birth to eight twins at once, and all of them are healthy. Two girls and six boys were born weighing between 700g and 1.9kg. True, the doctor who performed the IVF procedure was stripped of his license, and the clinic was fined.
In early 2012, scientists at the University of Bristol, having analyzed the results of 124,148 cycles of artificial insemination, resulting in the birth of 33,514 babies, published in The Lancet the results of an analysis of data on in vitro fertilization (IVF), which were provided by the Agency for Reproductive Technologies and Human Embryology (Human Fertilization and Embryology Authority, HFEA). The researchers concluded that implantation into the uterus during in vitro fertilization cycles of three or more embryos should be contraindicated, regardless of the woman's age.

How does this happen? And why does it not work for everyone, but only in very rare cases?

In this article I will not talk about various complications during multiple pregnancies, the frequency of premature births, intrauterine growth retardation, infant and maternal mortality. More about all this later, when we reach the 2-3 trimester of pregnancy. Although, if you have any questions, ask, I will try to answer.
I will only say that multiple pregnancy is a serious test for a woman’s body, and even more serious for her children. In the process of evolution, reproductive system women have evolved in such a way that very often female body unable to bear more than one child at a time. Is it good or bad - the question is too complex and multifaceted for me.
It is for this reason that humans have developed certain mechanisms of protection against multiple pregnancies. This includes the maturation, in most cases, of only one dominant follicle, and a change in the viscosity and density of cervical mucus after ovulation, which makes it difficult for sperm to enter the uterine cavity, and extremely rare cases of division of a fertilized egg to form monozygotic twins, and the phenomenon of “vanishing twins”.

Let's figure out what zygosity is.

Monozygotic twins (MB)- are formed from one zygote (fertilized egg) as a result of its division into several identical parts. Each part has the same genotype, such twins always have the same sex and a very similar phenotype. Scientists have not yet given a clear answer to the question of why exactly this happens. Among mammals, monozygosity is rare, with an incidence of about 1 in 1000, while polyzygosity is the norm in most species. An exception is the nine-banded armadillo, in which one embryo divides into four, forming monozygotic quadruplets. The incidence of monozygotic twins in humans is approximately 4 per 1000 births and is similar throughout the world, in contrast to dizygotic twins.
Di(tri)zygotic twins (DT)– respectively formed from two (three) fertilized eggs. Such twins have different genotypes, can be the same or different sexes, and are no more similar to each other than any brothers or sisters. The frequency of DB can vary greatly among different nations: 40 per 1000 births in western Nigeria, 6.5 per 1000 in Japan. According to The European Perinatal Health Report 2010, the incidence of twin births in European countries in 2010 averaged 16.75 per 1000.
The causes of DB have been studied much better, especially after the widespread introduction of assisted reproduction methods into practice. One of the main reasons for the formation of dizygotic twins is powerful hormonal stimulation of the ovaries. High level FSH can cause several follicles in one or both ovaries to mature and ovulate simultaneously, or the formation of two eggs in one follicle. It is possible that, as a result of different sensitivity of the receptors of the anterior pituitary gland, the level of FSH can vary widely, at times leading to polyovulation. Among women with multiple pregnancies, patients aged 35 to 39 years are more common. Those women who have already had dizygotic twins have a greater chance of having it again. Most likely, the predisposition to the development of dizygotic twins can be inherited along the maternal line in a recessive manner. If there is a history of monozygotic twins, the chance of its reoccurrence in the same woman is no greater than in the general population.
In general, the true number of multiple pregnancies is quite difficult to determine. As ultrasound scans in the first trimester show, the number of such pregnancies is much greater than the number of twins born. There is one "vanishing twins" phenomenon, when two (three) fertilized eggs are detected in the early stages, and subsequently, only one child develops and is born. It is also not uncommon for us to observe high numbers of hCG in the blood of a pregnant woman, more typical for twins, but in the future only one embryo is detected. The rate of “disappearance” of one of the twins at 6-8 weeks is approximately 21% for dizygotic twins (DT) and 50% for monozygotic twins (MB). The only complication of pregnancy in the presence of a “vanishing twin” is minor bleeding that does not threaten the life of the other fetus. After the first trimester, in another 12% one of the MB and in 2% one of the DB will disappear.

Now that we have defined zygosity, let's discuss chorionicity and amnionicity.

Determining chorionicity involves determining the number of placentas. If two fetuses have two placentas, then such a pregnancy is called dichorionic, if there is only one common placenta for two fetuses, then monochorionic. During twin pregnancies, about 80% of twins are dichorionic and their vasculature never communicates, 20% are monochorionic. Obviously, dizygotic twins are always dichorionic, each of them has its own placenta and is located in its own amniotic sac (amnion).
But with monozygotic twins, everything is much more complicated. They can be either dichorionic or monochorionic. In addition, monochorionic twins can each be in their own amnion or in one common amnion. It depends on the time when the zygote separated.
If separation of the fertilized egg occurs in the first 3 days after fertilization, then monozygotic twins have two chorions and two amnions. In this case, monozygotic twins will be dichorionic diamniotic. This variant occurs in 20-30% of all monozygotic twins. Therefore, if you were told on an ultrasound that two placentas are detected, this does not mean that the children will be “twins”.
If division of the fertilized egg occurs between days 4-8 after fertilization at the blastocyst stage, before the formation of amniotic cells, two embryos will form, each in a separate amniotic sac. The two amniotic sacs will be surrounded by a common chorionic membrane. Such monozygotic twins will be monochorionic diamniotic. The majority of monozygotic twins (70-80%) are represented by this type.
When separation occurs on the 9-10th day after fertilization, by the time the amnion is completed, two embryos are formed with a common amniotic sac. Such monozygotic twins will be monochorionic monoamniotic. Among monozygotic twins, this is the rarest type, occurring in approximately 1% of all monozygotic twins and representing the highest degree of risk in terms of pregnancy.

When an egg divides into more late dates, on the 13-15th day after conception (after the formation of the embryonic disc), the separation will be incomplete, which will lead to incomplete splitting of the twins and the formation of the so-called pugs(from the Greek pagos - fixed, chained), or conjoined twins. This type is quite rare, occurring in approximately 1:50,000-100,000 newborns.
I had to deal with just such a case, unexpected for me, relatively recently. At 7-8 weeks of pregnancy, I conducted an examination during which I did not see anything unusual - one fertilized egg, one embryo, one yolk sac, one beating heart.

And this is what my colleagues saw two weeks later. These are Siamese twins who are not separated at the level of the chest, abdomen, pelvis and, possibly, at the level of the heads.

What should be done in such cases, whether to continue such a pregnancy or whether it is better to terminate it, is an ethically very complex question and always individual. Modern medicine has already achieved a lot, and there are isolated cases of successful separation of conjoined twins. But these are extremely complex operations, which, unfortunately, rarely end successfully, and are almost always associated with the disability of both, or the inability to save the life of one of the twins.

The number of fetal eggs makes it possible to establish chorionicity already from the 5th week of pregnancy. Starting from 6-7 weeks, you can clearly visualize and count the number of yolk sacs and the number of embryos with an assessment of their cardiac activity. The number of amniotic cavities is determined from 8 weeks of pregnancy:
a) if each fertilized egg contains one yolk sac and one fetus, amnionicity corresponds to the type of placentation, i.e. dichorionic, trichorionic, etc.;
b) if the fertilized egg contains two yolk sacs and two fetuses with the presence of cardiac activity, subsequently the number of amniotic cavities may be greater than the number of placentas (monochorionic diamniotic) or the same (monochorionic monoamniotic). In this case, it is possible to accurately determine amnionicity after 8 weeks, when the amniotic membranes begin to be clearly visualized;
c) if the fertilized egg contains one yolk sac and two fetuses with cardiac activity, the number of amniotic cavities will correspond to the type of placentation (monochorionic monoamniotic).
From 9 to 10 weeks, rapid obliteration of the extraembryonic space begins due to an increase in the amniotic cavity.
In dichorionic twins, the amniotic septum between the fetal sacs is thick and contains chorionic tissue, which is located between the membranes in the form of a wedge. Echographically this sign is described as lambda-shaped placental tissue in the area of ​​the amniotic septum.
In a monochorionic diamniotic pregnancy, as the fetal egg grows and the extraembryonic space obliterates, the amniotic cavities begin to come into contact with each other, forming a thin dividing membrane. The separating membrane approaches the uterine wall at an angle of 90°, which is designated as T-shape connection.