Identical and human twins. Identical and fraternal twins

Cases when not one, but two or more babies settle in the mother's belly, are not so common - 1.5-2.5% of all pregnancies. How does it work? How is multiple pregnancy different? Can twins be scheduled? Our expert - Anna Romanovna PASTUKHOVA, obstetrician-gynecologist of the Zdravitsa family medicine centers answers all questions.

How likely to get pregnant with twins

First, some statistics. As we have already mentioned, this amazing phenomenon occurs in 1.5-2.5% of pregnancy observations in European countries. It is believed that the appearance of twins, triplets and twins is genetically determined - more often they are born in families where one or both parents were born as a result of multiple pregnancies. However, there are other reasons, among which .

• The birth of twins occurs with a frequency of geometric progression, obstetricians even have a special formula: for 87 births there is one twins, for 87 twins - one triplet, for 87 triplets - one quadruple, etc. True, today this formula needs correction - multiple pregnancies have become more common.

• Identical or identical twins are less common: on average, 1 case per 250 births.

• Most often, multiple pregnancies are observed in Africa, least often in Asian countries.

Types of multiple pregnancies

Let's start with a well-known fact. There are two types of twins: identical or monozygotic and fraternal (they are also called dizygotic).

• Monozygotic twins result from the division of one egg, fertilized by one sperm. They surprise everyone with their striking resemblance.

• Fraternal twins are the result of the fertilization of multiple eggs with multiple sperm. They can be either of the same sex or of different sex, often differ in character and are not always similar in appearance.

Expert commentary

Multiple pregnancy is a pregnancy when more than one fetus develops in the mother's body, but several at once. In medicine, there are identical or fraternal twins (triplets and so on). Identical twins are always the same sex, have the same blood type, and look very similar. Fraternal twins have phenotypic differences (eye color, facial features), there may be both the same and different blood groups. Fraternal twins can be of different sexes.

How identical twins are made

To be honest, we have no idea! The reasons why two (three, four) appear instead of a single baby from one egg and one sperm are not really known - a miracle, and nothing more. The mechanism of the emergence of monozygotic twins, however, is described in sufficient detail. It all starts as if nothing had happened: one egg, one sperm, meeting, fertilization ... Then the resulting zygote begins to divide, grow and, at some point, under the influence of not entirely clear factors, suddenly splits into two or more parts, each from which begins to live a life of its own.

Expert commentary

The development of identical twins occurs when one ovum is fertilized by one sperm, in the process of dividing the ovum into two, three or four parts at different stages of its development. Moreover, each part develops as a separate organism. It is assumed that such a separation can occur with a delay in implantation (attachment) of the ovum due to oxygen deficiency. Also, the occurrence of identical twins is associated with the fertilization of an egg that had two or more nuclei.

Do twins interfere with each other?

It is very important at what point the egg is split, since the further intrauterine development of the twins depends on it.

There are 3 options for the development of monozygotic twins

• The separation of the ovum occurred in the first five days after fertilization. In this case, both future babies begin to develop completely independently, each "grows" its own placenta and ... Until recently, it was believed that such a development is characteristic only of fraternal twins and identical twins in such cases were mistakenly identified as dizygotic (maybe this happened with the Olsen sisters?)

• The zygote split approximately 5-8 days after fertilization. By this time, the outer fetal membrane had already begun to form: remains common in babies, however, the inner fetal membrane (amnion) develops in each individual. If babies have a common placenta, then one child can suppress another - their competition begins with conception.

• Separation occurred on day 8 or later (1% of all monozygotic twins). At this time, the fetal bladder is already beginning to form, so the divided embryos will grow in the common amnion and common fetal waters. This situation is considered not the most favorable, there is a risk that babies each other.

The life of fraternal twins in the womb is organized with great comfort: each of them develops its own personal placenta (sometimes they grow together by walls), which means that babies do not need to compete seriously for a place “under the sun”.

So, if the embryos are in different placentas, then, most likely, they are heterogeneous, and if the babies "settled" with a common placenta, then these are identical twins who will be born similar like ... twins.

Expert commentary

There are the following types of multiple pregnancies:

• Monochorionic monoamniotic twins, when there is a common placenta and there is no septum between the twins. It's always identical twins
• Monochorionic diamniotic twins, when there is a common placenta and septum, i.e. each fetus has its own amniotic bladder
• Dichorionic diamniotic twins, when each fetus has its own placenta and its own amniotic bladder.

How fraternal twins are made

Non-identical twins develop from two or more fertilized eggs. How does this happen? The most common reason for this is the so-called "multiple ovulations", when one or both ovaries release several mature eggs at once. The reasons for this phenomenon may be different: genetic predisposition, artificial stimulation of the ovaries, the first cycle after the abolition of hormonal contraceptives.

Do you know that....

• The difference in time of conception of fraternal twins can be from several hours to several days.

• In very rare cases, a new one is added to an existing pregnancy. As a rule, after fertilization and implantation of the ovum, ovulation is immediately blocked. If, due to hormonal imbalance, this mechanism does not work immediately, a younger brother or sister is added to the older child in utero.

It is generally accepted that children born at the same time and similar in appearance are twins, and those having a different appearance are twins. In fact, this concept is erroneous: two fetuses developing simultaneously in the womb are called twins, they can be identical or fraternal. What is their difference - find out from our article.

Who are they: scientific concepts

The birth of twins is not so rare - once in 80 births. Multiple pregnancies result from the fertilization of two eggs by two spermatozoa that have matured at the same time: fraternal twins or dizygotic, as well as when two embryos develop from one fertilized egg - identical twins, or monozygous.

Monozygous (identical)

Monozygotic twins are always of the same sex. The intrauterine development of such fetuses is always associated with an increased risk, the degree of which depends on its formation.

At the heart of these problems is the sequence of changes associated with the division of the fertilized egg. It also determines the structure of the membranes.

Dizygotic (fraternal)

It accounts for 70% of all multiple pregnancies.

In the case of fraternal twins, the fertilized eggs develop on their own. After being introduced into the mucous membrane of the uterus, each fetus forms its own amnion and chorion, later a separate placenta with an independent network of vessels is formed.

In rare cases, in the circulatory system of the two placentas, there are anastomoses - branches that connect various vessels. This pathology can lead to uneven blood supply to the fetuses and a lag in one of them in development ().

Interesting! Fraternal twins are not always conceived during the same intercourse, sometimes the difference is several days. You can also conceive.

Dizygotic twins can be of different sexes or same sex.

Twins and twins - what's the difference?

In order not to overload people who do not have a medical education with terms, let us call monozygous fetuses - twins, and dizygotic ones - twins, especially since many are used to perceiving them this way.

What causes the differences - in simple words

How are twins different from twins?

Differences in fruits are determined by the characteristics of their conception:

1. Twins develop from a single egg that has been fertilized by a single sperm and just split. Accordingly, babies are very similar to each other, of the same sex, have the same genetic makeup and identical blood groups. It is known that such children have a special connection with each other, they feel each other's experiences at a distance, they suffer from the same diseases.
2. The development of twins in the womb is due to the fertilization of several eggs by different sperm. Therefore, they differ from each other not only externally, but also genetically. They may have different blood types, which makes it impossible to donate for each other. Thanks to modern medicine, twins are easily planned. If a woman has been taking oral contraceptives several months before conception, the likelihood of having twins increases. Often, multiple pregnancies develop after artificial stimulation of ovulation or IVF.

Difference in DNA

What are the differences in the DNA of mono- and dizygotic twins:

1. Twins have an identical genotype, which is why they are of the same sex. The similarity of their genetic composition is also associated with their origin - the early division of the egg into two cell masses, fertilized by one sperm and carrying one genetic information. They can be donors for each other, organ transplants from such siblings are always successful.
2. Twins are produced by fertilization with two different sperm containing different DNA. As a result, each embryo receives genetic material separately from mom and dad.

In the womb during pregnancy

In order to make it easier to understand how twins and twins develop in a mother's stomach, you need to know the basic concepts.

The fertilized egg consists of two shells:

1. Amnion - an aqueous membrane that protects the embryo from environmental factors.
2. Chorion - the outer shell with villi, which at a certain time is transformed into the placenta.

Twins in most cases, each has its own water membrane and chorion, and later a separate placenta with an independent network of vessels is formed. This option is ideal, each fruit receives the necessary nutrients and develops safely.

Twins usually have one chorion and a common placenta, large vessels in it are connected by small vascular branches, each has its own water membrane. In rare cases, the twins are surrounded by one water shell.

By appearance

1. Twins differ from twins in that they have a different appearance.They are no more alike than a brother or sister born at another time. They have individual facial features, eye and hair color. Often, babies are of different sex, have a different temperament, they have different tastes and hobbies. Twins can differ in skin color if their parents are of different races.
2. Gemini are born almost the same in appearance. Even parents can find it difficult to tell the difference. Mirror similarity is often found in monozygous fetuses, when one of the children is a reflection of the other: someone is right-handed, the other is left-handed, one has a mole on the right cheek, and the other has a mole on the left. This phenomenon is usually observed with late egg division. Monozygotic twins have the same shape of teeth and their location, eye color, and often suffer from the same diseases.

Fingerprint

Twins and twins have different fingerprints:

1. In identical twins, they are similar in many ways: the number of lines, patterns, but the detailed drawing is still different. More recently, it was believed that the patterns on the fingers of twins were the same, but with an in-depth study and examination, scientists discovered that the coincidence along the lines in such children is 95%.
2. Twins have completely different fingerprints. The drawing on the fingers is formed not only under the influence of the genotype, but also depends on the position of the fetus in the uterus, the rate of intrauterine development, blood pressure, etc.

By blood type

Another significant difference between twins and twins is the blood type:

• is always identical, since they were formed from one fertilized egg.
• In the case of twins, the group affiliation can be either the same or different. They receive one gene from each parent. Rh-positive couples may have Rh-negative twins if blood relatives carry Rh-. Rh-negative parents will have both children also Rh-.

Who will be born - twins or twins - is determined by the method of conception. Identical twins are identical in almost every way. Fraternal twins are regular brothers and sisters, only when they are born and born at the same time.

It doesn't matter which of them was born to you, it is necessary to bring up a personality and individuality in each child.

Gemini identical and fraternal

The appearance of twins aroused and still arouses great interest in those around them. The twins were always looked at with interest and some superstition. For example, in Ancient Rome twins became heroes of many myths and legends. And the twins Castor and Polux, the sons of Zeus, gave names to the two brightest stars from the constellation Gemini.

How often do twins meet in life? First, we need to clarify what kind of twins there are. If conception and implantation have occurred as a result of fertilization of two or more eggs with the corresponding number of sperm, then these are fraternal twins. They can be heterosexual and not very similar to each other. As a rule, observations of such twins confirm that these are two (three, etc.) different people who were simply born at the same time. External similarity is determined by the presence of some common genes (after all, mom and dad are the same) and similar living conditions.
Identical or monozygotic twins (MZB) are another matter. Everything is more complicated here. One egg fertilized by one sperm is divided into two (less often three, four) parts, and these "parts" begin to develop independently. It is quite important at what time stage in the development of the embryo this division took place. Separation can occur during the first two weeks of embryo development, resulting in multiple forms of monozygous twins. In 30% of cases, separation occurs within the first 72 hours. Then each embryo forms its own placenta and its own fetal bladder (dichorionic and diamniotic twins). If separation occurs at this stage, then children will develop as two independent organisms, but with an identical set of genes. In 2/3 of cases, the separation occurs on the 5-8th day of development. Then one placenta and two fetal bladders are formed (monochorionic and diamniotic twins).
In embryological practice, it is sometimes necessary to observe blastocysts with a doubled internal cell mass (ECM) (it is this part of the blastocyst, or the embryo of the fifth or sixth day of development that is responsible for the formation of the embryo itself after implantation and further development). In 5% of cases, separation occurs on days 9-13, resulting in monochorionic monoamniotic twins. Later separation can lead to the birth of Siamese twins.

Fraternal twins are born for every hundred births, and identical twins are born for every three hundred. For every 130 births of twins - one triplets, for two million regular births - one quadruplets. It is believed that the predisposition to have twins is transmitted through the maternal line. But all these statistics are for natural conception. For patients who have to resort to methods of assisted reproductive technologies (ART) for the birth of a child, the statistics are different.

To increase the efficiency of the IVF (in vitro fertilization) procedure, and sometimes just to induce ovulation in anovulatory form of infertility, it is necessary to resort to hormonal stimulation, as a result of which not one, but several eggs are obtained. After in vitro fertilization, several embryos are obtained. When two or three embryos are transferred into the uterus, fraternal twins can be obtained. The likelihood of having such twins is quite high, and this is one of the reasons for dissatisfaction with the results of IVF in some medical disciplines. Bearing, giving birth and the first stages of life of such children, as a rule, have difficulties.

Some statistics. When transferring two embryos (in developed countries this number is determined by law, as in many clinics respecting their reputation in Russia), the percentage of pregnancy is 40-45%, with the transfer of one embryo 35-38%. Moreover, the probability of twins is 32% and 0.8%, respectively. These data are confirmed by many authors. Thus, a slight decrease in the percentage of pregnancy during the transfer of one embryo is compensated for by obtaining a singleton physiological pregnancy. And this is precisely the main task of ART.

If everything is clear enough with fraternal twins, then not everything is so simple with monozygotic twins or, even less often, triplets. This is indeed a rare situation. And, nevertheless, the statistics of the birth of such children shows an increase from 0.42% (in the general population) to 1.2-8.9% (according to various authors) after the use of ART.

Many authors show a relationship between an increase in the birth of MZB with cultivation conditions and various manipulations with embryos. Indeed, the first days, even the hours of development, are very vulnerable, and the presence of embryos in sufficiently stressful conditions cannot but affect their future fate. Therefore, most authors suggest a relationship between the duration of cultivation and the appearance of MZB. Transfer to the third day in comparison with extended cultivation (fifth day) gave a higher percentage of MZB birth. The authors found 4 cases of MZB (1.57%) during blastocyst transfer. At the same time, the results indicated that the percentage of pregnancy on the 5th day was 67.8 compared to 36% of the transfer on the 3rd day, and it was noted that given such results, such an increase in the percentage of MZB can be neglected.

Some authors have shown a higher risk of MZB birth if the transferred blastocysts were obtained after ICSI, assisted hatching, or PGD (preimplantation genetic diagnosis). All these procedures lead to the formation of a hole in the embryonic membrane, which can affect the integrity of the ECM, but why then in all patients after ICSI, hatching and PGD MZB are not born, but at the same time they can occur in classical IVF options that do not involve manipulation with shells of the embryo. Moreover, an increase in MZB is also noted during the procedure of artificial insemination, which does not involve the stage of in vitro cultivation of embryos.

Other authors argue that ovarian stimulation itself can lead to an increase in the birth of MZB. The effect of increasing the MZB was found with the abolition of oral contraceptives and the onset of pregnancy in the first 3 months. In some works, the effect of clomiphene citrate on the level of MZB is noted. Back in 1987, work was carried out indicating that artificial induction of ovulation may be the first biological mechanism influencing the appearance of MZB.

Fraternal twins are born if two eggs were fertilized at the same time. Normally, a woman releases one egg during ovulation, however, depending on her body, there may be more of these cells, and this is not considered as any pathology. In fact, such children can hardly be called twins, since they are not identical in appearance, they rather resemble ordinary brothers and sisters who should have been born at different times, but were born at the same time. They have a different genetic structure, which will be 40-60 percent identical in both cases.
More often than not, fraternal twins are not only different from each other, but also have a different gender. Carrying twinsalways involves certain risks, because the female body is theoretically designed for the birth and feeding of only one child. If the mother twins are expected, then, most likely, she already had a multiple pregnancy in the anamnesis, or she has a genetic tendency to carry fraternal twins.
Every fifth multiple pregnancy ends with the death of one of the fetuses, but its remnants may be born with a living child. It is much more difficult for a woman's body to meet the needs of fraternal twins for vitamins and minerals, so there is a risk of developing iron deficiency anemia. Very often, multiple pregnancies provoke severe gestosis in the third trimester, but bed rest can help in this case.
In most cases, multiple pregnancies involve a cesarean section, since it is very difficult to predict the course of labor, especially since only one child can be turned correctly towards the exit - head down, and the second is perpendicular.

Identical twins or fraternal twins can be heard when people talk about twins and twins. What does the number of eggs and twins have to do with it? We offer at the everyday level, without delving into precise definitions, to figure out what it is when people say fraternal or identical twins. We will also briefly touch on a couple of difficult points. Let's tie "eggs" and zygotes plus pregnancy. We will deal with different and identical twins using everyday concepts and a school biology textbook G).

Identical twins and fraternal twins - the basics

To better understand what fraternal or identical twins are
let's remember some basics. Let's start with the fetus and multiple pregnancies.

Fetus - this is generally the result of pregnancy of the female body, in our case, the mother. There are different stages of fetal development. Sometimes for these stages the fetus is called an embryo or embryo. It's always about the same thing, about the result of pregnancy, i.e. about the fact that inside the mother's body lives and develops, and then, in a positive result, should be born in the form of a child, in our case, twins.

Egg Is a germ cell that develops in the female body. When merging with the male reproductive cell (sperm), the fetus develops from the egg during the process of breaking the egg into smaller other cells. The ovum is similar in structure and appearance to an egg, from which it got its name.

Multiple pregnancy

Pregnancy can be singleton or multiple
... Singleton - one child will appear, multiple pregnancy - two or more twin children may appear.

Twins (twins), triplets, etc. - These are the numerical designations of the number of children born with multiple pregnancies. Twins (twins) - two children, triplets - three, etc. However, they are all called twins, even if they were born in a different zodiac sign. They can also be identical and fraternal twins.

Twins - children born in one pregnancy and born as a result of one childbirth from one mother. At the household level, everything is simple. Twins are different children from the same mother, after the same birth, and twins are the same children under the same circumstances. And in essence, it doesn't matter whether they are similar or not, no matter what they are called twins, twins or twins - all the same they are twins, you just need to remember. Twins are children born as a result of multiple pregnancies.

Identical and fraternal twins or twins

So, we know that children born as a result of multiple pregnancies can be identical or fraternal twins, or, simply, twins. And there is another option when the so-called mirror twins appear.

Identical twins (twins)

Identical twins or identical twins are children born as a result of multiple pregnancies when one egg is fertilized by one male reproductive cell, which led to the appearance and development of two or more fetuses. Children turn out to be almost the same, i.e. identical.

How identical twins are made

Let's outline how identical twins are made. In fact, any fertilized egg (it can be called a zygote) in the process of intrauterine development is divided into two or more parts. This is how children become genetic copies of each other, i.e. identical. Roughly speaking, this is how natural cloning occurs. It is to this process that we owe the appearance of twins.

The division of a fertilized egg is directly responsible for how identical twins are obtained. .

• If division took place before 3 days, inclusively, after fertilization, then dichorial diamniotic twins are obtained, which is very similar to fraternal twins.
• If the division is between 4 and 8 days after fertilization, then monochorionic diamniotic twins are obtained.
• If on day 9 or 10 - monochorionic monoamniotic twins.
• If between 13 and 15 days, such a division will not be complete and you get a monochorionic monoamniotic conjoined twins - Siamese twins.

What do "chorial" and "amnion" mean will be below.

Mirrored twins - identical

By the way, identical twins can be mirrored.
, i.e. one of them has a location or appearance some body objects may be the same as if the other would look in the mirror. For example, a mole on different parts of the body, one on the right and the other on the left. One may be right-handed and the other left-handed. Identical mirror twins can only be monozygous (homozygous) and appear in 25% of twin births. They can be very similar in fingerprints, but still differ in small details. Their blood group is the same.

Fraternal twins (twins)

Fraternal twins or twins are children who appeared as a result of multiple pregnancies when two or more different eggs were fertilized with two or more different male sex cells, which also led to the appearance and development of two or more fetuses.

Simply put, there is a sperm for each egg. Children can be different in everything. A striking example is twins, a boy and a girl, they are also royal twins. In this case, fraternal twins are always dichorial diamniotic twins.

Identical twins and fraternal twins: "eggs" plus zygotes and pregnancy

Here, in fact, we will make an attempt to combine medical, biological and household terms into one whole, so that we no longer get confused and understand exactly what we are told about the same thing, only in different words. It's all about identical and fraternal twins.

All twins, twins, triplets, etc., as well as twins and twins, are twins in the biological sense. However, there are well-established terms and concepts in which the word "twins" is used.

In medical terminology, when describing pregnancy, the difference between twins in the placenta (one or more) and in the number, the presence of septa in one amniotic sac (one or more) is used. On this occasion there is a separate article monochorial diamniotic twins or monochorionic monoamniotic twins or diamniotic dichorial twins - WHAT IS IT or WHO IS IT ??? In it, we tried to explain as simply as possible what it is. At first glance, it seems that this is a repetition of all of the above, and it seems to be true. However, there are still those who get confused and think that this is all about different twins. Just for them it is written there, let them read.

When describing the birth or appearance of twins, the differences between twins in the number of eggs involved in their appearance are described. There will also be words from the field of genetics - zygotes, genes, chromosomes.

At the everyday level, twins are always different, and twins are always the same, everything is simple here.

Fraternal twins \u003d dizygotic twins \u003d dichorial diamniotic twins

Fraternal twins or twins are always and unequivocally dizygotic (heterozygous) and dichorial diamniotic twins. These are twins in the everyday sense. In fact, these are two different children, in everyday life they would say about them - twins.

They are the result of multiple pregnancies and will develop as two different babies. Children will resemble each other as close relatives and nothing more. We started using "different" hetero, di and monozygotes. In the article Monozygotic Twins and Dizygotic Twins - WHAT IS IT, you can familiarize yourself with them in detail and their relationship with twins.

It should also be noted that, unlike identical twins, for fraternal twins it does not matter when the separation occurred, because it was the default at the time of conception. The only option is that the placentas grow together. Then the dichorial diamniotic twins "become similar" to the monochorionic diamniotic twins, but still they are two different placentas.

Identical twins \u003d monozygous twins

Identical twins or twins are always monozygous, they are also homozygous twins, i.e. as a result of multiple pregnancies, children of the same sex and blood group are absolutely similar to each other. In everyday life, they are usually called twins. And then the differences begin, although at the everyday level they are always twins anyway. Identical twins (twins) differ in:

• monochorionic diamniotic, i.e. fetuses (embryos) are connected in the womb with matter by one placenta and each has its own amnion (amniotic sac). In fact, in the common amnion there is a partition separating the fruits, and it turns out, as it were, his own apartment for everyone in the common house. Then identical twins, as fruits, develop apart from each other. The probability of occurrence is 70-80% of all identical ones. The division took place between 4 and 8 days after fertilization. If division occurs between 13 and 15 days after fertilization, then it will not be complete and the fetuses (embryos) will grow together - a very difficult case.
• monochorionic monoamniotic, i.e. fetuses have one placenta for two and one common amniotic sac. This means that the fruits are not separated by a septum in the amniotic sac, and develop together. This is a very dangerous option for the development of identical twins. There is a possibility of the birth of Siamese twins. The probability of occurrence is 1% of all identical ones. Division took place after 9 days, inclusively, after fertilization.
• dichorial diamniotic, everything is like fraternal twins. The probability of occurrence is 20-30% of all identical ones. The division took place up to 3 days, inclusive, after fertilization.

Dizygotic twins are more common (in 66-75% of all twins). The birth rate of dizygotic (fraternal) twins varies from 4 to 50 per 1000 births. In African countries marked high frequency births of twins, in Europe and the USA - average, in Japan - low. The highest birth rate of dizygotic twins (40: 1000) was recorded in West Africa.

How are fraternal twins made?

• Dizygotic (fraternal) twins arise when two separate eggs are fertilized.
• The maturation of two or more oocytes can occur in one ovary or in two.
• Fraternal twins can be both same and opposite sex and are in the same genetic dependence as siblings.

Dizygotic twins are always characterized by a dichorial, diamniotic type of placentation. In this case, there will always be two autonomous placentas, which can touch, forming, as it were, one placenta, but they can be divided. The two fetuses are separated by two chorionic and two amniotic membranes.

One of the main reasons for fraternal twin pregnancies is the powerful hormonal stimulation of the ovaries. High FSH levels can cause maturation and ovulation of several follicles in one or both ovaries at the same time, or the formation of two oocytes in one follicle. Most often, two eggs come from one follicle.

It is also possible that the activation of hypothalamic releasing factors leads to stimulation of the pituitary gland and further poliovulation. It is possible that, as a result of the different sensitivity of the receptors of the anterior pituitary gland, the level of FSH can vary widely, sometimes leading to polio.

A similar picture of poliovulation against the background of an increased level of FSH can also develop during stimulation (hyperstimulation), ovulation with clomiphene citrate, clostilbegite, and chorionic gonadotropin.

There was a definite relationship between a number of factors and the frequency of pregnancy in fraternal twins. So, among women with multiple pregnancies, patients aged 35 to 39 years are more common. Among these women, re-pregnant women predominate, with a relatively large body weight and height. Those women who have already had dizygotic twins have a greater chance of having it again. Most likely, a predisposition to the development of fraternal twins can be inherited through the maternal line in a recessive manner. If there is a history of monozygous twins, the chance of their re-development in the same woman is no greater than in the general population.

What is the likelihood of having twins?

What are the odds of having fraternal and identical twins?

There is a higher frequency of twins with anomalies in the development of the uterus (two-horned uterus, septum in the uterus). With a bifurcation of the uterus, more often than with its normal structure, two or more eggs mature simultaneously, which can be fertilized.

Assisted reproduction technologies have had a direct impact on the increase in the frequency of dizygotic twins, which is a consequence of the stimulation of FSH and LH used in this process, which in turn leads to multiple ovulation.

It is assumed that almost all induced pregnancies are multizygous. This is due to the fact that as a result of stimulation, several follicles ovulate at once. When several eggs are fertilized at the same time, some of them can be cleaved and a "mixture" of dizygotic and monozygotic twins is formed.

Theoretically, there is a possibility of fertilization of two eggs of different ovulation periods, with the onset of a new pregnancy in the presence of an earlier one, since another ovulation may occur. In the first weeks of pregnancy, the sperm can penetrate into the uterine cavity, and from there into the fallopian tubes and fertilize the egg that was released during the last ovulation. A fertilized egg can be implanted into the decidual (falling off) membrane, since in the first weeks of pregnancy the uterine cavity is not yet completely filled with the ovum

In the literature, there is a description of the observation when a woman with a complete bifurcation of the uterus and vagina in each of the uterus was found to have simultaneously developing pregnancies: in one - 12 weeks, in the second - 4 weeks.

It is also possible that multiple pregnancies arise from eggs of the same ovulation period, but fertilized by different sexual partners.