Twin and identical twins.

A multiple pregnancy is one in which two or more fetuses develop simultaneously. If a woman is pregnant with two fetuses, she is called a twin, with three fetuses, she is called a triplet, etc. Children born from a multiple pregnancy are called twins.

Multiple pregnancy occurs in 0.7-1.5% of cases. Currently, there is a tendency for its frequency to increase due to hyperstimulation of ovulation in women with infertility when using assisted reproductive technologies. In this case, several follicles (3-4 or more) mature simultaneously and, accordingly, when several eggs are fertilized, multiple pregnancy may occur.

The frequency of multiple pregnancies can be determined using the Haase formula (1895). According to this formula, twins occur once in 80 births, triplets - once in 802 (6400) births, quadruplets - once in 803 (51200) births.

In multiple pregnancies and childbirth, complications occur more often than in singleton pregnancies. Perinatal mortality in multiple pregnancies is 3-4 times higher than in singleton pregnancies. Perinatal losses are directly dependent on the body weight of the children, averaging 10%. Perinatal mortality among monozygotic twins is 2.5 times higher than among dizygotic twins, and is especially high in monoamniotic twins.

Identical twins

There is no clear hypothesis for the formation of identical twins. Apparently, the most common cause of identical twins (triplets, etc.) is the fertilization of an egg cell that has two or more nuclei. Each nucleus combines with the nuclear substance of the sperm and an embryo is formed. Egg cells with two and three nuclei have been identified. There is also another mechanism for the origin of identical twins: a single embryonic rudiment in the cleavage stage is divided into two parts, and a fetus is formed from each part.

When the division before the formation of the inner layer (at the morula stage) and the transformation of the outer layer of blastocyst cells into chorionic elements occurs, which occurs in the first 72 hours after fertilization, two amniotic sacs and two chorions develop. The result is biamniotic bichorionic monozygotic twins.

If the division occurs on the 4th-8th day after fertilization, after the formation of the inner layer of the cellular membranes and the formation of the chorion from the outer layer, but before the formation of the amniotic cells, that is, before the appearance of the fetal bladder, two embryos are formed, each in a separate amniotic sac - monochorionic diamniotic monozygotic twins develop. Most monozygotic twins (70-80%) are represented by this type.

If the amnion has already been laid by the time of division, which occurs on the 9th-12th day after fertilization, then the division leads to the formation of two embryos in one amniotic sac, i.e. monochorionic monoamniotic monozygotic twins. This is the rarest type of monozygotic twins, which occurs in approximately 1% of cases of monozygotic twins. However, there is a significant risk in terms of the course of pregnancy.

After the 15th day, complete separation of the embryonic rudiments is impossible. In this case, conjoined twins develop. This type is quite rare, approximately 1 in 1500 multi-fetal pregnancies.

Identical twins are always of the same sex, have the same blood type, the same eye color, hair color, skin texture of the fingertips, shape and arrangement of teeth, and are very similar to each other. They tend to suffer from the same diseases at the same time, and have the same learning abilities.

In monochorionic diamniotic monozygotic twins, the membrane that separates the two amniotic cavities is fairly transparent, it has no vessels, as well as remnants of the decidua and trophoblast. At the same time, a certain type of connection of the fetoplacental circulatory systems of the twins is always observed to one degree or another, which contributes to the development of transfusion syndrome.

It should be noted that in monochorionic placentas, vascular anastomosis can be found quite often - arterioarterial or arteriovenous, which causes the development of fetofetal transfusion syndrome. Arteriovenous connection is carried out through the capillary system of the placenta. Due to such anastomosis, blood flows from the artery to the vein from one fetus to another. In bichorionic placentas, arteriovenous anastomosis develops much less often. The consequences of such anastomosis can be very serious. If the blood pressure in the vascular system of the placenta is symmetrical, both twins find themselves in the same living and developing conditions. However, in identical twins, this balance can be disrupted due to asymmetrical placental circulation, and then one fetus receives more blood (the recipient) than the other (the donor). The latter does not receive sufficient nutrition and finds itself in worse conditions for its development, which can cause intrauterine growth retardation of the fetus. With a sharp disruption of the balance in the placental circulation system, one of the twins (the donor) gradually becomes exhausted, dies and mummifies, turning into a "paper fetus" (fetus papyraceus). Often, the recipient twin develops polyhydramnios and dropsy, caused by heart failure.

Fraternal twins

Among all types of twins, fraternal twins occur in 70% of cases.

The formation of dizygotic twins is possible in the case of:

• simultaneous maturation and ovulation of two or more follicles in one ovary;
• maturation and ovulation of two or more follicles in both ovaries,
• fertilization of two or more eggs that have matured in one follicle.

Such variants of the origin of fraternal twins are indicated by the detection during surgical interventions of two or more corpora lutea of the same age in one or both ovaries.

Dizygotic twins are always characterized by a bichorial, biamniotic type of placentation. In this case, there will always be two autonomous placentas, which can be tightly attached, but they can be separated. Each fertilized egg that penetrates the decidua forms its own amniotic and chorionic membranes, from which its own placenta is subsequently formed. If the eggs penetrated the decidua close to each other, then the edges of both placentas are closely adjacent, which creates the impression of a single formation. In reality, each placenta has its own vascular network, each fetal sac has its own amniotic and chorionic membranes. The partition between the two fetal sacs consists of four membranes: two amniotic and two chorionic, and the decidua is common (bichorial twins). If the fertilized eggs have penetrated a significant distance, then the placentas develop as separate formations, and each fertilized egg has its own, separate decidual membrane.

Fraternal twins can be of the same or different sexes and have the same genetic relationship as siblings.

The difference in body weight in fraternal twins is usually small and ranges from 200-300 g. In some cases, due to different conditions of intrauterine nutrition, the difference can be quite significant - up to 1 kg or even more.