Multiple pregnancy. Etiology

Multiple pregnancy is referred to as pregnancy with appearance of two or more fetuses. Pregnancy with two fetuses is referred to twins, three fetuses — triplet, and so on. Each fetus at multiple pregnancy is referred to as a twin. In other words, twins are posterity of one mother who have appeared during one delivery.

Multiple pregnancy in a human being, unlike mammals in which it is common, is encountered rather rarely — on average once per 70-80 births. With increase of twin number, the incidence of multiple pregnancy progressively decreases.

Etiology. The reasons of multiple pregnancy are various, though they are not sufficiently studied. The possibility of fertilization of two or more ova is presumably created by a number of reasons:

- Two or even three follicles can simultaneously ripen in one ovary, that makes ovulation of not one, but two and more ovules possible;
- Ovulation can also occur simultaneously in both ovaries, hence, two or more ovules capable for fertilization are simultaneously released;
- Each mature follicle can contain two and even three ovules;
- Twins result from atypical process of cell division.

These data are testified by the similar facts during various abdominal operations.

Hereditary predisposition is of dominant significance in occurrence of multiple pregnancy. In some families, from generation to generation, delivery of twins and triplets can be observed.

The increased incidence of multiple pregnancy in women with endocrinal infertility after stimulation of ovulation, canceling of hormonal contraceptives, at extracorporal fertilization is established.

Multiple pregnancy is more often encounterted in elder women, and in those having more pregnancies.

Pathogenesis. Multiple pregnancy at a human being can be presented as two biological phenomena: monogerminal (monozygotic) and dizygotic twins. Dizygotic twins are fraternal twins. They develop from two ovules formed in one or different follicles, each of them having been fertilized by separate spermatozoon. And maturation of follicles in one or both ovaries not necessarily occurs simultaneously. In this connection there is an assumption of opportunity of supraconception (superfoetatio).

According to this hypothesis, difference in weight of the twins is explained by fertilization of two ovules in different ovulation periods. One of the twins, being "elder", is more developed. The fact, confirming possibility of supraconception, has been described in Russian literature by N. Sochava: in a woman with complete bifurcation of the uterus and vagina a simultaneously developing pregnancy was found out in each of uteri: in one — about 12 weeks of gestation, in the other — about 4 weeks. The possibility of fertilization of two ovules in various ovulation periods cannot be excluded, as, despite of onset of pregnancy, sometimes next ovulation can occur.

Fertilization of ovules develops in a usual way, independently from each other, forming two аmnions and two chorions. For each fetus its own placenta is formed.

Both placentas quite often remain separate, especially when the fertilized ovules have embedded into the mucous layer of endometrium at a significant distance from each other. In similar cases each fertilized ovum also receives a separate decidual capsule (decidua capsularis).

When both fertilized ovules are embedded into a mucous layer of uterus close to each other, the edges of both placentas so closely adjoin each other, as if merge in one whole; however chorion and amniotic membranes of each fertilized egg remain separate, while a capsular membrane is common. However the merging of placentas is apparent, it is not difficult to see it at more thorough examination of merged placentas: each placenta is easily separated from the other, and if to introduce into one of the umbilical vessels a water solution painted with a non-diffused substance, the vascular network of only one placenta becomes injected.

At fraternal (two-egg) twins, having common decidual capsule and separate for each twin chorion and amnion, each fetus lies in its own chamber, and the septum dividing both аmniotic cavities consists of four membranes: two аmnions and two chorions. Though each of these membranes adjoins intimately to the neighbouring one, it can easily detach from it. The two-egg twins may be unisex (both boys or both girls), and of different sexes (a boy and a girl). The blood group of the twins can be identical or different.

From the genetic point of view, dizygotic twins are similar, as usual brothers and sisters, i.e. they have approximately 50 % of common genes, however they differ from usual siblings by much greater commonness of mixed (both prenatal and post-natal) factors.

Monozygotic (one-egg) twins develop from one ovum, fertilized by one sperm cell. During the first two weeks after fertilization, there is a splitting of zygote into two symmetrically identical halves, which have an identical hereditary potential, but develop as two independent, very similar to each other individuals.

If the splitting of zygote occurs during the first 5 days after fertilization (up to a stage of morula), each embryo will form its own separate embryonic membranes (amnion and chorion). At splitting of zygote in the stage of developed morula, approximately on the 5-7th day after fertilization, the twins develop in one chorion (one placenta), but are separated from each other by amniotic membrane. If the splitting of zygote occurs after the 7th day, the process of division does not proceed anymore, and the twins develop in one amniotic cavity with presence of one placenta, that occurs rather seldom. The processes of splitting occurring after the 13th day of a zygote development, as a rule, do not result in complete separation of the twins. There are various variants of their growth and anomaly of development.

Theoretically monozygotic twins should be absolutely identical. They are always unisex — either both boys or both girls. They always have identical blood group.

The distinctions between them are explained by conditions of intrauterine existence, i. e. by environmental factors.

Visual inspection of afterbirth (expulsed placenta) and definition of newborn’s sex sometimes allow to establish a type of zygotic twins at birth (monochorionic twins are always monozygotic, those of different sex are always dizygotic). It is necessary to make a research of septum located between amniotic cavities. The presence in septum of two layers of membrane testifies to a monochorionic type of placenta, of four layers — to two placentas, probably joint (a dichorionic type). At birth it is practically impossible to determine a type of zygosity of dichorionic diamniotic liquid through the vessels of unisex twins, as they can be both mono- and dizygotic. In these cases the type of zygosity is determined by special laboratory and genetic analysis. If a water solution with a painting non-diffused substance is introduced into the vessels of placenta of monozygotic (one egg) twins, it is easy to see that the solution spreads all over the vascular system of both placentas. It testifies to presence of anastamoses in placenta between vessels belonging to system of blood circulation of each twin. Hence, in placental vessels there is a mixture of blood of both twins. If in the vascular system of placentas the blood pressure is balanced (“symmetric”), both twins are found in the identically favourable terms of nutrition and development. However, in оne-egg twins this balance is quite often broken owing to asymmetry of placental blood circulation: one of the twins receives more blood than the other, that causes distinction in their feeding, and consequently, in the development of twins.

Wherein the balance in the system of placenta blood circulation is sharply disturbed, one of the twins begins to provide own blood circulation and blood circulation of the twin. The heart of the latter becomes inactive, and it turns to the "heartless" monster (acardiacus). It is a shapeless mass, in which at closer examination it is possible to distinguish the outlines of separate parts of the body. In other cases under the same conditions, one of the twins is gradually exhausted, dies and becomes mummified, turning into papyraceous fetus (foetus papyraceus), which is born after the alive twin as appendage to him.

Triplets, quadruplets and other variants of multiple pregnancy can be of various origin. Thus, for example, triplets can result from the development of two fertilized ovules, one of which has given rise to one-egg twins, and the second — to development of one fetus. Another variant is possible as well: each of the three twins has developed from own ovule (a three-egg triplet).

The difference in weight in both twins is usually insignificant and ranges within the limits of 200-300 g. In some cases, owing to the above-mentioned distinctions in conditions of feeding, this difference can be rather significant — up to 1 kg and even more.

Diagnostics of Multiple Pregnancy. Clinical diagnostics of multiple pregnancy represents essential difficulties even at the end of second and third trimester of pregnancy. Sometimes (in 30-72 % of women), multiple pregnancy is determined only on delivery.

Diagnostics of multiple pregnancy is usually performed on the basis of clinical examination of a pregnant woman:
- Study of anamnesis;
- Estimation of height of the uterus fundus and circumference of the abdomen;
- Identifying on palpation of three and more large parts of the fetus;
- Early feeling of fetus’ movement in various parts of abdomen;
- Revealing on auscultation of two and more independent zones of cardiac activity of the fetus.

The measurement of fetus’ length is of great importance. So, if the size between the most remote poles of the fetus (the head and the buttocks), determined by pelvimeter, reaches 30 cm or more (instead of usual 24-25 cm), and the head is small (10 cm or less), the presence of twins is probable, because at measurement of fetus’ length the pelvimeter ends could locate on the buttocks of one twin and on the head of the other.

Of other signs of multiple pregnancy the following deserve attention:
- Arcuate uterus — a hollow in the middle of uterus fundus depending on protrusion of uterus corners by large parts of two twins;
- availability of longitudinal furrow on the anterior wall of uterus depending on tight adjoining of two fetuses to each other which are in a longitudinal position;
- availability of horizontal furrow on the anterior wall of uterus, depending on adjoining of both twins to each other in a transverse lying, one above the other;
- availability of slanting furrow, that depends on adjoining of both twins to each other, lying in oblique position.

However the listed above methods of examination allow only to assume the presence of multiple pregnancy.

The application of additional instrumental methods of examination (phono- and electrocardiography of fetus, cardiotocography, and ultrasound) has expanded the opportunities of diagnostics in multiple pregnancy. Thus, ultrasound examination is of particular significance.

In the first trimester, ultrasonic diagnostics in multiple pregnancy is based on revealing two and more echographic contours of fetal eggs beginning with the 6th week of pregnancy (counting from the first day after last menstruation). Ultrasound examination of multiple pregnancy in the II and III trimester of pregnancy is substantially facilitated. Ultrasonic diagnostics in this term is considered authentic at reception in one projection of images of two or more embryos, heads, trunks or buttocks of the fetus.

Determination of the fetus’ position in uterus at ultrasound examination is of particular significance before delivery, as it is decisive in choice of optimal method of labour. Most frequently at twins, fetuses are located in a longitudinal position (both in a head, one — in a head, the other — in a breech, both in a breech presentation). The combinations of longitudinal and transverse lying or only transverse lying of fetuses are less often observed.

At ultrasonic diagnostics it is of importance to visualize the anatomic formations and internal organs of each fetus. The use of ultrasonic biometry in combination with placentography provides a dynamic observation of growth and development of fetuses.