Multiple pregnancy is a pregnancy in which not one, but several (two, three or more) fruits develop simultaneously in the woman’s uterus. Usually the name of a multiple pregnancy is given depending on the number of fruits; for example, if the child is two, then they talk about the pregnancy of twins, if three, then triplets, etc.
Currently, the frequency of multiple pregnancy is between 0.7 and 1.5% in different countries in Europe and the United States. The widespread and relatively frequent use of assisted reproductive technologies (IVF) has led to an increase in the incidence of multiple pregnancies.
Depending on the mechanism of appearance of twins, distinguish between dizygotic (double-sided) and monozygotic (single-egg) multiple pregnancies. Children of a double-twin are called twins, and identical twins. Among all the multiple pregnancies, the frequency of the twins is about 70%. Twins are always of the same sex and as two drops of water are similar to each other, because they develop from one fetal egg and have a completely identical set of genes. Twins can be of different sexes and are similar only as a brother and sister, since they develop from different oocytes, and, therefore, have a different set of genes.
Dyadic pregnancy develops due to the fertilization of two eggs at the same time, which are implanted in various parts of the uterus. Quite often, the formation of double-sided twins occurs as a result of two different sexual acts, conducted with a small gap between each other not more than a week. However, twins can be conceived in the course of a single sexual act, but on condition that the simultaneous maturation and release of two eggs from one or different ovaries occurred. In the case of twins double, each fetus must have its own placenta and its own fetal bladder. The position of the fruit, when each of them has its own placenta and a fetal bladder, is called the bihorial biamniotic twins. That is, in the uterus there are simultaneously two placentas (bichorial twins), and two fetal blisters (biamnotic twins), in each of which a child grows and develops.