Multiple pregnancies (gestations) are associated with a higher risk of chromosomal abnormalities, fetal morbidity and fetal mortality than a single pregnancy. The risk gradually increases with the number of fetuses. Globally, twins naturally occur approximately once in every 85 pregnancies, triplets approximately once in every 902 pregnancies, quadruplets approximately once in every 903 pregnancies, and quintuplets approximately once in every 904 pregnancies.
Twins and Fetal Membranes
Twins that originates from 2 zygotes are “dizygotic” twins – fraternal twins (Fig. 1), whereas twins that originates from 1 zygote are “monozygotic” twins – identical twins (Fig. 2). The fetal membranes and placentas vary according to the origin of the twins. Approximately two-thirds are dizygotic twins, and dizygotic twinning rate increases with maternal age. A study of twins is important in human genetics, because it is useful to compare the influence of genes and environment to development. If an abnormal condition does not show a simple genetic pattern, the comparison of its frequency in monozygotic and dizygotic twins can show that heredity is involved.
Since they are the result of the fertilization of two oocytes with two sperm, dizygotic twins can be of the same sex or of different sexes. For the same reason, they are genetically no more alike than brothers or sisters born at different times. Dizygotic twins always have two amnions and two chorions (see Fig. 1A), but chorions and placentas can be fused (see Fig. 1B). The dizygotic twinning shows a hereditary tendency. The risk of relapse in families with one set of dizygotic twins is about three times higher than in the general population.
Fig. 1 Dizygotic twins developing from two zygotes. The relationship of the fetal membranes and placentas is shown for instances in which the blastocysts implant separately (A) and the blastocysts implant close together (B). In both cases, there are two amnions and two chorions.
Since they are the result of fertilization of one oocyte and develop from one zygote (see Fig. 2), monozygotic twins of the same sex are genetically identical and similar in appearance. Physical differences between monozygotic twins are caused by the environment, for example, anastomosis of the placental vessels, which leads to differences in the blood supply from the placenta. Monozygotic twinning usually begins at the blastocyst stage, around at the end of the first week, and is the result of dividing into two embryoblast embryonic germ (see. Fig. 2). Subsequently, two embryos, each in its own amniotic sac, develop in one chorionic sac, and share a common placenta, monochorionic-diamniotic twin placenta.
Fig. 2 Illustrations of how approximately 65% of monozygotic twins develop from one zygote by division of the inner cell mass. These twins always have separate amnions, a single chorionic sac, and a common placenta. If there is an anastomosis of the placental vessels, one twin can receive most of the nutrition from the placenta.
Unusually, early separation of embryonic blastomeres (e.g., at a stage from two- to eight- cells) leads to monozygotic twins with two amnions, two chorions and two placentas, which may or may not be fused. In such cases, it is impossible from the membranes alone to determine, whether twins are monozygotic or dizygotic.
Other Types of Multiple Births
Triplets can be delivered from:
One zygote and be identical,
Two zygotes and consist of identical twins and one singleton,
Three zygotes of the same sex or of different sexes, in which case children are no more similar than children of three different pregnancies.
Similar combinations occur in quadruplets, quintuplets, sextuplets, and septuplets.