Twin pregnancy is a common occurrence, representing approximately few of the pregnancies in Spain. The cause of a twin pregnancy can be natural, that is, there is fertilization of more than one oocyte in the same cycle, or iatrogenic cause, caused by doctor’s intervention. The natural cause is not foreseeable, except in older women, in whom the possibility of a twin pregnancy is greater than in younger women, since they produce double ovulations more frequently. This happens because older women have a hormone (FSH) that is ovulation inducing, in higher concentrations.
In any case, twin pregnancies produced by iatrogenic causes are more frequent, when ovaries are stimulated to achieve pregnancies, in the context of establishing a fertility support treatment and the impossibility of provoking a single ovulation, as in polycystic ovary syndrome.
In vitro fertilization has significantly decreased the incidence of twin pregnancies, as the number of embryos transferred can be controlled and the trend is increasingly towards single embryo transfer.
Importance of twin pregnancies
The importance of twin pregnancies is due to the fact that they are high-risk pregnancies, because of the possible complications for the fetuses, for the mother and for the newborns. Fetal complications are mainly due to the higher incidence of malformations, especially in cases where the fetuses share sac and placenta, which we call monoamniotic monochorionic gestations. However, problems of intrauterine growth retardation are more frequent, due to the limitation of the human uterus to feed more than one fetus, producing low birth weight newborns that will need more perinatal care. Another cause of low birth weight is due to prematurity, when contractions start prematurely, mainly due to the volume of the uterus, which cannot dilate beyond a limit and trigger labor earlier than desirable.
How to manage twin pregnancies
Twin pregnancies should be monitored in highly qualified centers equipped with systems for detecting fetal alterations and fetal development, especially with high-definition ultrasound equipment, vascularization Doppler systems, under the supervision of obstetricians and specialists in Gynecology and Obstetrics trained for this purpose. They require a greater number of visits, to control not only the fetuses, but also the mother in terms of weight gain, which must be very well monitored, blood pressure figures or the appearance of signs of poor evolution (edema, proteinuria, etc.).
A concept that is not always clear is whether twins come from a single oocyte. Bicygotic twins appear when two oocytes are fertilized by one sperm each. Two separate amniotic sacs, two chorions and two placentas are then formed. Placentas in bicigotic twins can fuse if the implantation sites are close to each other. Likewise, fused placentas can be easily separated after birth. These are siblings who have coincided in space and time.
The development of monozygotic twins, on the other hand, occurs when one fertilized egg splits into two during the first two weeks after conception. These twins are also called identical twins. If the division of the fertilized egg occurs within the first two to three days after fertilization, the twins will produce one amniotic sac and one chorion separate from the other. These twins have different placentas that may be separated or fused. Approximately one-third of monozygotic twins have bicorionic and biamniotic placentas.
If the division of the egg occurs between three and eight days after fertilization, it results in twins with monochorionic and biamniotic placentation, because the chorion has already formed but not the amniotic cavity. Approximately more than half of monozygotic twins are monochorionic and biamniotic. If the division occurs even later, i.e., during day nine or thirteen after fertilization, the placentation is usually monochorionic and monoamniotic. Monochorionic and monoamniotic twins are rare and only 1% of monozygotic twins have this type of placentation. These monochorionic and monoamniotic twins have a common placenta with vascular communications between the two circulations. These twins can develop twin-twin transfusion syndrome, which is very dangerous for the final viability of the gestation.