In Part 2, we discussed the different arrangements of membranes and of the placenta that occur in a monozygotic twin pregnancy, so the goal today is to explore the clinical implications, namely what does it all mean for your pregnancy, particularly when it comes to the the delivery of your babies. We’ll avoid diving into conjoined twins, since they are extremely rare, except to note, what you probably have guessed —that they require surgical delivery, known as cesarean section (C-section), that this has to be done fairly early and that, at some point, after delivery, they need to be separated with another surgical procedure (or a series of surgical procedures). Separating conjoined twins is so rare that one can go through an entire career as a pediatric surgeon and never witness such a procedure, let alone perform one.
Avoiding the scenario of conjoined twins, with monozygotic twins, we are talking about any scenario in which multiple gestation has resulted from a zygote (a fertilized ovum) splitting into two or more pieces at any point up to 13 days post fertilization. Additionally, one zygote can become the source of higher order pregnancy. For example, there can be monozygotic triplets, three embryos resulting from the fertilization of the same ovum by the same sperm cell. Furthermore, in higher order pregnancy, you also can have any number or monozygotic embryos developing alongside any number embryos formed from separate zygotes. With triplets, for example, most cases are trizygotic, meaning three babies are born, all fraternal and when there is monozygosity, usually it’s just two that are monozygotic (identical); that happens because there was a dizygotic pregnancy, in which two zygotes formed, but later one of them split.
As noted in Part 2, if the embryo splits into two at any point through the morula stage, meaning up to 3 days or so after fertilization, this produces a dichorionic, diamniotic pregnancy. Each fetus ends up with its own placenta and its own amniotic sac. This is just like in a dizygotic pregnancy and, clinically, this is the best situation. With each fetus floating in its own, separate amniotic sac, the umbilical cords cannot wrap around one another and tie up into a knot, causing great difficulty at birth, if not earlier. Also, since each twin has its own placenta, it avoids a potential scenario called twin-twin transfusion syndrome, in which one embryo/fetus essentially steals blood from the other, which we’ll discuss below. In cases of dichorionic, diamniotic pregnancy, the optimal time to deliver generally is in the range of 36-38 weeks gestation. A delivery any time from 37 weeks onward is not considered preterm.
Another thing about dichorionic, diamniotic twins is that vaginal delivery can be possible and safe, if at least the fetus closest to the birth canal is in a vertex (head down) position. If both babies are head down (vertex/vertex), vaginal delivery is most likely to succeed for both babies. If the first baby is head down and the other is bottom down (vertex/breech), vaginal delivery still might succeed, but there’s a fair chance the that the delivery will have to be converted to a C-section for the second baby. The same is true if the second baby is in a sideways (transverse) position. It might be possible to rotate that fetus to make it vertex. Now, if the first baby (the one closer to the birth canal) is breech or transverse, you will need a C-section. Your doctor will not even attempt to perform a vaginal delivery.
Now, if embryo splitting occurs any time from around four days to about eight days after fertilization, meaning during the early part of the blastula stage, the two twins end up sharing a chorion, and thus sharing a placenta. But they do not share an amniotic membrane, so each has its own amniotic sac. This is called a monochorionic, diamniotic pregnancy and it creates a risk for twin-twin transfusion syndrome. This relates to issues of blood vessels in the placenta causing unequal blood flow through one umbilical cord versus the other, causing one embryo to receive more blood while the other receives less. If the transfusion begins early in pregnancy, this may kill the donor, the embryo receiving less blood. If it begins later in pregnancy, one fetus (the recipient) will be larger, the other (the donor) smaller, and by receiving too much blood, the recipient may have too much fluid in the body. This is called hydrops fetalis, which can lead to heart failure and other serious problems.
While delivery of monochorionic, diamniotic twins may be possible as late as 38 weeks, the birth may need to be carried out as early as 32 weeks in some cases. Vaginal delivery may be possible if the first fetus is head down, but you are more likely to need a C-section than in cases of dichorionic, diamniotic pregnancy.
When an embryo splits between days 8-13 after fertilization, this produces a monochorionic, monoamniotic pregnancy. Within that single chorion, there is only one amniotic sac, and because there’s just one chorion there’s just one placenta. Since no membrane separates the twins, their umbilical cords can become tangled and knotted, so you need to have a C-section. With this type of twin pregnancy, obstetricians will try to keep the twins inside up to 36 weeks, if possible, but delivery might be needed much earlier, even as early as 31 weeks.