Multiple Gestations - 2 - Placentation in Twins
In singleton pregnancies, the baby is surrounded by the ‘bag of waters’ and the balloon that comprises that bag is actually made up of two separate layers that are adherent to each other. The layer on the fetal side of the balloon is the amnion and the layer on the placental side is the chorion. In dizygotic ('fraternal') twins, the babies are just like two separate singleton pregnancies and are always contained within completely separate sacs, each made up of both amnion and chorion. This is then termed diamnionic, dichorionic placentation. Under these circumstances, the babies may still have either completely separate placentas or they may have placentas that are ‘fused’- grown together to appear as one placenta. Even when the placentas are ‘fused’ in diamnionic, dichorionic, pregnancies, it is very rare for the blood vessels from one baby’s placenta to cross to the placental side of the other baby. The importance of this will become clearer as we continue our discussion over the next few posts.
The diamnionic, dichorionic placenta can usually be readily identified by ultrasound. The ‘membrane’ separating the babies contains 4 layers – two chorions sandwiched between two amnions. This appears fairly thick by ultrasound. Furthermore, if one looks at the actual insertion site of the membranes on the placenta (s), this will have a v-shaped appearance with the chorions and some placental tissue extending a small distance into the 4-layer membrane separating the babies. This ultrasound finding has been given various names, but I was taught to call this the ‘twin peak’. After the babies are born, the intervening membrane is very opaque due to its relative thickness and, if not known sooner, diamnionic, dichorionic placentation can usually be confirmed at that time.
With monozygotic (‘identical’) twins, the issue of placentation is a little more complicated and depends upon when the single embryo divides into two. If the embryo divides within 3 days of fertilization, the ‘identical’ twins will have diamnionic, dichorionic placentation just like the dizygotic twins mentioned above. This occurs in about 20-30% of all monozygotic twins. If the division occurs between days 4 and 8, the twins will most likely be diamnionic (two amnions) contained within a single chorion – diamnionic, monochorionic placentation. This occurs with 70-80% of monozygotic twins. If the division occurs between days 8 and 13, the placentation is most likely to be monoamnionic, monochorionic – both babies contained within a single ‘bag of waters’; and, division after this will often result in monoamnionic, monochorionic twins that are also ‘conjoined’, sharing various degrees of body parts and internal organs. The latter two stituations, fortunately, occur in only about 1% of monozygotic twins.
Diamnionic, monochorionic twins can usually be readily identified by ultrasound as well. The intervening membrane is very thin (containing only the two amnions) and there is no ‘twin peak’ evident where the amnions connect with the placenta. (In cases of monochorionic twinning, it is not uncommon for there to be vasculature connections between the two sides of the placenta as will be discussed in a later post). In monoamnionic, monochorionic placentation, no intervening membrane is seen and the babies are found to be freely floating together within a single sac, often having their umbilical cords intertwined or even knotted. The two umbilical cords in monoamnioic twins can usually be seen arise in very close proximity to each other at their placental insertion sites.
Pregnancy complications and perinatal mortality are directly correlated with the type of placentation. In the case of perinatal mortality, the rate in diamnionic, dichorionic twins is about 8.9%, still more than twice that seen in singleton pregnancies. Perinatal mortality rates in diamnionic, monochorionic twins are as high as 25%; and in momoamnionic, monochorionic twins this is usually in the range of 50-60%. Major contributors to these high rates of loss in twin pregnancies include prematurity, intrauterine growth restriction, congenital birth defects, maternal preeclampsia and a condition termed ‘twin-to-twin transfusion’ syndrome (sequence), all of which will be discussed in upcoming posts to this series…