We now know enough about the development and potential complications associated with monochorionic twin pregnancies that the term “twin pregnancy” is no longer precise enough to be used as a medical term. We must distinguish between monochorionic and dichorionic twins.
Monochorionic twin pregnancies have unique features that substantially increase the risk of fetal death, growth restriction, and other complications. The twins share a single placenta, and their circulations are essentially linked to each other through their placental anastomoses. These linked circulations allow blood to be redirected—sometimes very rapidly—toward one twin or the other. This is not typically the case in dichorionic pregnancies.
Thus, we must always take both fetuses in a monochorionic twin pregnancy into consideration, because when one fetus is in jeopardy, the other typically is as well. This interdependency is fundamentally different from the less-entwined relationship of dichorionic twins, and makes monitoring more complicated and all the more important.
We must make the distinction between monochorionic and dichorionic twins early on—optimally, in the first trimester. With the opportunity to make this critical distinction—as well as improvements in fetal therapy and advances in ultrasound assessment that allow us to detect potential problems early—we can lay the foundation for the effective, proactive management of these at-risk pregnancies from the first trimester on.
Once the diagnosis of chorionicity is made, medical reports should specify the type of twin pregnancy that is present, rather than using what should now be considered the layman's term “twin pregnancy.”
The Potential Risks
The potential risks of monochorionic pregnancies stem from:
▸ Unequal placenta sharing. In an ideal world, the twins' single placenta is equally shared. However, it is often the case that one twin will have just 30%–40% of the monochorionic placenta, while the other fetus has the much larger portion. Such unequal placenta sharing leads to an unequal sharing of nutrients, which can lead to growth restriction and severe low birth weight in one of the fetuses.
This type of growth restriction—known as selective intrauterine growth restriction (IUGR)—affects about 10% of all identical twins. It happens quite early in pregnancy and, as we know from singleton growth-restricted fetuses, can lead to a host of troubling complications.
That is why the fetuses in a monochorionic pregnancy can never be treated in isolation. With the early onset of growth restriction in a monochorionic pregnancy, for example, the twin with this complication faces a higher risk of in utero death—an outcome that always negatively impacts the other fetus as well.
In a dichorionic pregnancy, if a co-twin weighs 320 g at 26 weeks and is at high risk of in utero death, we typically would advise the parents to delay delivery. The extremely high likelihood of fetal death of the growth-restricted twin would not justify exposing the otherwise normally grown healthy twin to the risks of prematurity. Accepting the fetal death of the growth-restricted twin and allowing pregnancy to continue gives the larger fetus a very good chance of being healthy at birth rather than being born premature with a significant risk of prematurity-related complications.
However, in a monochorionic pregnancy, intrauterine demise of the smaller fetus could put the healthy co-twin at a significant risk for acute severe hemorrhage into the placenta and circulation of the growth-restricted twin. This carries the risk of brain, renal, and cardiac damage—or even death—of the co-twin. The option of delaying delivery beyond the point of demise of the smaller twin, therefore, is unacceptable in this setting.
Rather, the fetuses would need intensive monitoring by experts who are alert to all the potential signs of fetal deterioration. Additional options, including fetal therapy, might require even more subspecialty evaluation.
▸ Unequal blood volume. Blood volume also may be unequally shared. In uncomplicated pregnancies, blood is exchanged equally through the vascular anastomoses that characterize all monochorionic pregnancies. Sometimes, however, the exchange is unbalanced and blood is shunted in one direction without adequate return.
Anastomoses that are between artery and vein act as one-way valves and can lead to significant differences in volume. Artery-to-artery and vein-to-vein connections allow direct exchange in either direction, with the direction of blood flow determined by the difference in blood pressure on either side.
If one fetus develops an unstable circulation or dies, the instability or resultant drop in blood pressure causes the healthy or surviving twin to lose a large amount of blood volume across the connecting vessels and into the sick or dying twin. This is why, when one fetus dies, the risk of death for the co-twin can be as high as 60%. It also explains why a surviving co-twin has a significant risk of brain injury.