Are My Twins Identical?

“Patients with twins frequently want to know if their fetuses are identical or not. And there are several really simple ways to answer that question. For example, if they are different genders, we know they’re not identical. If they are the same gender, then we can look at the placentas and get some more information.”

Look at the Placentas

“So if the two fetuses share a single placenta, then they are in medical terminology monochorionic and by definition, they’re identical. If babies have the same gender, but two different placentas, then it’s possible that the egg divided before the formation of the placenta, and in that circumstance, they would have each have their own placenta and would look from the perspective of the placentas, like a set of fraternal twins.”

“If you look at it in a population that hasn’t been exposed to fertility drugs, a dichorionic, or two placentas set of twins that are the same gender have about an 18% likelihood of being identical. Most identical twins have a single placenta. About a third of identical twins have two placentas. And about 1% of identical twins have a single placenta and a single sack. So if you look at identical twins, if the division of that fertilized egg occurs in the first three days after conception, that is before the cells that are going to make up the placenta have developed, that pregnancy will have two separate placentas in two sacks.

Higher Risk Twin Gestations

“If division occurs between day three and day eight, that is after the placenta has formed. But before the cells that are going to develop the amniotic sac have formed, that pregnancy will have a single placenta. It will be monochorionic. And two sacks
Diamniotic. If division occurs between day eight and day 13, that is after the cells that are going to develop the (amniotic) sac have formed, and that pregnancy will have a single placenta. It is a single gestational sac and the amniotic sac with two fetuses within it, a monochorionic monoamniotic placenta.”

“If the division Occurs after day 13, it’s almost always incomplete. And those pregnancies end up with conjoined or Siamese twins. The reason that we care about the chorionisity, which is to say how many placentas there are, is because dichorionic twins aren’t at risk for complications that we commonly see in monochorionic twins.”

“So early identification of what is the placental structure of your twin pregnancy becomes very important for us to be able to identify the safest management plans for your pregnancy.”