Twin Pregnancy FAQ: Are My Twins Identical or Fraternal?

How to tell if twins are identical or fraternal in twin pregnancy.
Twin Pregnancy: Are Your Twins Identical or Fraternal?. © Pamela Prindle Fierro 2015

Question: Twin Pregnancy FAQ: Are My Twins Identical or Fraternal?

Get the answers to frequently asked questions about pregnancy with twins and multiples. Have a question that isn't answered here? Ask me.


One of the first things that people want to know about twins is whether they are identical or fraternal. The correct term for defining twin type is zygosity, which describes whether multiples form from a single fertilized egg (monozygotic, or "identical" twins) or several individual zygotes (dizygotic, also called multizygotic or "fraternal" multiples).

From the moment you announce that you are having twins, people will begin to question their zygosity, and they will continue to do so throughout your multiples' lives.

As described in this chart, monozygotic twins who form from a single, fertilized egg share the majority of their DNA, and thus tend to exhibit extreme similarities. Dizygotic twins, on the other hand, are no more alike than two siblings born at different times.

It is not always possible to determine zygosity during pregnancy. Sometimes, the only way to know for sure is a DNA comparison. However, there are some indicators that can offer clues during pregnancy.

Was your pregnancy the result of an in vitro fertilization in which the number of implanted embryos exceeds the number that were transferred? Occasionally, monozygous multiples result after in vitro fertilization. If a single embryo is transferred, yet two embryos have implanted in the uterus, it can be assumed to be monozygotic twins.

Are the babies both the same gender? If not, they are definitely dizygotic, unless there is some confirmation of a genetic disorder such as Turner's Syndrome. Monozygotic twins are always same sex twins, while dizygotic twins can be two boys, two girls, or one of each.

Is there one placenta or two? If there is a single, shared placenta, it is most likely that the twins are monozygotic.

However, there have been very, very rare cases where dizygotic twins share a placenta. More commonly, two single placentas can fuse together and appear to be a single placenta. It is often difficult to determine the placental structure using ultrasound, however, using the placenta to confirm zygosity is often unreliable until it can be physically examined after delivery.

Have the babies been diagnosed with Twin-to-Twin Transfusion Syndrome (TTTS)? TTTS only affects monozygotic twins who share a circulatory connection in the placenta. Doctors have identified a few, rare cases of dizygotic twins who experienced TTTS, probably due to the reproductive technology used to conceive them.

Have the babies been identified as monoamnionic? If so, they are monozygotic. Only monozygotic twins develop within a single, shared amnion.

Have you undergone prenatal testing that provides a comparable sample of DNA from each baby? It is possible that the results of prenatal testing such as amniocentesis can confirm the zygosity of the babies.

In most cases, zygosity will be revealed after the babies are born, perhaps by placental analysis, blood testing, or DNA testing. Sometimes though, twins never know for sure whether they are identical or fraternal.

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