Parents Misinformed About Zygosity

Do You Really Know If Your Twins Are Identical or Fraternal?

Are your twins identical or fraternal? Are you sure? Perhaps you have a suspicion based on your own observation, or maybe you were informed by your physician during pregnancy or shortly after birth. But a study found that nearly 15 percent of parents were misinformed about the zygosity of their baby twins. In most of the cases, the parents were mistakenly told their identical twins were fraternal.

Those results are not entirely surprising. There is a great deal of misinformation about zygosity, and not just in the general population. Although many people are confused about what identical and fraternal twins really are, even some twins and their parents don’t understand the basics of twin-type. And even worse ... some doctors mistakenly make incorrect assumptions about how to distinguish between monozygotic and dizygotic twins.

What Is Zygosity?

Most people are familiar with the terms identical and fraternal, used to describe types of twins. They assume that twins that look alike are identical, and twins that don’t are fraternal. But twin type is actually a classification of zygosity and defined by how twins form, not by what they look like.

Monozygotic multiples form when a single fertilized egg splits, resulting in the development of two or more individual embryos. Because they originate from the same set of cells, these individuals have the same DNA and often have remarkably similar physical appearances. That’s why they are known as identical twins. But appearance is not simply based on genetic traits; identical twins don’t necessarily look exactly alike. (Find out why identical twins are different.)

Dizygotic or multizygotic multiples are unique creations that occur when multiple eggs are fertilized and develop. Their genetic heritage is the same as any siblings, with about 50% of their DNA in common. They may look alike in the same way that siblings favor each other. Or, they may look completely different. They are commonly known as fraternal twins or multiples.

How to Know for Sure

The only way to determine zygosity with certainty is DNA testing. Monozygotic - or identical - twins or multiples will be a complete match, while dizygotic multiples will But there are other circumstantial clues that can provide conclusive evidence. These clues may be visible on an ultrasound during pregnancy or revealed shortly after delivery during an examination of the placenta.

During pregnancy, a fertilized egg implants in the uterus. As the weeks progress, the placenta develops on the wall of the mother’s uterus to nourish the fetus. In addition, the fetus is contained in an amnion, a sac of waters. Monozygotic twins may share a placenta, or even an amnion, depending on when the egg splits after conception. The earlier that the egg splits, the less the babies will share in the uterus; about 25% of monozygotic twins develop in separate sacs, with two placentas. But when an egg splits later, between 4 and 8 days post conception, the babies may share a placenta, chorion (the outer membrane of the amniotic sac) or amnion (the inner membrane of the amniotic sac).

Sometimes an ultrasound scan during pregnancy gives insight into whether twins are monozygotic or dizygotic. If it is apparent from the ultrasound that there is a single placenta, it can be concluded that they are monozygotic -- identical. However, the presence of dual placentas is not necessarily conclusive; both monozygotic twins and dizygotic twins can manifest with two placentas. Unfortunately, many doctors wrongly assume that two placentas indicate dizygotic twins. A 2004 study discovered that more than 80% of doctors believed that twins with two placentas were fraternal. Parents who assume their twins are fraternal based on this type of erroneous information from their medical provider may also be misinformed.

Why It Matters

For most people, determining zygosity is simply an issue of satisfying curiosity. Twins may want to confirm their origin, or parents may desire to provide an accurate answer to the ubiquitous question. But in some cases, establishing zygosity may provide some clarity in medical decisions, such as in the event of an organ or tissue transplant.

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