Two Sets of Twins, One Birth—How DNA Testing can Sort Out the Questions

On Valentine’s Day, a woman in Texas gave birth to two sets of identical twins. While 3 out of every 1000 births involve identical twins, the odds of delivering two sets of naturally occurring identical twins (that is, without the use of fertility treatments) is about 1 in 70 million.

Doctors can usually diagnose a twin pregnancy by examining the placenta. In this case, each pair of twins shared a placenta. However, in some cases, identical twins cannot be distinguished from fraternal twins using the placenta alone (see dichorionic diamniotic twins). Parents of twins can request a twin DNA test to determine if the twins are identical or fraternal.

A twin DNA test, also known as a twin zygosity test, compares the DNA of each sibling. Identical twins will have exactly the same DNA profile, and fraternal twins will not. In many cases, parents who request this test have doubts about what they’ve been told at birth as their kids grew up and exhibited identical/nonidentical features. Some parents also were simply not told by their doctor about their twins’ zygosity, or have no access to medical records (for example, if the twins were adopted).

Determining twin zygosity is an important component of a child’s medical history, since the predisposition to genetic diseases will be shared by both twins.

Visit our twin zygosity testing page for more information, or call 1-800-613-5768.