Are Identical Twins Always the Same Sex?

If you’re expecting identical twins, you can count on having a set of boy/boy or girl/girl twins. According to Sasha Andrews, M.D., a maternal-fetal medicine specialist at Obstetrix® of Colorado, part of Pediatrix® Medical Group, identical twins form when one sperm fertilizes one egg, which then divides into two embryos. Because they are formed from one fertilized egg, identical twins usually share an almost-identical genetic sequence, including sex chromosomes. However, there is one exception. “Identical twins can be different sexes due to a phenomenon called gonadal dysgenesis, where mutations or changes occur in the genes that determine the development of genitalia,” said Dr. Andrews.

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