Kim Brown heard about a twin contest at the county fair and decided to enter her 2-year-old boys. They won first prize in the Fraternal Twins Who Are Most Alike category. The following year, they took second place. Two years later, Brown learned her fraternal twins were actually identical – oops!

Before her twins were born, an ultrasound showed Brown's twins living in separate placentas in two different areas of her uterus. Her physicians informed her that this meant they were fraternal twins. They were wrong.

Misclassification of same-sex twins is not uncommon, according to Nancy L. Segal, professor of developmental psychology at California State University, Fullerton and author of Entwined Lives: Twins and What They Tell Us About Human Behavior (Plume, 2000). "What often happens is that when doctors or other medical personnel discover two placentas, it's easy to assume it's fraternal twins," she says. "But in fact, one-third of identicals also have separate placentas."

The opposite can be true, too. A single placenta, which usually indicates identical twins, can be formed if the sacs of fraternal twins fuse together to look like one. "You really need to go beyond placentas to know twin type for sure," Segal says.

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