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.

Guessing Game
It is much easier to determine twin type after the babies are born. If you have children of opposite sexes, you can be sure they are fraternal. If they are the same sex, an extensive blood typing test or DNA profile will provide the information you need.

DNA profiling can be done very simply and relatively inexpensively. For less than $150, a genetics institute will send the information and tools necessary to profile your twins. The process involves gently scraping the inside of each child’s cheek, placing the swabs in sterilized containers and returning the packet to the lab for analysis. The results are usually available within 10 days.

Segal recommends this type of testing rather than blood work because it is non-invasive and highly effective. “It looks at particular factors that would be so rare to match in fraternal twins,” she says. “If they matched, the chances are really, really, really high that they’d be identical.”

It was through a DNA test that Brown learned the truth about her twins. “They were born looking so different that we believed they were fraternal,” she says. But this changed as they grew. At their 5-year-old well check appointment, the doctor noticed their growth patterns were surprisingly similar and mentioned that it was unusual for fraternals to be so alike. That prompted their mom to do the test.

Why It Matters
Why is it important to know which type of twins you have? Segal says there are many reasons. First of all, it will give parents a much better understanding of their children’s physical and behavioral development. Fraternal twins generally develop at different speeds. One may talk sooner while the other walks earlier, and this is perfectly normal. On the other hand, identical twins have very similar developmental patterns. They usually accomplish milestones within days of each other. If there is a delay in one child, it may signal a problem.

Another important reason is for health issues. If you have identical twins and one develops a disease, you can take preventive measures to help the other one. You will also know that there is a perfect match if one child ever needs an organ or marrow transplant. You will be better prepared for what lies ahead if you are aware of the type of twins you have.

It is also helpful in social aspects. Identical twins like to do many of the same things. “They may be good at the same things, and they’re going to enjoy doing what they’re good at,” Segal explains. They may also choose many of the same friends, because they will probably like the same kinds of people. Fraternals, however, are often no more alike than ordinary siblings. "They may be closer in the sense that they’re the same age so they’re going through developmental events at the same time,” Segal says, but other than that, they are often very different.

Standard Practice
Many twin advocates believe that twin typing should be done at birth and the cost covered by hospitals or insurance companies. This is especially important for same-sex twins who have two placentas and fetal membranes because they can be either fraternal or identical. The earlier parents know what to expect, the better able they will be to understand their children's similarities and differences and detect potential problems.

Want to see more?

• Twins: The Science Behind Those Dynamic Duos
• Two at Once: Becoming a Parent of Twins
• The Home Stretch: Surviving the Last Trimester With Twins
• Talk about it!