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Two for One

How One Pregnancy
Makes Two Babies

By Melinda Copp

Ask any parent of twins and they'll tell you two is better than one. But finding out that you're carrying twins is shocking for just about everyone. You may wonder how it happened, how your pregnancy will be different and what's going on inside you.

So how does one pregnancy make two babies? Unless your pregnancy resulted from in vitro fertilization, then your babies were created the good old-fashioned way. And a twin pregnancy starts out in pretty much the same way.

After sexual intercourse, your partner's sperm travels through the opening of your cervix and makes its way through your uterus toward your fallopian tubes. If you're pregnant with fraternal twins, then two separate sperms will fertilize two separate eggs. And if your twins are identical, then there's only one egg and one sperm. In either case, after fertilization occurs the egg, or eggs, travel down the fallopian tube toward the uterus where it will attach to the wall and start to grow.

"Twin pregnancies are pretty much the same as any other pregnancy; there's just more than one," says Dr. Richard Chalsom, an OB/GYN at the Volunteers in Medicine Clinic on Hilton Head Island, S.C. "Either the woman has multiple ovulations, more than one egg comes out or the one egg splits and they are identical."

When you're pregnant with identical twins, they started out as one fertilized egg. But early in the pregnancy, probably before you had any idea that you were pregnant, the egg split into two separate zygotes.

"The split can occur any time between two or three days after fertilization and two weeks after," says Dr. Benito Alvarez, an OB/GYN at the Cleveland Clinic. However, an egg that splits later carries a higher risk for deformities and other challenges. The number of sacs also makes a difference.

Want to see more?

• From the Beginning: Reexamining the Birds and the Bees
• One of Each: Expecting Boy/Girl Twins
• My Hands Are Full? Stupid Things People Say to the Parents of Twins
• Check out our Article Library.
• Talk about it!

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