A surrogate mother thought she gave birth to twins—but it turns out the boys were half-brothers. Here's how this incredibly rare phenomenon can occur.

Yes, You Can Get Pregnant While Already Pregnant. We Asked a Doctor to Explain

Most people are familiar with the basics of how a woman gets pregnant: an egg is released from an ovary; sperm fertilizes the egg; the fertilized egg attaches to the uterine lining, where it eventually becomes a fetus. But what if the process occurs a second time, after one egg has already implanted in the uterus?

This is what happened to 31-year-old Jessica Allen from Perris, California. In December of 2016, Allen gave birth to what she thought were twin boys while acting as a surrogate for a Chinese couple. The babies were quickly delivered to their legal parents after the birth, but when Allen saw a photo of them later that night, she noticed they had different skin tones.

Sure enough, a DNA test revealed the babies were not twins, but half-brothers. One of the boys was the biological son of Allen and her husband, Wardell Jasper.

How does a double pregnancy happen?

Called superfetation, the phenomenon is known to occur in some mammals such as badgers and mink. But it's highly unusual in humans. "It's incredibly rare, and there aren't many reports on this," says Christine Greves, MD, an ob-gyn in Orlando, Florida. "It's so rare, in fact, that I can't even give you a statistic on it."

That's because once a woman is pregnant, changes in her hormones typically prevent her body from continuing to ovulate, Dr. Greves explains. But if a woman does ovulate again—and if that egg becomes fertilized and implants in her uterus, she can carry a double pregnancy.

The reasons why a woman might ovulate while she's already pregnant aren't well studied. "It's hard to find any literature on this poorly understood and mystical topic," says Sherry Ross, MD, an ob-gyn in Santa Monica, California. "Superfetation goes against what we know medically about getting pregnant spontaneously. The truth is not all medical phenomena have a logical explanation."

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How can you know if you have a double pregnancy, or you're carrying twins?

In short, it's very difficult. "Superfetation is controversial and hard to prove," says Dr. Greves.

She cites one previous case report in which doctors identified a four-week discordance between two fetuses. In other words, one fetus appeared to be four weeks older than the other, which suggested a superfetation pregnancy. "This would go with the fact that ovulation can occur every 28 days," she explains. But fetuses that appear to have different gestational ages aren't necessarily proof of superfetation.

There is such a thing as twin-twin discordance, Dr. Greves points out, which means the fetuses are growing at different rates inside the uterus. One baby may be getting more nutrients than the other, and therefore appear bigger (or older) in an ultrasound. "If that's the case, it could look like superfetation, but it wouldn't necessarily be due to that," she says.

If a pregnant woman has an early ultrasound that reveals only one fetus, and a subsequent ultrasound reveals a second fetus—that could indicate superfetation has occurred. But again, the evidence isn't definitive, since her doctor may have simply missed the second fetus the first time around.

Of course, if the fetuses have different fathers, as in Allen's case, a genetic test after they're born can provide a clear answer.