- In extremely rare cases, a woman gets pregnant while already pregnant.
- Normally, a pregnant woman's ovaries temporarily stop releasing eggs.
- But in a rare phenomenon called superfetation, another egg is released, gets fertilized with sperm, and attaches to the wall of the uterus, resulting in two babies.
In 2016, California mom Jessica Allen agreed to be a surrogate for another couple because she wanted to help them experience the joy of parenthood. Six months into that surrogate pregnancy, she got some surprising news from her doctor: She was carrying two babies.
At the time, the doctor thought the embryo made from the sperm and egg the intended parents had split in two, resulting in identical twins, Allen said in an interview with "Good Morning America."
Later, DNA tests revealed that wasn't true at all. The second baby was really the biological child of Allen and her husband. They had conceived the second baby naturally, even though she was already carrying a baby as a surrogate.
In simpler terms, she got pregnant while she was already pregnant.
Allen and her husband won full custody of their biological baby boy, who's now 10 months old. But their story has brought attention to incredibly rare phenomenon known as superfetation. Though it's common in other mammals, it's only been reported a handful of times in humans, according to the New York Times.
Normally, when woman becomes pregnant, hormonal changes temporarily stop her ovaries from releasing more eggs, TIME reported. The lining of the uterus also becomes much thicker, making it more difficult for another fertilized egg to attach, if one happened to wander down that far. Finally, a plug made of mucus forms in the cervix to prevent sperm from swimming up into the uterus, according to Scientific American.
But in a few out-there cases — including Allen's — those automatic defenses don't appear to have worked. In 2007, for example, one British woman gave birth to aboy and a girl conceived three weeks apart, according to TIME.
On Monday, ABC News chief medical contributor Dr. Jennifer Ashton told "Good Morning America" that we still don't know exactly why superfetation happens. So far, case reports indicate that the second pregnancy tends to happen roughly two to four weeks after the first one. And since the babies are so close in age, a mom who experiences superfetation faces similar risks to a mom expecting multiples: A higher risk of preterm birth, high blood pressure, and preeclampsia.
Most importantly, she stressed that superfetation is really rare. As she said on the show: "I want people to know that you have a better chance of being struck by lightning or winning the lottery than having this happen to you, if you're a woman with a uterus."
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