Surrogate learns one of the twins she was carrying is her biological child

A California woman was shocked to find out that, in an extremely rare occurrence, she became pregnant with her own son while carrying a baby as a surrogate for another couple.
4:36 | 10/30/17

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Transcript for Surrogate learns one of the twins she was carrying is her biological child
Now to our "Gma" cover story. A mother's battle for her baby. Listen to this a woman who oftened to be a surrogate giving birth to two babies then discovering one of them was actually her biological child. We first saw the story in "The New York post" and linsey Davis is here with this incredible story. It really sounds surreal. Good morning. We're talking about two babies in the same womb and both have two completely different sets of parents. One of the babies is Asian and the other is half white and half black. They are even slightly different ages. You're about to hear from a woman who agreed to be a surrogate. She became pregnant with one baby as a result of ivf and then managed to become pregnant naturally with another baby at the same time. For Jessica Allen and her husband Wardell jasper there is no greater blessing than their children. After two healthy pregnants the California couple decided to pay it forward and Jessica signed up to be a surrogate. No woman in the world should have to live their life without experiencing the love and the bond from a mother and a child. Reporter: Jessica was matched with a couple and in April of 2016 she says one of their embryos was successfully implanted into her uterus. Six weeks into the pregnancy she says her doctor noticed something rather unexpected. He goes, well, I definitely see that there's another baby, the chance of embryo splitting is very small but it does happen and, you know, I just thought, you know, I was very surprised. Reporter: Jessica delivered what she believed to be identical twin boys via c-section. She says she never got to see them in person but was shown a photo. I did notice, you know, one was much lighter than the other. You know, obviously they were not identical twins. Reporter: One month later some shocking news. She says a DNA test confirmed the second baby was not a twin to the implanted embryo but actually Jessica and her husband's biological son. It's called superfetation and while rare, it occurs when a woman continues to off late after becoming pregnant resulting in two babies with different gestational ages and in this case two different sets of genetic parents. I didn't even know I was pregnant with you. I carried my own child. I didn't know he was mine. Reporter: Jessica says after a complicated process she and Wardell got custody of their son in February. Now 10 months old he they say he's the piece that completes their family puzzle. The concept of superfetation essentially becoming pregnant while pregnant is not all that rare in certain animals but in humans it's said to be so unusual we're talking 1 in several million pregnancies. This is something maury povich would have a field day with. You are not the father but you are the father of this one. That's true. Thank you, linsey and Dr. Jennifer Ashton is here. What? I know. This is incredibly, incredibly rare, so rare there are in fact maybe 11 case reports in the world literature. Most ob/gyns myself included will never see it. Most have never heard of it. It has nothing to do with in vitro fertilization, it happened to be a coincidence so I want you to know 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. What's the medical reason this doesn't happen more often? Well, we don't understand how this could possibly happen because to your question, when -- as soon as conception occurs there are processes that actually prevent the woman from releasing another egg. So in this case the babies are conceived at different times. They're usually a couple of days to a couple of weeks apart and as we heard in the piece, different eggs in this case different sperm but different in terms of the time line. Any rhysing to the woman. You have the risk of multiple gestation, twins, the risks are predetermine delivery, usually the second baby will be born slightly premature and riches like high blood pressure, pre-eclampsia, anything you can see with twins or triplets. Adding to the complexity of surrogacy at times. Very complicated medically, legally, financially, emotionally and then you get these rare cases here but obviously very, very emotional. I would imagine you've heard of it. We've heard of it but it is literally so rare and this is where, again, the legal documentation, anyone going through surrogacy don't really have to worry about this but you have to have that legal documentation in place. Good thing to keep in mind. Thanks so much.

This transcript has been automatically generated and may not be 100% accurate.

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