Surrogate mom says she became pregnant with her own child while carrying another couple's baby "I carried my own child. I didn't know he was mine."

 -- A California woman said she was shocked to find out that after an extremely rare medical incident, she became pregnant with her own biological child while she was carrying another baby as a surrogate for a Chinese couple.

Jessica Allen told ABC News that doctors initially thought the second baby was an identical twin, and it was not until much later that they found out that it was her biological son.

While extremely rare, this process of superfetation can occur during some pregnancies when a woman continues to ovulate after becoming pregnant. In such cases, fertilization and implementation of a second embryo can result in two babies with different gestational ages, and in this case, two different sets of genetic parents.

Allen said that after she gave birth to two healthy children she decided to become a surrogate to help other women who may be struggling to have a child.

"No woman in the world should have to live their life without experiencing the love and the bond from a mother and a child," Allen told ABC News.

She was then matched with a Chinese couple and in April of 2016, one of their embryos was successfully implanted into her uterus.

Six weeks into the pregnancy, however, she said that her doctor noticed that there were two babies during an ultrasound.

"He started moving the ultrasound around and he goes, 'Well I definitely see that there is another baby,'" Allen said. "The chance of an embryo splitting is very small, but it does happen, and I just thought ... I was very surprised."

Allen said she went on to deliver what she believed to be identical twin boys for the Chinese couple via Cesarean section.

She said that she never got a chance to see the babies in person but she was later shown a photo.

"I did notice that one was much lighter than the other," Allen said. "You know, obviously they were not identical twins."

Allen said it was not until months later that she received the news that the second baby was not a twin to the implanted embryo, but actually her and her husband's biological son. She added that a DNA test confirmed that the second baby was in fact their child.

"I don't know how to describe it ... we were floored," Allen said. "We were like, how did this happen?"

Allen said that after a complicated process, she and Jasper got custody of their son in February. Now 10 months old, they say he completes their family.

"He's just so smart, so intelligent," Allen said of their youngest son. "He's learning fast. He's got two big brothers to run after and learn from."

Her husband, Wardell Jasper, added that it has been an "emotional situation."

"You got to think like, 'Wow, we didn't know you were coming,'" he added. "We didn't plan this."

Allen added, "I carried my own child. I didn't know he was mine."

ABC News' chief medical contributor, Dr. Jennifer Ashton, stressed that superfetation is incredibly rare, saying on "Good Morning America" today that there is not a good hormonal or physiological explanation for how it can happen that a woman becomes pregnant with two babies, who aren't twins, at the same time.

Ashton said that when it does occur, however, the risks are minimal, aside from the same factors associated with carrying twins or multiples.

Ashton added that while this medical incident is extremely rare, it can occur with natural pregnancies, or with a pregnancy achieved through in vitro fertilization.

Finally, Ashton said that this case emphasizes the importance of making sure your legal contracts are airtight if you decide to become a surrogate or use a surrogate, saying contracts need to be in place to account for the expected, as well as the unexpected.