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.

Allen was 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.

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

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.

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.