A woman from California named Jessica Allen shares her story of how she became pregnant again while she was already pregnant. The rare medical incident is known as "superfetation."

Jessica Allen's Story

Jessica Allen, a 31-year-old mother of two children, has revealed how she unknowingly gave birth to her biological son while she was carrying a surrogate child for a Chinese couple.

The Chinese couple agreed to pay Allen $30,000 in exchange for her service, that is, to carry and deliver the couple's baby. After undergoing in vitro fertilization (IVF), Allen then found out that she was going to deliver identical twins for the couple, but it turns out that one of the baby boys is her very own.

After the delivery, Allen noticed the differences of appearance between the two boys and then realizes the whole truth after conducting a DNA test.

"He started moving the ultrasound around and he goes, 'Well I definitely see that there is another baby,'" Allen told ABC News.

Superfetation: How Did It Happen?

The very rare medical incident is known as "superfetation." Superfetation takes place when a pregnant woman becomes pregnant for a second time with another younger fetus.

The process can take place when a woman continues to ovulate after becoming pregnant. Ovulation occurs when mature eggs are released from the ovaries, pushed down the fallopian tube, and are made available for fertilization.

The fertilization and implantation of a second embryo then result in two babies with different "gestational ages." What actually took place with Allen was that the Chinese couple's embryo was implanted and then her body released another egg that was later fertilized. It is believed that superfetation is common in some species of animals, but it is extremely rare in humans.

"The chance of an embryo splitting is very small, but it does happen, and I just thought ... I was very surprised," said Allen.

Allen And Her Child Finally Reunited

As a result of Allen's realization, she decided to bring back her biological son. However, the Chinese couple was still legally responsible for the child and had demanded compensation from Allen. The incident was then brought to court.

After a long legal battle, Allen manages to win over her son and did not have to pay any compensation as was requested by the couple.