A woman has recently told her story of becoming pregnant whilst already pregnant, resulting in her giving birth to two children who were not conceived at the same time and had different sets of parents.

Jessica Allen, a 31-year-old from California, underwent in vitro fertilization in April 2016 after she agreed to become a surrogate mother for a Chinese couple for the sum of $30,000, The New York Post reports. California is one of the few US states with legal commercial surrogacy programmes where you can be paid to be a surrogate mother.

Jessica was told she was actually carrying twins following a routine ultrasound in her sixth week of pregnancy. Her payment was increased by $5,000 for the second child as doctors and the surrogacy agency simply assumed she had unexpectedly become pregnant with twins.

She went on to give birth to two boys via cesarean section later that year in December. A month later, she received a photo of the two boys from the Chinese couple with a message saying: “They are not the same, right? Have you thought about why they are different?”

"I did notice that one was much lighter than the other," Jessica told ABC News. "You know, obviously they were not identical twins."

DNA tests confirmed this suspicion. It showed that one baby was Allen's biological child and the other baby was the Chinese couple's child.

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

This is a phenomenon known as “superfetation”. It’s extremely rare because, in the most majority of cases, a pregnant woman’s body releases hormones to stop ovulation. In this case, however, Jessica's body continued to ovulate, releasing an egg that managed to become fertilized. Since superfetation is rare in humans, scientists and practitioners actually know very little about how and why it happens.

Fortunately, both children are now fit and healthy. After a complicated and pricey legal process, Jessica and her partner Jasper received custody of their son in February and named him Malachi.

In an article for the New York Post called "I rented out my womb – and they almost took my own son", Jessica finished by saying: "I don’t regret becoming a surrogate mom because that would mean regretting my son. I just hope other women considering surrogacy can learn from my story. And that a greater good will come out of this nightmare."