A woman has recently told her story of incredibly becoming pregnant while she was already pregnant, resulting in her giving birth to twins who were conceived by different parents and not at the same time.

Jessica Allen, a 31 year-old from California, agreed to become a surrogate mother for a Chinese couple and underwent in vitro fertilization in April 2016. She was paid $30,000 for doing so, according to The New York Post. California is one of the few states in the United States where you can pay someone to be a surrogate mother.

In sixth week of pregnancy, Allen was told she was carrying twins after a routine ultrasound. Her payment was increased by $5,000 for carrying the second child as the doctors assumed she had become pregnant with twins.

In December 2016 she gave birth to two boys, and one month later 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.”

Subsequent DNA tests confirmed her suspicion that they weren’t twins. It showed that one of the babies was Allen’s biological child and the other 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?”

It happened due to the extremely rare phenomenon known as “superfetation”. In most cases, when a woman becomes pregnant they release hormones to stop ovulation. But in some rare cases the woman’s body continues to ovulate, releasing an egg that can become fertilized. This is  what happened to Allen, with the new egg becoming fertilized by her partner. Supferfetation is so rare in humans that scientists know very little about it.

Fortunately, both children are now well and healthy. After a harrowing and expensive legal process, Alllen and her partner now have custody of their son and have renamed him Malachi.

Allen doesn’t have any regrets about the process: “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.”