A surrogate mother carrying a Chinese couple's twins discovered one of the babies is her biological son.

Jessica Allen, 31, was already a mother of two when she decided to become a surrogate. She would be paid $30,000 which would allow her to be a stay-at-home mom and save for a new house. Allen's partner, Wardell Jasper said it would also be a "chance to give a family the blessing of a child."

After becoming a surrogate for the San Diego-based Omega Family Global, Allen was matched with the Lius (a pseudonym) and became pregnant in 2016, ABC News reports.

She quickly realized she was carrying twins and the Lius were thrilled. Allen's surrogacy fee was upped by $5,000.

Allen told the New York Post the medical staff provided by the agency never mentioned the babies were in separate sacs.

"As far as we were concerned, the transferred embryo had split in two and the twins were identical," she said.

After Allen went into labour in December 2016, the biological parents reneged on their agreement, which would have allowed the California-native an hour with the children. It wasn't until later in the evening Allen realized something was wrong. When she saw the picture of the twins she immediately noticed their contrasting appearances.

"One, his skin tone was much fairer than the other. One looked full Chinese, the other didn't look full Chinese," she said. "It was very clear that they were not identical ... but I didn't ask questions."

Nearly a month after Allen gave birth, she received a message from Mrs. Liu, which included a picture of the twins.

"They are not the same, right?" Liu's message said. "Have you thought about why they are different?"

A week later, the babies underwent a DNA test, proving one of the children were biologically Allen's and Jasper's.

It was discovered Allen went through a rare medical phenomenon, called superfetation - when a woman continues to ovulate after becoming pregnant. The couple were dumbfounded.

"Wardell and I did not have sexual intercourse until we were given permission by the IVF doctor, who recommended the use of condoms," Allen said.

A lengthy and expensive custody battle ensued, where the Lius requested $22,000 from the couple to give the child back.

"To my disgust, a caseworker from the agency lined up parents to adopt him and "absorb" the money we owed the Lius. Or, if that didn't work out, the Lius were thinking of putting Max up for adoption, as they were still his legal parents," she said.

Allen said the couple desperately wanted their son back, but the agency treated their son as a commodity.

A caseworker from Omega Family Global further added insult to injury when she said the pair also owed her $7,000 in expenses the agency incurred for the bureaucracy and for looking after their son.

"We spent $3,000 on an attorney, and there was a lot of strained negotiation between us, our lawyer and Omega. It was an uphill battle, but the agency finally reduced the "fee" we owed the Lius to zero," Allen said.

In February 2017 the couple were reunited with their son, and changed his name to Malachi.

"The moment was incredibly emotional, and I started hugging and kissing my boy," Allen said. "It's now been nearly nine months since we got Malachi, and he is doing well. He's beautiful. He's healthy and his personality is hilarious. He loves his big brothers, is learning to walk and is starting to speak."