Surrogate mum Jessica was sent a photo of the twins she birthed. Then her world exploded.

When 31-year-old Jessica Allen, from California, decided to become a surrogate for a couple unable to have children, she had no idea she would soon be locked in a legal battle to be able to raise her own biological son.

That’s because, in a rare circumstance, Jessica and her fiancé, Wardell Jasper, had fallen pregnant with their own baby at the same time the other couple’s embryo was transferred into her uterus.

Speaking to New York Post, Allen said she had no idea the twins she had given birth to were not related to each other until a month later.

On January 10, 2017 – nearly 30 days after babies Mike* and Max were born – the woman whose babies she carried sent Allen a picture of the boys.

This woman’s story sounds like an absolute nightmare https://t.co/tWmtSwyeop

— New York Post (@nypost) October 26, 2017

“She sent me a picture of the babies and said: ‘They are not the same, right?’ followed by, ‘Have you thought about why they are different?'” Allen told the New York Post.

The babies were remarkably different: Mike was of clear Asian descent, like his parents, while Max was half-white and half African-American.

“A week later, the boys… had DNA tests, and the agency told me the results. Mike was a biological match to the [couple], while Max had my genes.”

Allen decided to become a surrogate just six months after welcoming her second son. She and fiancé (now husband) Wardell wanted to give another family the blessing of having a child.

"Besides, I wanted to stay at home with my sons rather than return to my job as a senior caregiver, and we decided we'd put the money toward buying a house," Allen said.

She was matched with a Chinese couple who wanted to hire an American surrogate, as the practice is illegal in China. In April 2016, the woman's frozen male embryo was transfered into her uterus.

At her six week scan, a doctor remarked that there were "two babies" in Allen's womb.

"Not once during the pregnancy did any of the medical staff provided by the agency say that the babies were in separate sacs," she said.

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

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Once it was confirmed that one of the babies was biologically hers, Allen realised what had happened.

"It turned out that, in an extremely rare medical incident called superfetation, we had gotten pregnant naturally, despite using condoms, after the IVF cycle during which [the couple's] embryo was transferred into my uterus," she said.

But the process of getting Max back wasn't simple: the surrogacy agency revealed the couple Allen had carried for wanted "nothing to do with him", and a new family was lined up to adopt him.

The couple demanded between US$18,000 and US$22,000 as compensation for the fact Allen had fallen pregnant with a child of her own - even though she had followed doctors orders by abstaining from sex until she was allowed, and used protection.

"It was like Max was a commodity and we were paying to adopt our own flesh and blood," she said.

After hiring an attorney who did "a lot of strained negotiation" with the surrogacy agency, the amount was finally reduced to zero and Allen and Wardell were handed back their son - who they have renamed Malachi - on February 5, almost two months after he was born.

"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.

"I don't regret becoming a surrogate mum because that would mean regretting my son."

* Names have been changed.