An extremely rare case of a woman becoming pregnant while already pregnant has occurred in the US, with a mother unwittingly giving birth to 'twins' who were not conceived at the same time.

In an even more unlikely twist, four distinct parents were ultimately involved in producing her two babies, as the woman in question, California resident Jessica Allen, was acting as a surrogate for a Chinese couple when she became pregnant a second time via her own partner.

Allen was already a mother of two boys when she entered into a surrogacy arrangement to carry a child for a Chinese couple who wanted to become parents.

In exchange for a US$30,000 payment, Allen received IVF treatment and was successfully implanted with one of the couple's embryos. The payment would help Allen and her partner (now husband) Wardell Jasper save for a house, but money wasn't the only incentive.

"No woman in the world should have to live their life without experiencing the love and the bond from a mother and a child," Allen told ABC News.

The pregnancy proceeded smoothly, until at six weeks, during a routine ultrasound, Allen's doctors noticed she was actually carrying two foetuses, not one.

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

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

At the time, everybody assumed the two babies were identical twins – as a result of the Chinese couple's embryo splitting – with nobody considering a far less likely explanation called superfetation.

Superfetation is an extremely rare phenomenon in humans where a pregnant woman continues to ovulate after becoming pregnant, and so is able to conceive a second child, who gestates simultaneously alongside the first foetus.

While rare in humans – having only been reported 10 or so times in medical literature – it's a more common event in the animal kingdom, and has been observed in rodents, rabbits, horse, sheep, and kangaroos.

In the case of Allen and her partner, despite using condoms during the surrogate pregnancy, they had unwittingly managed to conceive a second child.

Allen went on to deliver what she thought were identical twin boys via caesarean section, and while she never got to hold or meet the children in the hospital, a photo she was shown indicated something was up.

"I did notice that one was much lighter than the other," Allen told ABC News.

"You know, obviously they were not identical twins."

Weeks later, while the mother was still recovering from the C-section, a DNA test confirmed that one of the babies was a genetic match for the Chinese couple, but the other child had Allen and Jasper's genes.

After a legal battle with the agency that organised the surrogacy – and which was demanding fees to relinquish the baby to his genetic parents – Allen and Jasper were finally reunited with their child, Malachi, who was then nearly three months old.

While instances of superfetation in humans are incredibly rare, this case was even more so because it ultimately involved two different sets of genetic parents due to the surrogacy.

All in all, an amazingly improbable medical event that caused no small amount of surprise and heartache for those concerned – but luckily, there was a happy ending for Malachi, who at 10 months old is living with his parents, despite the incredible, unlikely journey he took to get home.

"The [reunion] was incredibly emotional, and I started hugging and kissing my boy," Allen wrote in a column for the New York Post.

"[We] weren't planning to expand our family so soon, but we treasure Malachi with all our hearts."