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A pregnant US woman named Jessica Allen became pregnant again and gave birth to two boys. She conceived a baby while she was already pregnant, which makes this case rare.

Allen, who is already a mother of two sons, was being a surrogate mother for a Chinese couple and was paid $30,000. She underwent IVF treatment, which helped in getting pregnant with the couple's baby.

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She became pregnant for the second time by her partner Wardell Jasper, who is now her husband despite being pregnant by the Chinese couple's baby.

Allen and Jasper needed the money to save for a house, so she had agreed to become a surrogate mother.

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

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No complications were observed in Allen's pregnancy, and the revelation about her being pregnant with two foetuses was made at six weeks by her doctors.

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

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

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The babies were initially considered to be identical twins, a result of the splitting of the Chinese couple's embryo. But it was caused by a rare condition -- superfetation – a condition in which a second conception takes place despite the woman being pregnant.

This phenomenon occurs because the woman continues to ovulate despite being pregnant. The second foetus grows along with the first foetus.

In this case, Allen and Jasper revealed that they had used protection while she was pregnant due to surrogacy but they still conceived the second child.

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Allen underwent caesarean to deliver the babies and thought that they were identical twins. She didn't get to hold the babies, but a picture of the babies revealed what really happened.

"I did notice that one was much lighter than the other. You know, obviously, they were not identical twins," Allen revealed.

Soon after this, a DNA test was carried out and one of the babies matched the Chinese couple genetically, while the other one matched Allen and her partner.

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Allen and her husband got their child named Malachi almost after the baby turned three-months-old. The couple had to fight a legal battle with the agency which had arranged this surrogacy. The agency was claiming fees for giving their baby to them.

This condition is very rare among humans, only 10 or so cases of this kind have been reported in the medical literature so far. But the phenomenon is found to be more common in the animal kingdom and has been seen in rabbits, rodents, horse, kangaroos and sheep.

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"The [reunion] was incredibly emotional, and I started hugging and kissing my boy," Allen wrote in a column for New York Post. "[We] weren't planning to expand our family so soon, but we treasure Malachi with all our hearts," she added.