A surrogate mother’s emotional ordeal of unknowingly giving birth to her own child alongside a surrogate baby has reignited the debate over whether a woman can fall pregnant while already expecting.

Californian woman Jessica Allen, 31, delivered surrogate twins, only to later discover one of the children she gave birth to was in fact her own.

But she is not the first woman to fall pregnant twice, simultaneously.

A handful of other women from around the world have been known to give birth to two babies at the same time, to the same or different fathers, before finding out it is possible their children are not technically twins.

Stephen Robson, president of the Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG), confirmed with The New Daily that it was a real, albeit “very rare”, phenomenon.

“Usually only one egg is released each month, but it is quite possible that when an embryo is placed within the womb of a surrogate, the woman will ovulate herself and have relations with her partner and … voila!” Dr Robson said.

“I have only seen this once in my 26-year career, or at least only once that I’m aware of.”

Professor Mark Umstad, maternity services director at Melbourne’s Royal Women’s Hospital, said there were two proven explanations for a scenario where a woman has two separate pregnancies simultaneously.

‘Superfecundation’ is the fertilisation of two separate eggs from two episodes of sexual intercourse or artificial insemination, occurring within the one ovulation cycle.

Meanwhile, ‘superfetation’ is when fertilisation of the second egg occurs in a different cycle when the first embryo is already developing in the uterus.

In both cases, the babies develop in two separate placentas. The two conceptions must occur within two to three days of each other, Dr Umstad said.

He recalled one superfecundation case where the woman indicated she was not aware of who the father was, before giving birth to one baby with pale skin and another with darker skin.

It quickly became apparent that the ‘twin’ babies had been born to different fathers.

Dr Umstad said claims of superfetation occurring through spontaneous conception only – where the woman was not undertaking IVF at the time – were questionable.

“It’s never been proven and it would be very hard to prove. It’s an extraordinarily unlikely if not impossible event,” he told The New Daily.

“The hormonal changes after conception stop subsequent ovulation [the releasing of another egg].

“Cervical mucous thickens almost like a back-up to further prevent sperm from entering the uterus to create a second pregnancy.

“There are also changes to the lining of the uterus during pregnancy, acting as another natural protecting mechanism.”

However, Dr Umstad said he was aware of at least a dozen cases where superfetation had occurred within an in-vitro fertilisation (IVF) cycle because, in IVF, the woman does not possess the aforementioned natural barriers to falling pregnant a second time.

Dr Umstad added that it was not uncommon for twins to be different in size and this could be mistaken as being superfetation.

He suspects that this was the case in an Australian scenario last year when a Brisbane woman gave birth to twins who appeared to have been conceived about 10 days apart due to their differences in size, weight and gestational development.

However, the Californian surrogate case would be considered superfetation as it involved inserting the embryo in the surrogate, Ms Allen, using IVF.

Weeks after the birth, the Chinese couple who paid Ms Allen to be a surrogate mother began questioning whether one of the babies belonged to them, and a DNA test confirmed their concerns.

The couple initially demanded $US22,000 ($28,500) to return her baby. The legal dispute was settled last week, with no fee, but not before Ms Allen and her husband had spent $US3000 ($3900) on a lawyer.