A US woman who was applying to make a man pay child support for her twin daughters had a shock when it was discovered he was only the father of one of them.
The woman, named in court papers only as "TM", applied for public assistance in Passaic County, New Jersey, telling the Board of Social Services that a man she had been involved, "AS", was the father of her twins born in January 2013 and should pay child support.
The Board ordered AS to undertake a DNA paternity test and the medical oddity was discovered.
It transpired that TM had a brief liaison with another unidentified man while involved with AS, who impregnated her.
Obstetrician-gynaecologist Jennifer Wu explained that sperm can remain viable for up to five days and ovulating women can produce more than one egg, meaning a woman could become impregnated twice. When both sets have the same father, normal fraternal twins result. When two different men are involved, the phenomenon is known as superfecundation.
Wu said that the phenomenon is on the rise due to a combination of factors, including the rise of fertility treatments such as IVF, particularly when gay men both contribute sperm towards a pregnancy.
"That's why we're seeing it more often than we were in the past, when we were relying on nature and women who have more than one sexual partner in the same cycle around the time of ovulation."
Passaic County Superior Court Judge Sohail Mohammed said he had researched the subject and discovered two similar cases in the US, though experts believe the phenomenon is more common than many people realise.
Fertility expert Dr Cynthia Austin told CBS News that superfecundation is sometimes obvious, particularly when the babies are of different races, but that "the vast majority of times, twins with different fathers, it goes unnoticed".
As a result of the court ruling, the father of one of the two girls, AS will only have to pay towards the upkeep of one child – believed to be $28 (£15) a week.