An Australian woman managed to conceive twice in 10 days, after having sex just once, Australia's Seven Network news media reported.
The woman, whose age is not known, has since given birth to a pair of twin girls.
Brisbane couple Peter and Kate Hill had been struggling with infertility after Mrs Hill was diagnosed in 2006 with polycystic ovarian syndrome. The condition affected her ability to ovulate.
Mrs Hill underwent hormone injections, which were so successful that her body released a fresh egg even after a first egg had been fertilised.
She told Seven Network's news outlets that "my husband and I only had intercourse one time - his sperm stayed alive for 10 days to fertilise the second egg released".
The first egg split to form identical twins, but only one of the embryos survived.
Her subsequent pregnancy with two babies conceived at different times, a scientific phenomenon called superfoetation, is almost unheard of.
It is unusual for ovaries to release fresh eggs after conception has occurred, and sperm do not usually survive in the female reproductive tract for longer than a week.
The Hills' daughters, Charlotte and Olivia, are now 10 months old. They were originally given separate due dates and a Caesarean birth was scheduled, but they ended up being delivered vaginally on the same day.
The babies have different heights, weights and even blood types.