Known as superfetation, Kate and Peter Hill are one of ten reported cases in the world.
Speaking to Australia’s Today Tonight, Kate said her daughters - non-identical twins Charlotte and Olivia - 'are definitely little miracles.'
As Time explained, the phenomenon occurs when a woman continues to ovulate after becoming pregnant. This allows a second egg to fertilise and implant itself in the the womb.
'Typically, hormonal changes prevent further ovulation and thicken the lining of the uterus to preclude a second embryo from attaching.'
Struggling to conceive after Kate was diagnosed with polycystic ovarian syndrome in 2006, the mother-of-two underwent hormone treatment with better than great results; despite only having sex once during the ten-day period.
'We actually did not realise how special that [form of conception] was until they were born,' Kate said.
'What makes this case even more rare, is that my husband and I only had intercourse one time.'
Kate’s obstetrician, Dr. Brad Armstrong, from Greenslopes Private Hospital in Brisbane, said he hasn't seen anything like this. 'I could not find any literature in the medical review websites at all.'
'I had to go and Google it,' he added.
Kate and Olivia were initially given separate due dates, December 20 and December 30, however they were both born via planned caesarean section two days prior.
This article originally appeared on Practical Parenting.