After struggling to fall pregnant, Kate Hill was delighted to discover she was expecting twins - but surprised to learn the babies were conceived 10 days apart.
Mrs Hill had been diagnosed with polycystic ovarian syndrome 10 years ago. The condition meant she didn't ovulate and required hormone treatment in order to fall pregnant.
After having the treatment early last year, Mrs Hill conceived naturally twice in 10 days.
The double conception is known as superfetation, which means carrying two unborn babies at different stages of gestation. It was possible because Mrs Hill ovulated for a second time after she had already conceived - which is very rare.
"What makes this case even more rare is that my husband and I only had intercourse one time – his sperm stayed alive for 10 days to fertilise the second egg released," Mrs Hill said when sharing her family's story on Today Tonight this week.
During the pregnancy the twin girls measured differently and were given due dates which were 10 days apart.
Superfetation is so rare there are only 10 documented cases of it in the world - but despite the unusual circumstances, Mrs Hill and her husband Peter didn't realise how unique the pregnancy was until the babies were born.
Baby girls Charlotte and Olivia were born on the same day last December. They had different birth weights and different blood types.
Mrs Hill's obstetrician Dr Brad Armstrong told Today Tonight he had to consult Google to research information on the Hills' conception.
"Superfetation is so rare that I could not find any literature in the medical review websites at all," he said.
The story of Mrs Hill's pregnancy is reminiscent of the experience of fellow new parents Krystal and Luke Findlay.
Earlier this year the South Australian couple became parents to twins after one baby was conceived via IVF and the other naturally.
Mrs Findlay was undergoing fertility treatment last year and had only one embryo transferred. Two months later the couple learnt they were expecting two babies - a girl who grew from the transferred IVF embryo, and a boy conceived naturally around the same time.
Natural conceptions which happen during successful IVF embryo transfer programs are so rare there are no Australian statistics on them.
"My wife and I think we are the luckiest people in the world," Mr Findlay told the Adelaide Advertiser earlier this year.
"They have brought us love and joy and we want to share our story to give other couples in a similar position hope that miracles really do happen."