Parents-to-be are usually surprised when they find out they are having twins, but Krystal and Luke Findlay had more reason to be shocked than most.
The couple was undergoing IVF treatment when Krystal had only one embryo transferred. Two months later they learnt she was pregnant with two babies.
But the twin pregnancy wasn't a result of that embryo splitting and implanting - instead, one of the babies was the result of the IVF treatment, while the second baby was conceived naturally.
The South Australian couple's twins, Ethan and Ava-Rose, were born via caesarean section this week. Their little girl started life as as the transferred IVF embryo, while her twin brother was conceived naturally.
Mr and Mrs Findlay told the Adelaide Advertiser the operator at their eight-week ultrasound asked if they wanted to "look for more" babies after discovering the twins.
"We said 'don't you dare, two is enough'," Luke told the newspaper. "We were in shock and thought the embryo had split and we were having identical twins. But it turns out one of the babies was naturally conceived. Doctors say the odds are astronomical.
"Just to be able to conceive naturally after all we have been through is amazing, but to have it occur in that small window when an embryo was successfully implanted is unbelievable."
Natural conceptions which happen during successful IVF embryo transfer programs are so rare there are no Australian statistics kept about them.
For the Findlays this embryo transfer was possibly going to be their last attempt at parenthood after three years of trying to conceive.
According to the Advertiser, the couple had been through two IVF cycles. The first did not produce any embryos, but the second cycle produced three.
The first embryo was transferred but did not implant, and the second embryo was frozen but did not survive thawing.
"We were pretty sure the third embryo would be our last attempt as it was taking a heavy toll on Krystal," Mr Findlay told the newspaper.
"We were preparing ourselves for a life without children. Then we got the news that Krystal was pregnant, we were delighted.
"My wife and I think we are the luckiest people in the world, we just can't get enough of them.
"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."