Q: Can a woman have two foetuses in different stages of development in the uterus at the same time?
A: It may sound odd, but yes, it is possible for a second fertilisation to occur when one pregnancy is already progressing.
The condition is called superfetation, and while it is fairly often seen in mammals, including cats, only a few case studies have been reported in humans.
A French report of one apparent case, published in 2008 in The European Journal of Obstetrics & Gynaecology and Reproductive Biology, said that a review of the scientific literature had found fewer than 10 such report up to that time.
"Superfetation is defined by the fertilisation and the implantation of a second oocyte in a uterus already containing the product of a previous conception," the report said.
In this case, each foetus had a separate amniotic sac. The difference in their size was obvious in the first trimester of pregnancy and continued, leading the doctor to offer superfetation as a diagnosis.
Ordinarily, the release of eggs ceases once a woman is pregnant, and the hormonal and physical changes of pregnancy work together to prevent another conception. If a second pregnancy does later occur, it often results in a risky premature birth.
But in at least one US case, in Arkansas in 2009, both children were born healthy by caesarean section, though they were conceived more than two weeks apart.
"They're twins, but they're not twins," explained their mum, Julia Grovenburg, in 2011. "And if it sounds confusing that's because it is."
Closer to home, Australian mum Kate Hill last year welcomed two healthy baby girls who were conceived 10 days apart following superfetation.
"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.
New York Times with staff writers