A UK woman has given birth to twins conceived three weeks apart in a medical marvel so rare her doctor struggled to diagnose it.
Rebecca Roberts and her partner had struggled with infertility for more than a year - so when an at home test returned a positive result they were overjoyed. A visit to the obstetrician confirmed the single baby - but at the 12-week scan Roberts' doctor made a shocking discovery.
"The sonographer looked at me and was like, 'Do you know you're expecting twins?" she told the Washington Post.
But not just any twins - Roberts' second child had been conceived weeks after the first in what's called superfetation - an ultra-rare condition where an already pregnant person conceives again.
The pregnancy is one of few superfetation cases recorded in medical literature, said her obstetrician, David Walker.
Superfetation is so uncommon he struggled to diagnose it. In his 25 years as an obstetrician, it was something he had never seen before.
"It just doesn't happen," he's quoted as saying.
It was only discovered when constant scanning of the twins revealed the second baby was consistently three weeks behind the first twin.
The true number of superfetation cases is not known, but according to a report published in 2008 in the European Journal of Obstetrics & Gynecology and Reproductive Biology, fewer than 10 instances of the phenomenon had been recorded at the time.
It's "miraculous," says Walker.
"Instead of stopping ovulation, she released another egg about three or four weeks after the first one, and the egg somehow miraculously managed to fertilize and implant in her uterus."
With one baby three weeks behind the other birth was almost guaranteed to be complicated.
"But with a three-week difference, you don't want to compromise the smaller twin by delivering too early. You have to keep a really close eye. The delivery was crucial in this case," said Walker.
Both twins were delivered via Cesarean on September 17.
Noah came first, weighing 2.4kg. Two minutes later, his younger sister, Rosalie, arrived, weighing 1.1kg.
"We got to see both of them as soon as they were born," Roberts said. "It was absolutely beautiful."
Both babies were quickly whisked away to the neonatal intensive care unit - Noah stayed for three weeks and Rosalie 95 days.
Now both home safe and growing well Roberts' documents her children's development on an Instagram page with more than 44 thousand followers.
"We want people to be able to continue to watch as they grow up," said Roberts. "Miracles can happen, and my children are proof of that."