Story at a glance

  • A U.K. woman conceived a second baby three weeks after conceiving the first.
  • This rare condition is known as superfetation.
  • Up to 2008, there had only been approximately 10 cases of superfetation worldwide.

After struggling with infertility for years, 39-year-old Rebecca Roberts and her partner were thrilled, though a little shocked, when an at-home pregnancy test came back positive.

During her first sonogram appointment, the obstetrician noted she was carrying a single baby and everything looked normal. Then a second surprise came.

When Roberts, who lives in Wiltshire, England, returned five weeks later for her 12-week ultrasound, she received some shocking news. She told the Washington Post, “The sonographer looked at me and was like, ‘do you know you’re expecting twins?’”

However, she wasn’t carrying your quintessential set of twins. Roberts was diagnosed with a rare condition known as superfetation, where a woman who is already pregnant conceives a second baby.

The condition is so rare, her obstetrician, David Walker, ran a multitude of scans before he could confirm it, saying he’d never seen it in his 25 years as a doctor. 

“It just doesn’t happen,” Walker said.


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Due to its rarity, there isn’t an exact figure on how many cases of superfetation have occurred, but a 2008 study in the European Journal of Obstetrics & Gynecology and Reproductive Biology estimated fewer than 10 cases had been reported at that time.

Typically when someone becomes pregnant, the body undergoes hormonal changes that impede it from conceiving again at that time. 

“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,” Walker said.

While Roberts had been undergoing fertility treatments to help her conceive, drugs which are known to increase a person’s chances of conceiving multiples at one time, Walker isn’t convinced that that is the cause of her superfetation, and the rarity of the condition means little is known about its exact causes instance to instance.

Roberts’s second baby was three weeks younger than the first she conceived. Given the rate of developmental differences between the two, doctors were unsure if the second baby would survive.

“Anything that can go wrong with a pregnancy is more common with twin pregnancies. 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,” Walker said. “The delivery was crucial in this case.”

A full-term baby is typically delivered around 40 weeks, but at 33 weeks doctors found the younger fetus’s umbilical cord wasn’t functioning correctly and decided the babies had to be delivered.

Roberts had a Caesarean section on Sept. 17, 2020, to deliver both babies. The oldest, Noah, was 4 pounds, 10 ounces, and the youngest, Rosalie, was 2 pounds, 7 ounces.

Due to their premature state, both babies were taken to the neonatal intensive care unit, where Noah spent three weeks and Rosalie for 95 days.

Now, healthy and at home, Roberts tracks the twins development via Instagram.

“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.”


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Published on Apr 09, 2021