You can fall pregnant when you’re already pregnant. Now that’s something you probably didn’t know. 

There are few known instances of the rare phenomenon, called superfetation, but one of them is now in the UK. Rebecca Roberts, from Bath, fell pregnant with her daughter, Rosalie, when she was already pregnant with her son, Noah.

The 39-year-old found out was expecting twins during her 12-week scan – her partner, Rhys, wasn’t allowed to join the scan due to Covid restrictions. 

The mum-to-be was informed one baby was larger than the other and, as her pregnancy progressed, Rosalie was consistently three weeks behind Noah in development. 

Test after test ruled out any other complications, so Roberts was eventually diagnosed with superfetation – she’d fallen pregnant, while pregnant. Noah was born at 4lb 10oz, while Rosalie was 2lb 7oz.

At first, the family worried about the health implications for the twins, especially Rosalie. But they’re now all home and doing well. Roberts has even set up an Instagram account – @roberts.supertwins – for those who want to see them growing up.

“Words can’t explain how happy we are to have both the twins home,” the mum tells HuffPost UK. “The care from the NICU staff towards Noah, Rosalie and ourselves was outstanding.

“Noah is really thriving and so is Rosalie, she’s growing well.”

While it is possible to fall pregnant when you’re already pregnant, Asma Khalil, professor of obstetrics and a spokesperson for the Royal College of Obstetricians and Gynaecologists says it’s “incredibly rare” – so isn’t something most people need to consider when trying to conceive. 

“Superfetation is when non-identical twins are conceived at different times,” she tells HuffPost UK. “It’s so rare, it’s impossible to put an accurate figure on how many occur, but we would estimate around one to two occur a year.”

It’s unclear why some women experience superfetation and others don’t. “It’s rare because once someone falls pregnant, further ovulation – when eggs are released from the ovaries – is suppressed, so it’s very unlikely eggs will be released, let alone an egg being fertilised and a further pregnancy taking place,” Prof. Khalil adds. 

Roberts’ pregnancy was not plain-sailing and doctors needed to closely monitor the babies after they were born. Noah spent a total of three weeks and three days in the neonatal intensive care unit. Rosalie spent 95 days there and came home a few days before Christmas.

Complications from superfetation don’t always occur, adds Prof. Khalil.  “Although there may be a risk that one baby is born prematurely and could therefore be at higher risk of medical conditions such as low birth weight and trouble breathing, it’s possible for both babies to be born at term without any medical concerns,” she says.