British woman Rebecca Roberts, age 39, was shocked to find out that she was pregnant again just a few weeks after learning about her first pregnancy.
She was diagnosed with superfetation after her 12-week ultrasound scan, when it was revealed that she was carrying another baby who was less developed. Superfetation is the formation of a fetus while another fetus is already present in the uterus.
Roberts’ obstetrician David Walker said: “We were concerned because the second twin was much smaller. It was only by regularly scanning and seeing that the rate of growth was consistently three weeks behind that we realized it was superfetation.”
Dr. Ilina Pluym, an assistant professor of maternal and fetal medicine at the University of California said that the condition is so rare that doctors don’t know the prevalence of the phenomenon.
She said: “There’s only been a handful of reports of it in the world’s literature. It’s more common in fish, rodents and bunnies.”
A 2008 report published in the European Journal of Obstetrics & Gynecology and Reproductive Biology noted that, at the time of writing, fewer than 10 cases of superfetation had been recorded worldwide.
The condition is rarely diagnosed because when a woman becomes pregnant, her hormones tell her body not to conceive again. Women who are undergoing fertility treatment are at a slightly higher risk of superfetation, but the chances of it happening are still slim.
Not only are the chances of being diagnosed with the condition very rare, but it is also extremely difficult to prove superfetation.
“We have common answers to why twins can be different sizes,” said Pluym. “The last thing on our list is superfetation.”
A more likely cause of different sized twins in the uterus is one fetus’ placenta not being as healthy as the other one.
As with all twin pregnancies, Rebecca was monitored closely. The babies were delivered by C-section, with big brother Noah weighing in at 4lbs 10oz and little sister Rosalie weighing a dainty 2lbs 7oz. The little ones are now thriving at home.
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