An Australian mother of two little miracles has shared the story of the phenomenon that resulted in her daughters being conceived 10 days apart.
Kate and Peter Hill are one of only 11 reported human cases of superfetation in the world, which brought them their 10-month-old daughters Charlotte and Olivia.
Superfetation occurs when a woman carries on ovulating after conceiving, and the second fertilised egg successfully implants itself in the womb's lining. The Brisbane parents only had sex once during the period of conception, but Peter's sperm stayed alive for 10 days, meaning the second egg Kate released could be fertilised.
It's so rare that Kate’s obstetrician Dr. Brad Armstrong, from Greenslopes Private Hospital in Brisbane, had to Google it to find out more about it. "I could not find any literature in the medical review websites at all," he confessed.
Charlotte and Olivia's original due dates were 10 days apart: Dec. 20 and Dec. 30, but both were delivered two days before the C-section they'd originally planned.
Another case of superfetation made the news in 2009 when American mother-to-be Julia Grovenburg went for a routine ultrasound and was told by her doctor that next to the baby girl she was carrying was a little boy, conceived days after his sister.
Julia and her husband Todd then had to prepare for the arrival of two babies: a daughter on Dec. 24 and a son a fortnight later on Jan. 10. Both babies ended up arriving on the same day: Dec. 2 by C-section.
Despite being so rare, superfetation shouldn't be a case for concern, according to Dr. Robert Atlas, chairman of the obstetrics and gynecology department at Baltimore's Mercy Hospital. Although he has never come across a case of superfetation during his career, he told Time that superfetation babies should behave much as twins do (although they're not actually twins because they were conceived at different times), with the second baby being only slightly premature.
In the animal world superfetation is far more common, with many documented cases in badgers, buffalo, mink and panthers.
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