Asked by: Ashley Martin, Hampshire
Getting pregnant while already pregnant is called superfoetation. It’s been reported in mammals, including mice and rabbits, and there have been a handful of possible cases in humans. In 2017, a US woman acting as a surrogate mother was reported to have discovered that one of the twins that she was carrying was in fact her biological son. She had apparently become pregnant roughly three weeks after her surrogate conception.
For superfoetation, a woman has to ovulate again after becoming pregnant. This is rare because hormonal changes after conception usually prevent the ovaries from releasing another egg. What’s more, a mucus plug forms in the cervix during pregnancy, which should prevent sperm from swimming up.
Dr Emma Davies is a science writer and editor with a PhD in food chemistry from the University of Leeds. She writes about all aspects of chemistry, from food and the environment to toxicology and regulatory science.