What is superfetation?
When a foetus is conceived days or weeks after one has already been conceived, it is referred to as superfetation. This phenomenon is more common in the animal kingdom, but since humans are mammals, this condition is technically also possible in humans.
One foetus is gestationally older than the other, but the babies will share a birthday. Doctors will have to induce labour for the younger sibling or deliver the babies via c-section. In some cases, the older one will even be much bigger than the younger twin because they have had more time to grow and develop in the uterus.
How common is this?
Doctors are still researching if superfetation is physically possible in humans, for they believe that falling pregnant while you're already pregnant could be due to another medical condition that as yet remains unexplained. Only 10 cases have been reported in the United States.
In 2016, an Australian couple, Kate and Peter Hill, conceived babies 10 days apart. The doctors even gave them two different delivery dates! What is even more rare about their case is that the couple only had intercourse once and the sperm were able to stay alive inside her body for up to 10 days (usually sperm live for up to 5 days). Moreover, she must have ovulated again shortly after the first ovulation.
In 2010, an American couple, Todd and Julia Grovenburg, also welcomed superfetation babies.
How does it happen?
For pregnancy to occur, ovulation must take place. So for superfetation to happen, the woman must ovulate while she is already pregnant. Now, this is nearly impossible because when someone falls pregnant, she stops ovulating. Pregnancy triggers a rise in the progesterone hormone, which prohibits the ovaries from releasing another egg.
Furthermore, when you fall pregnant, a mucus plug is formed in the cervix, blocking semen from passing through to the uterus.
So to recap, for superfetation to happen, the following needs to occur:
- ovulation must take place during pregnancy (highly unlikely)
- semen must be able to penetrate through the mucus lining (again, not very likely)
- fertilisation has to be successful in an already occupied uterus.
According to the Scientific American, "reports of superfetation in humans are more commonly associated with women who have undergone fertility treatment."
Studies are still being done about this rare condition, as Kate and Peter's doctor came to realise, when it was close to impossible to find any information about superfetation and he had to turn to Google for answers.
Have you heard of more cases similar to this one?
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