Fell pregnant while pregnant? Here’s how it can happen

Superfetation might seem a myth, but here are the facts behind the bizarre medical phenomenon

07 December 2023 - 13:33
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In rare cases, women have been found to be pregnant twice by one or more men.
In rare cases, women have been found to be pregnant twice by one or more men.
Image: 123RF/paylessimages

One of the things people surmised would come out of the Covid-19 pandemic was a baby boom — mirroring the many other events that saw people have babies after a global tragedy.

The assumption was many couples would emerge from the pandemic with multiple children, but Rebecca Roberts did not just have one baby but also a surprise baby conceived after she fell pregnant.

The New York post reported Roberts and her husband welcomed their newborns on September 17 2021 with their youngest needing an extra 95 days of medical care. 

The occurrence is known as superfetation, affecting about 0.3% of women, where they fall pregnant while already carrying a baby. 

According to Roberts' sonographer, the second baby was too small to spot in scans and tests and deduced she had experienced superfetation. The phenomenon occurs when an egg is released from a woman’s ovary after she is already pregnant and implants alongside the first embryo which Healthline breaks down in the three unexpected occurrences:

Three steps to superfetation

1. Ovulation (release of on ovum by an ovary) during an ongoing pregnancy. This is incredibly unlikely because hormones released during pregnancy function to prevent further ovulation.

2. The second ovum must be fertilised by a sperm cell. This is also unlikely because once a woman is pregnant, their cervix forms a mucus plug that blocks the passage of sperm. This mucus plug is the result of elevations of hormones produced in pregnancy.

3. The fertilised egg needs to be implanted in an already pregnant womb. This would be difficult because implantation requires the release of certain hormones that wouldn’t be released if a woman were already pregnant. There is also the issue of having enough space for another embryo.

While this has often been the stuff of myths or old wives' tales, it is not always a happy ending for some women when the second foetus rarely survives. In the Healthline report, most recorded cases of superfetation occur in women who had fertility treatment such as in vitro fertilisation (IVF). Because IVF transfers fertilised embryos into a uterus, superfetation can then occur if the woman ovulates and the egg is fertilised by sperm a few weeks after the embryos are transferred into her uterus.

However, these supertwins are not always a blessing.

In a case best suited for a front-row ticket on The Jerry Springer Show, Fox News reported Mia Washington conceived another baby while pregnant with a man she had a one-night stand with. 

Washington came clean to her fiancé James Harrison after their birth when they noticed one of the children did not have matching facial features. They went for a DNA test and found he was not the father of the baby.

“Of all the people in America and the world, it had to happen to me. I'm shocked,” Washington told Fox.

In this case, Washington had a heteropaternal superfecundation that happens when women release more than one egg during ovulation, which can result in a pregnancy if they have intercourse with more than one partner during that time.

While summer is often jokingly called baby-making season, perhaps new parents should be a bit cautious as superfetations are on the rise.

The most recent case reported by the Daily Mail occurred this year with a woman in Perth who “fell pregnant twice in three weeks” years after an IVF treatment that found her pregnant with twins who were weeks apart.

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