In a disturbing twist that goes against everything we learned in sex education, a recent New York Times Q&A confirmed that it's actually possible for a woman to get pregnant… while she's already pregnant.
As far-fetched as that sounds, the science on this one actually lines up. In a creepy quirk of the human body, under very exceptional circumstances, a woman can continue to ovulate while pregnant and can conceive another child - something known as 'superfetation'.
That means a woman can have two foetuses developing inside her at the same time, both at different stages of development. And we sort of wish we could immediately scrub that knowledge from our brains.
Before you panic, this is highly unlikely to happen to you or anyone you know. In fact, according to a paper published in 2008 in the European Journal of Obstetrics and Gynaecology it's only happened 10 times in the scientific literature… ever.
Many claims of double pregnancies over the years that have gone on to be debunked, but, according to the researchers, at least 10 of those seem to be the real, evidence-backed deal.
And since that paper came out, there have been a few other cases reported.
In 2009, a couple from Arkansas became pregnant after already conceiving two and a half weeks earlier. Both babies were delivered healthily by c-section on 2 December, with one baby measurably a fortnight more premature than the other.
And in 2015, an Australian couple gave birth to two girls that were 10 days apart in age at birth.
So, how is this even possible? Superfetation is actually quite common in mammals outside of humans, and has been seen in species including rodents, rabbits, horse, sheep, and kangaroos.
Sometimes these mammals have two uteri to facilitate the double pregnancy, or sometimes their menstrual cycle simply continues during pregnancy. It's even considered a handy reproductive strategy in some species.
But, in humans, superfetation appears to be a very rare accident - that's because, as soon as a woman becomes pregnant, her body actively blocks a second pregnancy from happening.
"Ordinarily, the release of eggs ceases once a woman is pregnant, and the hormonal and physical changes of pregnancy work together to prevent another conception," C. Clairborne Ray explained in a New York Times' science Q&A this week.
But for some reason, in superfetation, a pregnant women still manages to ovulate. A male's sperm then manages to fertilise that egg, somehow bypassing the the mucus plug that blocks up a woman's cervix once she's conceived.
Finally, implantation has to occur - which is an incredibly delicate process even in ordinary pregnancies. And when a woman is already pregnant, her hormones should make the uterus an unfavourable environment for another fertilised egg to implant (not to mention that there wouldn't be much room).
"In order for superfetation to occur in humans … it would appear that three seemingly impossible things need to happen," Khalil A. Cassimally reported for Scientific American back in 2011.
"Ovulation must take place during an ongoing pregnancy, semen must somehow find its way through the blocked cervix to the oviduct, via the occupied uterus and finally, the conceptus has to successfully implant itself in an unsuspecting already-occupied uterus."
Unlike twins - which occur either when a fertilised egg splits into two, or when two eggs are fertilised by two sperm at the same time - superfetation leads to a woman being pregnant with an additional foetus that's younger than the existing pregnancy.
So far, no cases have been reported of the age gap being greater than a few weeks.
According to the 2008 paper in the European Journal of Obstetrics and Gynaecology, whenever superfetation was confirmed to occur, the two foetuses had a separate amniotic sac. And they differed in size throughout the pregnancy and after birth.
Given the small sample size scientists have to work with, it's not yet clear why superfetation sometimes occurs, and whether whether there are any risk factors that can increase the odds of the phenomenon.
But Casimally notes that reports of superfetation are more common in women who've undergone fertility treatments, which could explain how one or two of those checkpoints get passed.
Despite how rare superfetation is, however, surprisingly, most babies conceived through the strange accident end up surviving.
The main risk is the fact that the babies are still born at the same time, despite their age difference - so one of the babies has the additional challenge of being born premature.