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There's so much about the human body that we think we understand, but then weird stuff happens that changes everything. Take baby-making, for example.

Two fertile people have sex, their egg and sperm meet, and then about nine months later, a new human enters the world. Or so the story goes. Well, hold on to your ovaries, folks.

Because rarely, after someone gets pregnant, they can get pregnant again. And no, I don't mean shortly after having a baby. I'm talking about getting pregnant days or even weeks after conceiving, and carrying both fetuses at the same time—what scientists call superfetation.

In humans, superfetation is super rare, but also super real. And scientists have been studying these strange cases of twinning for insights into how reproduction works. Normally, a person releases one egg per month from their ovaries.

If sperm fertilizes that egg within about a 24 hour period, its cells begin to divide. As that happens, part of the ovary produces hormones like progesterone which help prepare the uterus for implantation. And then, if those hormones do their job, about six days later, the quickly-growing clump of cells burrows into the thick layer of tissue lining the uterus and implants.

After that, the body continues to suppress ovulation, and the lining of the uterus changes to accommodate the growing fetus— changes which also make it less receptive to implantation. But the uterus isn't completely unreceptive for several weeks. Finally, about a month after implantation, the opening at the bottom of the uterus called the cervix forms a mucus plug.

This protects the fetus from pathogens and prevents sperm from coming in and fertilizing another egg. Now, obviously, twins and other multiples are a thing, so we know that more than one embryo can implant. But that's typically because an embryo happens to split itself into multiple identical copies at an early stage, or because the person released multiple eggs to begin with.

That means multiples generally occur before the suppression of ovulation, the decrease in uterine receptivity, and the formation of the mucus plug. For superfetation to occur, allllll of these barriers have to be circumvented or go awry. Which obviously doesn't happen often.

But it does happen. Like, in a case from the late ‘90s. A women had a set of twins that consistently measured four weeks apart based on ultrasounds, which use the length of a fetus's spine to determine its gestational age.

Doctors working on the case ruled out other reasons for this difference in size, like genetic conditions that could restrict growth. And other than there being two fetuses of different ages in her womb, the pregnancy was unremarkable. The person delivered both vaginally when the oldest was forty weeks of age.

We don't know exactly what happened in this case, but her doctors suggested atypical levels of hormones that could have encouraged her to ovulate after implantation. Then, so long as some sperm managed to make it past the cervix before the mucus plug locked them out, the second embryo could have settled into the uterine lining before receptivity was totally gone. Or, the woman may have released more than one egg during ovulation, both of which were successfully fertilized.

In that case, the only reason she didn't have normal twins was that the second experienced delayed implantation. Basically, the doctors thought it may have pressed pause on its development and hung around in the uterus for a few weeks before implanting, getting in just under the wire in terms of uterine receptivity. Other known cases of superfetation have involved people with uterus didelphys: a rare condition where a person has two uteruses… and therefore, can become doubly pregnant.

One such case gave birth to two healthy babies whose ages were more than a month apart! From what doctors can tell, it seems like the two uteri can function independently—so implantation in one doesn't prevent implantation in the other. And sometimes, these uteri even connect to different vaginas, so there's no mucus plug blocking sperm from reaching the egg if the person ovulates!

Scientists also hypothesize that the use of assisted reproductive technologies can make superfetation more likely. Still really rare, but, you know, a little less so. And this probably has something to do with hormones taken during the process.

With in-vitro fertilization, the egg donor often receives hormones to induce more than one egg to mature at the same time. The mature eggs are then retrieved with a needle and fertilized with sperm in a lab. Once fertilized, two or three of these embryos are transferred into the uterus, often while the person receives supplemental progesterone.

That's the hormone we mentioned earlier that helps prep the uterine lining for implantation. It also maintains that lining during pregnancy, so people undergoing IVF often stay on it for awhile after conception. The thing is, that progesterone might keep the uterus receptive to new implantations longer.

And that seems to be exactly what happened in a 2005 case where one triplet was estimated to be eight weeks younger than its two siblings. In fact, the doctors involved in that case suggested growth discrepancies between multiples could be due to superfetation more often than we think. Even though known cases of superfetation are vanishingly rare, it's possible that some have gone undetected— especially in relatively older people carrying fraternal twins.

That's because as people with ovaries age, there's a slightly increased chance of what's called a luteal out-of-phase event, or LOOP event: an abnormal surge in hormones 1 to 3 weeks after conceiving. These hormones could spur an additional ovulation, and even make the cervical mucus plug porous enough for sperm to get through and fertilize the egg, just in the nick of time to implant. And, well, it's not uncommon for fraternal twins to be different sizes, so scientists are trying to suss out whether some seemingly-normal twins are actually cases of superfetation.

In any case, there's no need to worry about abstaining from sex while pregnant if your doctor has given you the go-ahead. This phenomenon is super unlikely to happen. Still, researchers are continuing to study these rare cases to help us better understand how the human body normally conceives and progresses through pregnancy, and how it can sometimes throw a curveball instead.

Thanks for watching this episode of SciShow! And a special shout out to our channel members. Those are the people in the comments and chats with the cool badges and using special emojis — like Hank's head.

Their support helps us keep making fun, educational videos like this one, so we really appreciate it. You can learn more about becoming a channel member by clicking the “join” button below. And, if you're not sure what to watch next... well, we have a whole episode about what usually happens during pregnancy.

So maybe start there! And let us know the weirdest thing you learn in the comments. [♪ OUTRO].