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  • [♪ INTRO]

  • There's so much about the human body

  • that we think we understand, but then

  • weird stuff happens that changes everything.

  • Take babymaking, 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 timewhat 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 independentlyso 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!

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  • Their support helps us keep making fun,

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  • so we really appreciate it.

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  • 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]

[♪ INTRO]

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