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 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.

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And, if you're not sure what to watch next...

well, we have a whole episode about

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So maybe start there!

And let us know the weirdest thing you learn in the comments.

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