What stops women from becoming pregnant while they’re already pregnant?


Is there a mechanism in the body that stops this, or is it that the sperm can’t get to an egg anymore?

In: 33

There’s no egg anymore. The body stops ovulating each month.

Before the woman becomes pregnant, each month she would ovulate then have her period (shed the unused egg along with other tissue). When she’s ovulating, an egg is released to the Fallopian tube that’s ready to receive sperm – if it successfully does so, the egg becomes a zygote and the woman becomes pregnant. She then stops having her period (because otherwise she’d shed the fertilized egg) and stops ovulating.

When a woman is pregnant she stops ovulating, this is essentially how hormonal birth control works, it “tricks” the body into thinking (but not literally thinking) it is pregnant.

Also, mucus forms a plug in the cervix and prevents stuff from getting into the uterus.

Several things.

A woman’s menstrual cycle stops while she’s pregnant. Since her cycle is paused, the follicles in her ovaries do not mature, and thus she does not normally produce any eggs during pregnancy. (People will sometimes loosely say that women are born with all of their eggs, but more properly they’re born with all of their ovarian follicles, and those mature into eggs later.)

And in case that weren’t enough, there’s a thick layer of mucus called the *cervical plug* at the bottom of the uterus during pregnancy. That plug seals off the interior of the uterus from the outside world, protecting the embryo/fetus from e.g. infection. This mucus is present normally, but it’s thickened enough during pregnancy that sperm can’t swim through it. (In fact, artificially thickening it is part of how some birth control works.)

Hormonal changes (high estrogen and progesterone) prevent ovulation (preparing an egg for fertilization). There will also be thick mucus acting like a plug to the uterus preventing sperm to pass to where the egg might be

In the vast majority of cases, the hormones that mature an egg are overwritten by the hormones that sustain a pregnancy. This mostly has to do with relative levels of nearly a dozen different hormones, most of which are present in a woman’s body at some low level from puberty onward, regardless of being pregnant or not. These same groups of hormones are responsible for the thickening and shedding of the uterine lining during a full menstrual cycle, thickening and thining the cervical mucal “plug”, and maturing an egg cell from a “pre-egg” into an egg, then releasing one into the Fallopian Tubes.

In rare cases more than one egg matures (at least we think it is rare, reasonably certain), and if both are fertilized and implant you end up with fraternal twins (sometimes triplets, and other variations may occur).

In even rarer cases (and these we are very certain are very rare) enough of the hormones that cause a pre-egg to ripen and release along with the matching “mural thining” hormones end up in her bloodstream and another new egg is sent “downstream” with a thinned cervical plug as it were. Given how hard it is for pregnancy to occur in the first place (timing and hormonal balances are everything) it is *super* rare for this second egg, which has been released several weeks to potentially months after the earlier one fertilized and implanted, to implant, but when it does it results in an extremely rare form of twins. This is somewhere in the order of one in several hundred thousand twin births, possibly one in a few million twin births iirc.

But under normal pregnancy circumstances, the same hormones that promote and make a pregnancy “healthy” suppress or at the least flood out the hormones that are normally used to either flush the uterine wall or create the conditions that cause a new egg to mature, release, and become easily accessible to sperm.