Is there a mechanism in the body that stops this, or is it that the sperm can’t get to an egg anymore?
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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
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.)
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.
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.
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.
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HCG (bHCG) is a hormone that is released by the placenta to stop ovarian action (Releasing eggs) as well as preventing the mothers immune system from attacking what is essentially a foreign invader.
around 20% of it is excreted in Urine, which is why if a pee test shows the presence, then a blood test is needed to accurately determine levels.
HCG is also produced by some cancers.
HCG (bHCG) is a hormone that is released by the placenta to stop ovarian action (Releasing eggs) as well as preventing the mothers immune system from attacking what is essentially a foreign invader.
around 20% of it is excreted in Urine, which is why if a pee test shows the presence, then a blood test is needed to accurately determine levels.
HCG is also produced by some cancers.
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