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
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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