As above. Also, how would the answer to this title question explain babies that are born premature, or babies that are born so late that the labour has to be induced?
Thank you in advance! I have wondered this for a while!
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It’s really hard to say. Human reproduction is, to put it lightly, a giant hot mess. Much of it doesn’t make sense, and there’s always more factors at play than we realize.
There’s a lot of strict control over what research can be done on humans, particularly on fetuses and surrounding pregnancy. Much of what we have learned about early development is from IVF. I doubt we will ever really know.
There are some facts we do know, although they don’t really form a cohesive picture at this time.
Generally speaking in human pregnancy, the mother has almost no control. This is unique among mammalian pregnancy, most mother’s have at least some control over what the fetus can take, some can even terminate pregnancies that they feel won’t be successful. Humans can’t. It’s actually a reason humans menstruate, to ‘filter out’ weak embryos, and to preemptively build a lining that can prevent the placenta from breeching maternal blood vessels. So by that logic, it’s unlikely that a mother’s body has much say in when labor starts. Except for certain situations such as extreme dehydration, food poisoning, infections of the reproductive or urinary tract.
There’s some indication that as the placenta begins to wear out, it emits certain hormones that initiate labor. As others have stated, lung development seems to play a role. It’s believed that sex, orgasm, and nipple stimulation might trigger labor, and people at risk for preterm labor are recommended to avoid it. However, for people not at risk for preterm labor, it is generally considered a myth that those activities will trigger labor. Certain hormones produced in response to those activities are used in labor induction so maybe they could play a role. But so does natural light, blue light, and melatonin.
Typically, the beginning of labor is marked by loss of the mucus plug/”bloody show”, and/or rupture of the amniotic sac/”water breaking”. It’s possible to have both of these events without going into labor. It’s also possible for a mucus plug to regrow, and for a ruptured amniotic sac to heal. It’s possible for a cervix to dilate without a single contraction. In fact, I delivered my premature stillborn daughter without contractions, and the doctor only induced mild contractions to deliver the placenta.
But what about twins and triplets? Nearly all triplets are born early, and twins are frequently premature. It couldn’t be lung development or an old placenta. It’s thought that a contributing factor is excessive uterine distension. Literally there’s no more room to grow.
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