Has anyone read "Clan of the Cave Bear" ?
Great story. Film is rubbish, but novel is fantastic.
A young human girl is rescued by a tribe of Neanderthals. Their inital plan is to leave her, but she had been attacked by some kind of lion - leaving deep symmetrical cuts on her thigh. The lion was a powerful spirit, by their reckoning, and they reason that the gods/spirits/whatever had sent her to them to look after .. despite her humanity.
They tolerate her for the most part. As she matures, they describe her as having long, flowing blonde hair. Bright blue eyes. Firm breasts and an ample rear. Plus being ridiculously tall (remember, they're Neanderthals - 5'6" is tall for them).
One of the characters laments that, despite all her intelligence and wit, she is SO DAMN UGLY she will never find a mate.
Now, for these Neanderthals, they have never quite understood procreation. Males could mate with any female at pretty much any time. You might have a wife, for example, but she could be commanded by any other male to ... shall we say, surrender herself?
And this happened, frequently.
They believed that you had a spirit inside yourself, a totem. This totem would war with other totems around it (your own spirit would fight another). When the women's totem fought with a man's, she would be wounded - hence, menstruation. It would be wounded and the blood would flow.
When a man's totem had won a battle easily, a woman's totem was simply dead. Dead things don't bleed. His powerful totem had won, and it was at this time that she would become with child.
Ayla, the human child, possessed of intelligence miles ahead of even the brightest Neanderthal, started to ponder. She had noticed cause and effect - she noticed that the females mated regularly were the ones more likely to become pregnant. Eventually, the brute of the tribe took an interest in her (most definitely not romantic - he did it because he frankly enjoyed rape) and, after a while, she fell pregnant. One of the key scenes of the story is that a Neanderthal female had had a seriously messed up baby. Ayla figured out that the female was, although 'bonded' with one male, had been 'servicing' another - her brother.
Look, the point of what I'm trying to say is thus: primitive peoples were primitive, not stupid. Generation upon generation upon generation of pregnancies .. daughters becoming mothers becoming grandmothers ... they would have seen this. They would have known that when their male comes for them, it means 'this'. When she stops bleeding, it means 'this'. When her belly swells, it means 'this'. When the pain comes, and the birth begins, it means 'this'.
Give your ancestors some goddamn credit.
NB: I find evolutionary biology frightfully interesting. Don't get me started.
EDIT: Uh oh you got me started. Once humanity developed reliable forms of communication - something beyond monkey squeals and grunts - probably the first thing they talked about was sex. The second thing was probably how to kill the neighbouring group of humans. So they could have more sex.
EDIT EDIT: Something I'd just like to throw in for the hell of it - Caesarians are being performed with growing frequency. Babies are being born bigger and bigger. I wonder ... are we allwoing this to continue too far? Should Caesarians be restricted to only life-and-death situations BY LAW? Developed countries are likely to soon evolve into a species that cannot physically give birth to themselves without extreme medical intervention. As a closet nutjob survivalist, it freaks the shit out of me. Imagine a world where even 10% of women die in childbirth, let alone 50% due to gargantuan babies.