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The reproductive systems of all reproductive-aged women are suddenly altered to make them capable of being bred with almost any animal species.
The change to every woman's reproductive system comes with a slow, tortuous slide into animal heat, during which women's bodies are also changed so that they no longer menstruate but will slide into another 3-4 period of nearly-uncontrollable animal lust instead.
Should women survive their first "heat" without allowing themselves to be knocked up by an animal, they will lose more and more of their free will with every subsequent estrus, until during that 3-4 period they are completely reduced in intelligence to that of an animal.
A woman who goes an entire year without allowing herself to be knocked up (and there are very few of them - mostly women who allow themselves to be locked up during estrus like literal bitches in heat by men they can trust), women will spontaneously generate a litter of animal young in their bellies, with another litter due every year afterwards should women not be impregnated by more conventional means again.