For what it's worth, none of the grandmothers are actually 70 or over.
Nana is 61.
Mable is 66.
Minnie is 68
Ophelia, who has no family we know of, is 71.
Also
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has a list of women within these age ranges carrying pregnancies to term.
Yes they were nearly all though IVF and egg donation, but we have an IVF specialist practically in the family so that should be no problem. As for the egg the game is not short of fertile women who could donate if needed, but again this is a world where we have super sperm that is just as unrealistic as most of the elder women still being naturally fertile