A few years after the first crop of men (for their adventurous spunk) and women (for popular appeal, lower mass, and because they quite rightly insisted) began living together in space–really actually living in space–eventually nature took its ageless course.
As had been known from the earliest NASA and Soviet missions, everything “works” in space. But there’s a problem. On Earth, embryos develop under an effectively uniform acceleration, allowing them to develop such necessities as skeletons and brain tissue. The first (official) pregnancy in zero-gee ended in tears all around–and some rather graphic footnotes in ontogenic texts. But, lesson learned, fetuses need gravity.
That’s sortof a problem if you’re living in free-fall.
Under the circumstances, the nascent U.N. issued a surprisingly uncontroversial mandate barring more than one week of a pregnancy to be below 0.12g. This seemingly arbitrary cutoff allowed colonies on the larger moons to be sustained (but of course, citizens were encouraged to return to Earth for child-rearing). In those early years, no one really knew exactly how much gravity was required, although it seemed to fall in a range. Martian children seemed normal. Toddlers from Ganymede had trouble breathing.