The biggest effect on family size is that there are clone lines. The Jessicas, for example, start a cohort of sixteen, on schedule, every eleven years. They prefer natural birth, with the previous cohort currently twenty-two years old carrying the children. But if there’s a problem with that, or if the individuals decide they don’t want to do it that way, they rent gestators and go ahead with the next generation. The clone line works as a huge, extended family and mostly do their best for each other the way family does.
Because they plan it out in detail and have some economies of scale that individual parents don’t, the clone lines can do extensive micro-edits of their genome before every generation - something was discovered that we can correct, we found a way to increase life expectancy, something else is a direction we want to go in, the next generation ought to be an inch taller or shorter, etc… and sometimes differences more serious than that.
In addition to just bringing up clone offspring though, a lot of clones get married to individuals as adults, so these micro-edits slosh around and spread in the general population. By the time of our story, just about everybody in the city is zero-gravity tolerant, highly cancer resistant, can take easily ten times the radiation we’d think of as borderline-dangerous, has life expectancy 20% to 50% longer than us, and has a few other traits that they’ve come to think of as “normal.” And some of the clone lines, where these traits haven’t been crossed back into the general public, are more extreme - like being able to live about three hundred years, or having tetrachromat vision that picks up near-infrared, or having an immune system that can laugh at every pathogen so far encountered, etc.
This idea of “normal” has become rooted and the lack of these traits are considered severe problems that any responsible doctor, clinic, etc would correct before implantation. Finding out someone isn’t zeegee resistant or isn’t cancer-resistant makes them think, my god what kind of barbarians were her parents, if they didn’t seek out basic medical care when having a child?!