I was thinking about the generation ship in my sci fi story and I am working on a book about specifically the generation ship. I thought this thing about pregnancy deserved a topic here so here I go. Starting with a few weeks before mating, half of the people on the generation ship will stay diurnal while the other half transition over those few weeks from diurnal to nocturnal. The aliens on the generation ship have already mastered this but this is partly due to how their sleep schedule is determined by photosensitive proteins and genetics. This all happens in the primary sleeping quarters. The secondary ones are reserved for mating. Now Diurnal-Diurnal couples will most likely have Diurnal children but there is a possibility that their children will be Nocturnal instead. And very little emotional upset occurs(at least that caused by sleep schedule) Nocturnal-Nocturnal couples are similar in the fact that very little emotional upset occurs. Diurnal-Nocturnal couples on the other hand can only have sex during the 2 twilights(morning and evening) because those are the only times when both the diurnal humans and the nocturnal humans are awake. When the time for mating comes, each M-F pair will be selected randomly to increase genetic diversity. Once those pairs are selected there can't be any deviation from that for a week(fertile window). Now of course this could cause issues like for example there is an equal chance that a woman will be in her fertile window and that a woman will be on her period. But anyway, after that 1 week everyone goes back to sleeping in the primary sleeping quarters. 2 weeks later all the women that aren't on their period head to the medical bay for pregnancy tests. Those that come up negative are asked to come back in a week if they still have a missed period. If their period comes within that 1 week, it is a true negative. If however the test comes up positive or a transvaginal ultrasound shows an embryo, that negative was false. Of course there is a grey area. Very rare but possible is there being no sign of pregnancy via either pregnancy test or symptoms(known as cryptic pregnancy) until labor. And then those women with cryptic pregnancy could easily be mistaken for those people with appendicitis. A few months later they are tested again but this time to see if the baby has any external defects. If the baby does have external defects, there are 2 options. 1 option is for the baby to adapt to the defect(like for example hand walking if a leg is missing). The other option is for the aliens to fix the external defect which could be as simple as cutting an extra limb off and skin grafting or as complicated as forming a new limb for several years. After birth, all babies are tested for internal defects and if there are internal defects, they are diverted right to surgery to fix it. They have all kinds of machinery to sustain homeostasis during surgery. Do you think that this is reasonable or not and if not, why not? I certainly think it is reasonable.