When I first heard of Githyanki being oviparous and having Crèches on the prime material plane, I must admit that I added it to my AD&D(2e) home campaign canon immediately. It didn't conflict with earlier material and made sense from the perspective that time doesn't pass normally on the Astral. This makes mammal-style pregnancy quite complicated indeed. According to Lae'zel, the Githyanki also choose who is becoming a mother (and probably fathers too) which would likely mean giving some of their best fighters, mages and other castes and classes several months off and producing less offspring since it has to fit a belly instead of having a whole hatchery to grow eggs. As for the size of the eggs, I guess they grow with the fetus which would also fit the iron bands having that organic look, as it is then magically grown along with the egg. Losing features like breasts and bellybuttons could easily be done by magic. Doing it through selection/evolution would take aeons and shift the focus from selecting their best to selecting those for who these features become vestigial. From how I understand the Githyanki, I can also easily see how keeping these mammalian features would be a matter of pride and distinction from mind flayers.
