@igazor I used to try to get different genetics from my sim babies by saving right before they were born, then quitting and reloading if I didn't like what I saw. I always got exactly the same sim or sims as a result; the only things that changed when I briefly aged one up in CAS were the name, hairstyle, and clothing (I know that the last two are random regardless). Granted, I haven't done that in a while without using MC, but I did try a number of times a few years ago without any success. Also, on a few more recent occasions, I've quit without saving for other reasons right after a birth, and the baby grows up to be the same sim I saw before I quit, despite the fact that he or she had to be born a second time in-game. Of course, I'm not playing with SP, so that may make the difference here.
@Naito_Shade I have MC and MC Cheats, and the options for pregnancy are under sim>basic>baby. (You can click on the pregnant sim directly, or if she's already in labor, on city hall and select her after picking which command you want to run.) There are four options, including "baby gender" (pick it or set it to random) and "gender offset" (tilt your chances one way or the other). But the only ones I use are "set multiple," which sets the chance rather than picking a number of babies, and "reroll genetic seed value," which claims to "alter the random values used during inheritance." Using this, I see changes in gender (if I haven't forced it one way or the other), hair and eye color, facial features, weight, etc. The number of babies born can change as well—once, I wanted twins, and I got a single, then triplets, then a single again, before the game decided to cooperate. Running the command even changes which traits the baby receives if you don't get to pick them yourself (definitely—I did this a week ago); otherwise, this outcome is set the moment the mother-to-be realizes she's pregnant (at least when I tested it a few years ago).