In the Medical Career Guide it says
"Sims that reach the World Renowned Surgeon job will find they are able to determine the gender of a baby, and see if it's a boy or girl. It's not that useful however, because once Mom is pregnant the baby's gender is locked and you can't use fruit to change it."
This isn't actually true, the baby's gender can indeed be changed with fruit after the mother is visibly pregnant, the act of determining the gender is what locks it, there is a method of getting mixed gender twins or triplets that relies on these two facts. I didn't discover this method myself, I read it in someone's legacy blog and they got it from someone else and so on, but I have used it on multiple occasions and it works. What you do is avoid apples or watermelons entirely until the mother is visibly pregnant, then you get a level 5 or higher medical professional Sim to determine the gender (you can do this with uncontrolled medical Sims by having the mother ask them to determine the gender, but you have to watch for the pink or blue baby in the speech bubble because the game won't actually tell you), this locks the gender of the first baby to be born, however if you know your Sim is likely to have a multiple birth, you can now change the gender(s) of the second (and third) babies to be the opposite gender by eating the appropriate fruit.
How much more useful that makes the skill is of course dependent on how much you'd like to have boy / girl twins, but I thought I'd pass the information along.
