I go with Pam on this one, as many babies as my sims have had, and I love for them to have babies, when I go to pick the gender and remember to use the fruit, if I don't have her eat at least nine, then I do not get what I want. And the nine is pushing it, it seems that the magic number is 10 of whatever fruit is needed.
One of the reasons I stay with the after the twirl and bump is that I have had her eat six or seven fruits before the bump and then four or three after and not gotten the one I wanted. It could just be luck of the drawl, but for me, I just go with better safe than sorry and have her eat it after she does the twirl, and eat 10 fruits; five the day she twirls and five the next day. I don't think I have even not gotten the one I wanted doing it that way.
Others will have different results, so it is trial and error I think for each person, on what they think works.