Golfer – I heard (but I don’t have a source) that one is supposed to or at least can daven for at least one girl in order to be able to be mekayim the Mitzvah of pru u’rvu, but they should not daven for a specific child to be a girl (I guess this meant before 40 days, or even before conception – don’t daven that the NEXT child should be a girl.)
It’s interesting though – I know a family who only had one girl near the top of the family followed by many boys, and the mother was chalishing for another girl (which she finally has as her last child, ba”h). I wonder if she was allowed to daven to have another girl since she already had one.
I have heard that there is an opinion that you need 2 girls and 2 boys to be mekayim pru u’rvu, so maybe that would make it muttar.
Also, maybe the reason why it’s different when you are not davening about a specific child is that then by davening for a girl, you are not asking for a girl baby INSTEAD of a bad baby. Who said that you would be zoche to another baby at all? So it’s not instead of anything – it’s in addition. So maybe you would be allowed to do so in that case even if you were already mekayim pru u’rvu.