The gemara 30b cites a braisa that says that a child is taught כל התורה כולה while in utero and is made to forget it right before he leaves. It isn't clear whether this is only for a male child or even for a female (since she isn't obligated in talmud torah, it is possible that she wouldn't need to be taught it). The Taz writes in Hilchos Mila that the reason we have a sholom zachor after the birth of a boy is to mourn the lost torah. Being that the minhag is not to have a sholom bas (or at least there is no minhag to have a sholom bas), it would imply that the Taz understands that the teaching of kol hatorah kula in utero, is limited to boys and not girls.
However, Rashi 31a clearly understands that the Braisa which speaks about the learning of the entire torah in utero applies to girls as well. Rashi (d.h. derech) is speaking about a girl who is assumed by the gemara to be facing up down inside the uterus and must turn around before birth (because the gemara understands that girls are born facing up), supports this notion from the braisa which says ראשו בין ברכיו, implying that a babies including girls inside the uterus are facing down. Rashi seems to understand that the braisa is not limited to boys, but applies to girls also. Based on this, the minhag to have a sholom zachor and not a sholom bas indicates that the source for this seudah must be for some other reason other than mourning over the torah as the Taz suggests.