A fetus can not go either way. The nipples are there becuase men have all of the physical parts required to lactate. Some medication, particulry those used to treat psych disorders can cause men to lactate. I have seen it in person. We still make fun of him. Basicly it is hormones that cause women to grow lager breasts or spur the glands that produce milk in gear. Really no evolutionary reason for men to be prefered without them since it does not hinder use and lets face it a man with out nipples would look weird.