The Machaber in OC Siman 470 writes that there is an opinion that holds a woman that is a bachorah should fast on Taanis Bechoros. The Rema adds there however that this is not the minhag. The Rema however in Sif Beis does say that when a Father is himself a bachor and this couple has a son that is also a bachor but still a katan that the Mother should fast in place of her son (Since the Father is already fasting because of himself.) Why is it that in this case the Rema says a woman should fast for her son (when the Father is unable to do so) but not for herself?
(The M"B in Sif-Katan 9 does write that poskim say the woman doesn't have to fast in this case as well. However I'm asking according to the Rema himself paskens.)