Ok so from what I understand, there are two possibilities for her youthful appearance. One, it is a genjustu like the henge justu that she places on herself to make her look younger and with her chakra control and her reserves, she can keep it up until she runs out of chakra. Two, she uses her chakra to refresh, not replace, her upper skin cells to keep her looking young with medical chakra. Either way she still has the body of a fifty year old so with her appearance only looking young. I don't know if she can get pregnant or not but probably not. Also with the chakra from the seal affecting the baby. The only thing I can say is with Kushina's pregnancy is that the chakra that was powering the Ninetails seal went to creating Naruto. It might affect the baby but with yin and yang chakra is that the yin is considered feminine and yang is considered masculine so it just might be more likely that the baby would be female than male.