Unless the mother has a history of preterm labor and/or the nursing is causing contractions, cramps or discomfort there is no reason to wean the nursing baby.Just know that your body will feed the fetus first, nursing baby second and yourself last so you need to rest and drink lots of water and eat well. Your body may cease making milk or making as much milk, it may change the taste of the milk or maybe nothing will happen. As long as you and unborn baby are healthy and you and nursing child want to nurse then there is no problem. I did it with my first child when he was a toddler and I was pregnant with #2 and now I am pregnant with #3 and my second baby is 1 so she is still nursing. It hurts my nipples but I want to keep it up for her because she is in no way ready to wean. My son was older and he was ok with weaning at 20 months, this younger baby is just not there! I did have issues with preterm labor when I wsa pregnant with my second so I will probably have to cut back or wean her in my second trimester if not before. I want to nurse her as long as possible and I might tandem nurse her and the new baby if she wants to and I want to also.