Praise be to Allaah.
The question of breastfeeding a child during pregnancy should be referred to a capable and trustworthy doctor. If he says that breastfeeding the child will not harm the foetus or the nursing infant, then there is no reason not to do it and it is permissible.
But if it will harm your child or the foetus, then do not breastfeed him. And Allaah knows best.
Sheikh Muhammed Salih Al-Munajjid