You can continue to breastfeed when pregnant.
However, do consult your gynae about it. There are cases whereby premature births caused by cervix dilating too early, or signs of premature labour during pregnancy, or unusual bleeding caused by the contractions of the uterus from breastfeeding.
Some mothers may experience morning sickness and can be aggravated by nursing.
You can find out more about handling breastfeeding during pregnancy from here:
http://www.todaysparent.com/pregnancy/top-13-tips-for-breastfeeding-during-pregnancy-and-beyond/
Read more