While most mothers who are nursing a toddler stop breastfeeding if they find out they are pregnant, it is an individual choice to decide whether to keep breastfeeding during the pregnancy.
It is not unsafe for the unborn child if you continue to breastfeed an older child during this time. But, if you are having some problems in your pregnancy such as uterine pain or bleeding, a history of preterm labor or problems gaining weight during pregnancy, your doctor may advise you to wean.
Your child also may decide to wean on his or her own because pregnancy changes the amount and flavor of your milk.
Some women also choose to wean at this time because they have nipple soreness caused by pregnancy hormones, are nauseous, or find that their growing stomachs make breastfeeding uncomfortable.