High blood pressure during pregnancy poses the following risks:

Less blood flow to the placenta. If the placenta doesn't get enough blood, the fetus might receive less oxygen and fewer nutrients. This can lead to slow growth (intrauterine growth restriction), low birth weight or premature birth. Babies born early can have breathing problems, increased risk of infection and other complications.
Placental abruption. In this condition, the placenta separates from the inner wall of the uterus before delivery. Preeclampsia and high blood pressure increase the risk of placental abruption. Severe abruption can cause heavy bleeding, which can be life-threatening for you and your baby.
Intrauterine growth restriction. High blood pressure might result in slowed or decreased fetal growth.
Injury to other organs. Poorly controlled high blood pressure can result in injury to the brain, eyes, heart, lungs, kidneys, liver and other major organs. In severe cases, it can be life-threatening.
Premature delivery. Sometimes an early delivery is needed to prevent life-threatening complications from high blood pressure during pregnancy.
Future cardiovascular disease. Having preeclampsia might increase the risk of future heart and blood vessel (cardiovascular) disease. The risk of future cardiovascular disease is higher if you've had preeclampsia more than once. It's also higher if you've had a premature birth due to having high blood pressure during pregnancy.