This depends a lot on what happens during the pregnancy right at the end.
There are many reasons for a pregnancy to be called high risk. Some of these have to do with the health of the mother (high blood pressure
, diabetes, heart disease, etc) and some with the health of the baby (for example a baby who is not growing well). Initially when your OB GYN doctor
tells you that this is a high risk pregnancy, this generally means that you will requiring more frequent office monitoring of the health of both mother and baby.
As the time approaches for delivery, a decision about caesarean section will have to be made. Most OB GYN doctor will prefer to facilitate a normal vaginal birth if possible. However, if the mother's health is very poor (for example if the blood pressure is very high) she may not be able to tolerate a prolonged labor and vaginal birth, and so a c section will be performed for the health of the mother. Similarly, if there are any signs that the baby cannot tolerate the labor (for example slowing down of the heart rate), this might also lead to a c section.
Talk to your OB GYN doctor if you have more questions.