There are a few risk factors that cause blood pressure to increase with pregnancy. Firstly it's the woman's lifestyle. Unhealthy lifestyle choices may lead to high BP during pregnancy. Being overweight or obese and not staying active are the major risk factors. Secondly, it can increase with the type of pregnancy. During the first pregnancy, a woman is more likely to have high BP. Luckily, there's a lower chance of this in subsequent pregnancies. Carrying twins makes it more likely for a woman to develop hypertension. As the body is working hard to nourish more than 1 baby. Age is another factor. Pregnant women over the age of 35 are at a higher risk.