Normally, a pregnant woman's blood pressure is slightly lower than normal during the second trimester and then gradually returns to normal throughout the remainder of her pregnancy. However, in 10% of pregnant women, blood pressure begins to increase to abnormally high levels (hypertension) sometime after 20 weeks of pregnancy.⁸ This is occasionally referred to as pregnancy-induced hypertension. Less commonly, this change in blood pressure develops during the first days after childbirth.

At the first sign of high blood pressure during pregnancy, your health professional cannot predict whether it will remain mild, become severe, or turn out to be an early sign of preeclampsia. If you are developing preeclampsia, your urine test (urine screen) will probably show increased protein levels before long. This sign that your kidneys are being affected by the condition doesn't develop right away.

If you aren't certain that you had normal blood pressure before pregnancy, it is possible that you have preexisting chronic high blood pressure. If so, your blood pressure may remain high after your pregnancy.

High blood pressure that develops during pregnancy

High blood pressure that develops before the 20th week of pregnancy is usually a sign of ongoing (chronic) high blood pressure or short-term (transient) high blood pressure. On rare occasions, it is an early sign of preeclampsia.

High blood pressure that occurs after midpregnancy is more likely to be a sign that you are developing preeclampsia. This can be anytime after the 20th week.⁹

Chronic high blood pressure and pregnancy

Women with chronic high blood pressure (hypertension) who become pregnant normally have a drop in blood pressure during the first two trimesters. During the late second or in the third trimester, however, blood pressure returns to higher-than-normal levels. Following delivery, their blood pressure remains high. For more information, see the topic High Blood Pressure (Hypertension).

Chronic high blood pressure increases your risk of preeclampsia during pregnancy. Of women with chronic high blood pressure who become pregnant, about 1 in 4 (25%) develop preeclampsia during pregnancy.⁹ (Of all pregnancies, only about 5% of women develop preeclampsia.⁵)

Most women with chronic high blood pressure who are otherwise healthy have a low risk for other cardiovascular problems during pregnancy.

Preeclampsia

Preeclampsia affects your blood pressure, placenta, liver, blood, kidneys, and brain. Preeclampsia can be mild or severe, and it may get worse gradually or rapidly. Both you and your fetus can potentially suffer life-threatening problems involving the following:⁹

• Blood pressure. Blood volume doesn't increase as much as it should during pregnancy. This can affect fetal growth and well-being. The blood vessels also increase their resistance against blood flow (vasospasm), increasing blood pressure.
• Placenta. The blood vessels of the placenta don't grow deep into the uterus as they should, nor do they widen as they normally would. This makes them unable to provide normal blood flow to the fetus.
• Liver. Impaired blood circulation to the liver can cause liver damage. Liver impairment is related to the life-threatening HELLP syndrome, which requires emergency medical treatment.
• Kidneys. During a normal pregnancy, kidney function increases by up to 50%.¹⁰ When affected by preeclampsia, kidney function is usually higher than before pregnancy but not as high as necessary for a healthy pregnancy. This is called mild renal insufficiency.
• Brain. Vision impairment, persistent headaches, and seizures (eclampsia) can develop, probably in relation to reduced blood flow to or within the brain. Less than 1% of women with preeclampsia suffer one or more seizures.¹¹ Eclampsia can lead to maternal coma and fetal and maternal death. This is why women with preeclampsia are often given medicine to prevent eclampsia.
• Blood. Low platelet levels in the blood are common with preeclampsia. In rare cases, a potentially life-threatening blood-clotting and bleeding problem develops along with severe preeclampsia.¹ This condition is called disseminated intravascular coagulation (DIC). After delivery, DIC goes away. In the meantime, you may be given a medicine (clotting factor), blood transfusion, or platelet transfusion.