Blood Pressure & Hypertension
Kelsey McLaughlin, PhD
Measuring blood pressure is a standard clinical test for pregnant women. However, the terms 'blood pressure' and 'hypertension' are not very well explained or understood by most people.
Keep reading to learn more about what these terms mean and why blood pressure is an important test for pregnant women.
What is blood pressure, exactly?
Blood pressure is the physical pressure that the blood is exerting on the walls of your blood vessels.
Let's think back to our discussion regarding the maternal cardiovascular system first - the overall goal of the cardiovascular system is to make sure that all the tissues and organs of the body are receiving proper amounts of blood properly; this is done by maintaining a good blood pressure level. In cases where blood pressure is too high, blood vessels can be damaged, especially the delicate little capillaries. Conversely, if blood pressure is too low, tissue and organs may not receive proper blood flow. This is essentially a Goldilocks situation - the body doesn't want blood pressure to fall too low (hypotension) or rise too high (hypertension). Maintaining proper blood pressure is critical.
The heart beats in a pattern that is called the cardiac cycle - one cycle is the time from one heart beat to the next. Although there are many complicated actions happening at the molecular levels throughout the cardiac cycle, it is essentially divided into two basic phases: systole (period when the heart is contracting, which pushes blood out of the heart) and diastole (period when heart is relaxing to fill back up with blood to pump for the next beat).
When blood pressure is measured, you hear it reported as a higher number 'over' a lower number; these numbers represent the blood pressure in the arteries during systole and diastole.
Blood pressure is higher in the systole phase of the cardiac cycle because there is great force and pressure generated when the heart beats. Blood pressure is lower in diastole because the heart is resting between beats.
Blood pressure in non-pregnant adults
The definition of 'normal' blood pressure is complicated. For many years, high blood pressure in adults was defined as pressure over 140 mmHg during systole and/or over 90 mmHg during diastole. However, new guidelines from the American Heart Association have redefined high blood pressure as pressure in non-pregnant adults over 130 mmHg during systole and/or over 80 mmHg during diastole.
Blood pressure can be measured through manual or automatic machines. The old-school manual methods of measuring blood pressure used a column filled with mercury to measure blood pressure, and a doctor or nurse would listen to the sounds of the blood flow in your arm with a stethoscope. These sounds are refereed to as Korotkoff sounds. Automatic methods of measuring blood pressure depend on the pressure the blood in the arm exerts against the arm cuff at different pressures.
Side note - historically, devices used to measure blood pressure were glass columns filled with mercury, referred to on the periodic table as Hg. mmHg is the unit for blood pressure, referring to millimeters of mercury. This refers to the pressure exerted on a column of mercury that’s one millimetre high. Canada and many countries are phasing out the use of mercury devices due to environmental toxicity concerns.
Side note - blood pressure machines in drug or grocery stores may not be as reliable as there can be variation between different automatic machines. Always have your blood pressure double-checked by a medical professional.
Blood pressure patterns in normal pregnancy
As discussed in our maternal cardiovascular system article, the maternal heart and blood vessels undergo significant change to adapt to pregnancy and keep blood pressure at normal levels. The maternal blood vessels relax early in pregnancy to help keep blood pressure at normal levels during healthy pregnancy.
Healthy pregnant women typically experience a small decline in blood pressure of approximately 10 mmHg during the first trimester of pregnancy, compared to their pre-pregnancy blood pressure. Systolic blood pressure remains slightly lower until delivery, when it rises back to pre-pregnancy levels. Following the initial drop early in pregnancy, diastolic blood pressure then slowly rises to normal levels throughout pregnancy, normalizing by the third trimester.
The average women with a normal pregnancy will see their blood pressure return to pre-pregnancy levels by term, and remain stable for at least one year postpartum.
High blood pressure in pregnancy
Approximately 10% of pregnant women have high blood pressure, clinically known as hypertension. Hypertension is diagnosed in pregnant women when blood pressure is above 140 mmHg systolic and/or 80 mmHg diastolic. Women are classified as having severe hypertension when blood pressure is above 160 mmHg systolic and/or 110 mmHg diastolic.
Hypertension in pregnancy is typically categorized into 3 pregnancy disorders:
Chronic hypertension - pre-exisiting high blood pressure before pregnancy that continues into pregnancy
Gestational hypertension - the new development of hypertension after 20 weeks of pregnancy
Preeclampsia - the new development of hypertension after 20 weeks of pregnancy with signs of organ injury
Although the exact causes of these disease are not well know, scientists hypothesize that the new development of hypertension in pregnancy is due to improper adaptation of the maternal cardiovascular system to the huge demands of pregnancy. Lack of adaptation, either in the heart or blood vessels, can then cause imbalance in the cardiovascular system, leading to a rise in blood pressure.
Does high blood pressure in pregnancy effect mom & baby's health?
If a woman develops hypertension in pregnancy, she is at higher risk of adverse pregnancy outcomes, compared to pregnant women with normal blood pressure.
Gestational hypertension is typically a more mild, less complicated hypertensive disorder of pregnancy than preeclampsia. However, gestational hypertension is associated with increased risk of adverse pregnancy outcomes, including preterm birth, compared to pregnant women with normal blood pressure.
Although less common than gestational hypertension, preeclampsia is a more severe disease that is associated with increased risk of significant adverse outcomes for both mom & baby, including placental abruption, preterm birth, and low birthweight.
Scientists are not quite sure if gestational hypertension and preeclampsia are different severities of same disease, as almost half of women with gestational hypertension will progress to develop preeclampsia. However, these disorders have unique risk factors and pregnancy outcomes, suggesting that they may be different diseases with different origins.
High blood pressure in pregnancy has a larger impact in developing countries with limited access to adequate medical care - hypertension in pregnancy is the second leading cause of maternal death worldwide.
There has been a huge push in the medical & research worlds in the last 5-10 years to figure out what happens to the maternal cardiovascular system during normal and complicated pregnancies. However, this is really a short period of time in the research world and there have not been too many changes to the clinical care of hypertensive pregnant women.
Wait...hasn't tons of money been invested in hypertension research?
There has been significant progress for the treatment of hypertension in non-pregnant vs pregnant women, due to a variety of scientific, social and political reasons. As nicely summarized in the Heart & Stroke 2018 Heart Report, women have historically been 'under-researched' and excluded from clinical trials.
As a result, there are currently few screening, treatment and clinical strategies specifically designed for pregnant women with hypertension.
But there is hope! Major funding agencies in Canada now require consideration of sex as part of the application process, with more funding available for research in women's health. There are teams of scientists, especially in Canada, Italy, United Kingdom and United States, who have made remarkable progress understanding the cause of hypertension in pregnancy and may be close to developing pregnancy-specific treatments and clinical strategies.
Mom's cardiovascular system undergoes large adaptations during pregnancy to maintain normal blood pressure levels. Approximately 10% of pregnant women have high blood pressure in pregnancy and are at higher risk of adverse pregnancy outcomes. Further research is required to develop pregnancy-specific screening, treatment and clinical strategies to maximize the health of mom & baby.
Every woman and every pregnancy is unique. Pregnant women should speak to their healthcare provider to ensure maternal and fetal safety. This article is meant to provide readers with current information and opinions. All medical and treatment decisions should be discussed with your healthcare provider.