Blood Pressure in Pregnancy

I was recently asked some questions regarding blood pressure during pregnancy by my colleagues at Physiquality. In preparing material, I wrote this blog, which includes very basic clinical information and explanations about this topic.

What is the normal range for blood pressure for pregnant women? What readings would fall under high blood pressure?

Blood Pressure (BP) in pregnancy is a complex topic.

First, we need to know: What are the classifications of BP?

The chart below is from the evidence-based 2014 Guidelines of the American Heart Association and the National Heart, Blood and Lung Institute of the NIH. The first number is Systolic BP or during the heart beat. The second number is Diastolic BP or between beats. These numbers are relevant for women of childbearing age.

  • Normal                        <120 mm Hg and <80 mm Hg
  • Pre-hypertensive          120-139 or 80-89
  • High BP Stage 1            140-159 or 90-95
  • High BP Stage 2            ≥ 160 or ≥ 10

Why does low BP (hypotension) occur in a healthy pregnancy?

A healthy pregnant woman with normal BP and no cardiovascular or immune system complications, will have pregnancy BP lower than her non-pregnant BP due to increased progesterone relaxing her vasculature. To create the placental and uterine blood flow, blood volume (V) expands rapidly increasing by around 40%, but stroke volume increases less, so beats per minute (pulse) may increase, systolic BP may drop 5 mm Hg and diastolic may drop 10-15 mm Hg. If V is not adequate with this relaxed vasculature, BP may drop even lower. To help maintain normal BP, women are encouraged to drink sufficient water (about 8 glasses/day) and eat enough protein (about 20-25% of daily intake) to produce a blood volume that will sustain an adequate BP. Other more severe conditions – often genetic – may also be relevant, such as postural orthostatic tachycardia syndrome.

Other causes of hypotension include lying still on the back with legs extended for long periods of time after the first trimester. The weight of the uterus impinges on the vena cava returning blood to the heart, thus reducing BP and blood flow to the uterus and placenta. Also, standing for long periods of time with a minimum of motion, as happens with teachers, cashiers, line workers and nurses in the second half of pregnancy when increasing relaxin and elastin cause further softening of vasculature. This results in difficulty returning blood from the lower limbs and reducing blood flow to the uterus and placenta.

What are hypertensive disorders of pregnancy?

According to the National High Blood Pressure Education Working Group on High Blood Pressure in Pregnancy, hypertensive disorders of pregnancy are presently classified into four categories:

  • Chronic hypertension (pre-existing)
  • Preeclampsia-eclampsia
  • Preeclampsia superimposed on chronic hypertension
  • Gestational hypertension

[The Society of Obstetricians and Gynecologists of Canada categorize these disorders as pre-existing or gestational, with the addition of preeclampsia to either category.]

Chronic hypertension is BP >140/90 prior to pregnancy or before 20 weeks. New onset of high BP after 20 weeks may indicate preeclampsia (PE), which requires further consideration. PE involves other symptoms and organs. It occurs in about 5% of all pregnancies, 10% of first pregnancies and 20-25% of women with a history of chronic hypertension. It is a serious disorder and major cause of adverse maternal and fetal outcomes, including strokes, seizures and restricted fetal growth and development.

The underlying pathogenesis of preeclampsia-ecclampsia is not yet fully understood, but is a fundamental dysfunction of the placenta leading to endothelial dysfunction and vasospasm. Possible causes include pre-existing endothelial dysfunction, metabolic dysfunction, auto-immune responses and infection. It is likely that the placenta is affected very early on, during implantation, trophoblast invasion of the uterus and opening of the spiral arteries to form the blood pool on the maternal side of the placental circulation.

Gestational hypertension is the onset of BP >140/90 after 20 weeks without other features of preeclampsia. About 1/3 of these women develop preeclampsia. Gestational hypertension is highly associated with hypertensive disorders later in life. Diabetes can also be a factor associated with hypertension.

Whenever a woman has elevated BP in pregnancy, she needs to be evaluated and have a follow up course of observation and treatment. At its most severe, a hypertensive disorder can affect all the body’s organs and systems, and can be fatal.

What can pregnant women do (diet, exercise, healthy habits) to keep their blood pressure within a normal range?

Some risk factors for hypotension or hypertensive disorders of pregnancy are inherited, others are a consequence of behavior, and many are a combination.

What can a woman do before pregnancy?

Because the events that pre-dispose a woman to hypertensive disorders may occur before she knows she is pregnant, some efforts at prevention may be helpful in the six months to a year prior to pregnancy. Preparing for the implantation period by maintaining optimal health and fitness is likely the most helpful behavior. Cardiovascular or aerobic fitness, which prevents or reduces the severity of endothelial dysfunction is highly valuable. An adequate daily nutrient intake along with sufficient water, and maintaining a BMI <25 are important factors. Women with elevated blood pressure should discuss with their care provider the balance of sodium and potassium intake, along with the total allowable amounts.

Avoiding infections or illness around the time of conception may be a factor. Hypertensive disorders are mediated by inflammation. Unfortunately, another factor may be the maternal immune response to the fetal DNA. This may also be dependent on combined maternal/paternal immune system responses.

What can a woman do once she is pregnant?

Once a woman is pregnant, maintaining optimal health and fitness continue to be important. Even if there are pre-disposing factors for disorders, she may be able to reduce the severity by staying fit, well nourished and well rested. A balanced and colorful diet, along with avoidance of alcohol, drugs and unsafe behaviors are critical.

The ability to achieve the Relaxation Response, meditation, deep breathing and hypnosis are valuable for acute BP reduction. Each of these skills is mediated by the parasympathetic nervous system response (or alpha brain rhythm) and mitigates the effects of stress on a temporary basis. Cardiovascular or aerobic fitness is effective for long-term BP reduction, as well as cardiovascular health.

Resting on the left side maximizes circulation and – if possible – finding 15 or 20 minutes to rest this way during the day is beneficial, especially if a woman’s work involves standing for long periods of time. Avoiding lying on the back or standing for long periods of time is advisable. Finding a community of support for having a healthy pregnancy can be a great asset, as well.

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