The Bottom Line

The cardiovascular system is the first major organ system to change during pregnancy and is one of the most important in ensuring the success of the health of both mother and baby.

A pregnant woman's blood vessels change in various ways, and blood volume, flow, and pressure, along with heart rate and cardiac output – all change significantly and almost immediately.

These cardiac changes can put stress on the mother's body. Therefore, regular prenatal appointments, awareness of personal and family medical history, and knowledge of risk factors, as well as signs and symptoms of potential complications can help prevent unforeseen cardiac concerns during pregnancy.

Note: Women who experience shortness of breath, extreme fatigue, dizzy spells, changes in their vision, or feeling faint, need to call their healthcare provider (HCP) right away or seek emergency care, especially if symptoms are accompanied by chest pain.

Postpartum: Pregnancy can cause adverse long-term cardiac changes in high risk women. It is critical for women with heart disease before pregnancy – or heart disease that develops during pregnancy – to continue follow up care with their provider. Learn more below.

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Background

The cardiovascular system undergoes significant structural and hemodynamic (blood flow) changes during pregnancy, as the heart has some of the highest metabolic demands in the body.

The purpose of these major changes is to carry enough oxygen and nutrients through the blood to supply both the mother’s organs and the developing fetus.

The main cardiovascular changes begin as early as 5 weeks, which include decreased blood pressure (overall), increased blood volume, increased heart rate, increased cardiac output, and slower blood return to the heart.

These changes can cause numerous symptoms such as swelling, dizziness, fatigue, varicose veins, increased risk of blood clots, and sweating.

Cardiac Stress

A pregnant woman’s heart and blood vessels undergo a remodeling – or “transformation” – which promotes proper fetal growth and development.

Remodeling is important for placental development, the prevention of high blood pressure, and the storage of blood in anticipation of future delivery.

Blood flow to all organs during pregnancy increases (except for the brain) to meet additional oxygen requirements. In normal, healthy pregnancies, the major arteries of the body handle changing blood volume and blood pressure very well, contracting and expanding as needed.

Genetics are suspected to play a large role in a woman’s ability to overcome the cardiovascular stresses of pregnancy –induced by this remodeling. 

Pregnant women should learn their family medical history and share it with their HCP.

Since women with preexisting cardiovascular conditions can become pregnant, and cardiac conditions can be suspected or diagnosed during pregnancy, HCPs regularly assess and monitor women for risk factors or indicators of potential future problems.

Therefore, it is very important for pregnant women to know and share their medical history with their HCP.

It is not uncommon for pregnancy to expose underlying or previously silent cardiac problems in women, which is why pregnancy has been called “nature’s stress test”.  For example, women may learn they have blood pressure problems for the first time during pregnancy.

Cardiac Output

One of the first cardiovascular changes during pregnancy is the increase in cardiac output – the amount of blood the heart can pump in one minute.

Cardiac output is the product of heart rate and stroke volume – the amount of blood ejected with each contraction.

During pregnancy, women experience an increase in heart rate that could result in an extra 14,000 to 28,000 beats per day.

TR Easterling, et al. (1990)

Women can experience a slight increase in resting heart rate (10 to 20 beats per minute) which could start as early as 4 weeks of pregnancy, continuing until term due to the body’s requirement for extra oxygen.  This increase in heart rate could result in an extra 14,000 to 28,000 beats per day.  Twin pregnancies may accelerate heart rate (and overall cardiac output) even further.

Due to the early dramatic rise in blood volume during pregnancy, stroke volume increases.  Since both heart rate and stroke volume increase, cardiac output therefore increases: the pregnant woman’s body has more blood (mostly plasma), more blood is pumped out with each heart contraction, and overall heart rate increases to carry more oxygen.

More than 50% of this change takes place before 8 weeks of pregnancy, but continues to rapidly increase until 20 to 24 weeks, before slowing down for the rest of pregnancy. The highest increase in cardiac output is seen during labor.

It is hypothesized that women who have faster and earlier increases in cardiac output during pregnancy could be at risk of gestational high blood pressure (hypertension) or preeclampsia, and that early heart rate and blood pressure may be key indicators of potential future risk.

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Blood Pressure

Although debated, a general agreement appears to be that – overall – blood pressure decreases during pregnancy. However, blood pressure fluctuations can be quite variable per woman, and blood pressure may actually increase during pregnancy for certain women, based on weight, nutrition, genetics, and cardiac history.

HCPs will check a pregnant woman's blood pressure at every appointment. Some women (high-risk or pre-existing condition) may need to monitor their blood pressure at home as well.

Some researchers report blood pressure drops as early as 6 to 7 weeks of pregnancy, likely due to the increased circulation to the fetus and placenta, which is a likely contributor to feelings of fatigue, exhaustion, and dizziness in early pregnancy. The largest decrease has been reported to occur between 16 and 29 weeks of pregnancy, with a return closer to normal near term.

Further, a decrease in the resistance of the circulatory system that is used to create blood pressure occurs as early as 8 weeks of pregnancy. This triggers the kidneys to retain both sodium and fluid, leading to swelling.

Since blood is slower to return to the heart, pregnant women are also at an increased risk of deep vein thrombosis, varicosities, and varicose veins in the lower limbs/groin.

The growing uterus adds to this problem by obstructing the large veins in the pelvic region which further traps blood and fluid in the lower body. This obstruction has been observed in almost 90% of women who were studied lying flat on their back.

Women who lie on their backs or stand up too quickly after being in a certain position for an extended period can experience light-headedness, dizziness, black spots in their vision, nausea, and potentially even fall or faint. This is known as supine hypotensive syndrome of pregnancy.

Supine hypotension (low blood pressure) syndrome of pregnancy.
open.osmosis.org. CC BY-SA 4.0

This can be remedied almost immediately when women lie on their left sides, as this position takes the pressure off the pelvic veins.

Women should also aim to stay hydrated, change positions slowly, and avoid lying flat on their backs for an extended period.

Lying flat on the back in late pregnancy can decrease blood flow to the uterus and placenta.

An MRI study published in January 2021 highlighted blood flow changes that occur when women in their third trimesters lie flat on their backs instead of on their sides.

Maternal supine position in late pregnancy was associated with a 6.2% decrease in oxygen transfer across the placenta and an average 11% reduction in fetal umbilical venous blood flow. Further, there was a 23.7% reduction in total arterial blood flow to the uterus (Couper et al. 2021).

Note: Women should not panic if they wake up on their backs, and not all studies have found an increased risk of adverse outcomes from this position (Silver et al. 2019).

However, if concerned, women should call their health care provider with questions and monitor fetal movements. Learn more.

Additional Changes

As the uterus grows, the diaphragm gets pushed up, the rib cage expands, and the heart is shifted upward and to the left.

At term, there is enlargement in all four chambers and all valves of the heart as a result of the overall increased load on the cardiovascular system from the past 40 weeks.  Left ventricular mass increases by as much as 50%, peaking around the third trimester.

Note: Heart murmurs are frequently found among pregnant women, but how or why pregnancy causes murmurs is unknown. Unless a woman has a pre-existing cardiac condition, onset of a murmur is not expected to be a concern.

Postpartum

Although some cardiac output changes can be reversed as early as two weeks postpartum, most normal cardiovascular changes reverse by six months after delivery. However, not all studies indicate complete reversal by that time.

According to the American Heart Association (AHA) and American College of Cardiology (ACC), adverse pregnancy outcomes such as hypertensive disorders of pregnancy, preterm delivery, gestational diabetes, small-for-gestational-age delivery, placental abruption, and pregnancy loss increase a woman’s risk of developing additional cardiovascular disease (CVD) risk factors and of developing subsequent CVD.

According to AHA, "adopting a heart-healthy diet and increasing physical activity among women with adverse outcomes, starting in the postpartum setting and continuing across the life span, are important lifestyle interventions to decrease CVD risk". Breastfeeding may also reduce risk.

Women need to continue following up with their health care provider well after the initial standard six-week postpartum appointment to potentially prevent future cardiovascular concerns. Read more detail in the below section.

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Cardiac Concerns During and After Pregnancy

Cardiac Disease

While very uncommon, cardiovascular disease is one of the leading causes of maternal mortality in the United States (U.S.), Canada, United Kingdom (U.K.), Ireland, Australia, New Zealand, and the Netherlands, and these incidents are increasing.

In the U.S. and U.K., there are also significant racial disparities, as Black, American Indian/Alaska Native, and Hispanic women are significantly more likely to die from a pregnancy-related cause (in general) than White women. Black women are also more likely to experience high blood pressure.

Potential risk factors for possible cardiovascular-related mortality during pregnancy include:

  • Race (Black/Hispanic)

  • Age (risk increases as age increases)

  • Congenital heart disease (in the mother); CHD is currently the main cause of heart disease in 80% of pregnant women with heart disease

  • High Blood Pressure

  • Superimposed preeclampsia

  • Elevated body mass index

  • Diabetes

  • Smoking

  • Family history of cardiovascular disease

It is estimated that cardiovascular disease complicates up to 4% of pregnancies in the U.S. However, most of these conditions may go unnoticed or undetected until a problem occurs. Therefore the true prevalence may be hard to determine.

Photo by Ben Iwara on Unsplash

The most important aspect of this issue: It estimated that up to 68% of pregnancy-related deaths caused by cardiovascular conditions are preventable. This has led to the emerging interest of a new medical specialty – cardio-obstetrics ("pregnancy heart team").

Complicating prevention and fast diagnosis is that most early signs of cardiovascular problems mimic otherwise "normal" pregnancy-related signs and symptoms such as shortness of breath and leg swelling.

According to a November 2019 report from the U.K., researchers found that many of the women who died from cardiovascular disease had classic symptoms that would have been flagged in a non-pregnant person, but instead were attributed to symptoms of a normal pregnancy.

Therefore, although chest pain (in some cases), shortness of breath, leg swelling, dizziness, and fatigue are common in a normal pregnancy, they can mask cardiovascular conditions and can be the "first-line" signs to prevent a cardiac event.

According to ACOG, women who present with symptoms such as shortness of breath, chest pain, leg swelling (blood clot), or palpitations and known cardiovascular disease whether symptomatic or asymptomatic (or both) should be assessed for possible cardiac concerns. 

Although cardiac events do occur in pregnancy, heart attack – either during pregnancy or in the first year postpartum – is very rare.

It has been estimated that pregnancy itself due to the above noted physiological stressors increases heart attack risk 3- to 4-fold in women of reproductive age (James et al., 2006).

Despite this risk, heart attack during pregnancy is still exceedingly rare. An October 2020 study identified 913 instances of heart attack among more than 11 million recorded pregnancies in the National Inpatient Sample between 2003 and 2015. However, during this time frame, heart attack rate rose from around 0.006% to 0.011%; most of these deaths occurred in the postpartum period (Balgobin, et al. 2020).

High Blood Pressure

In November 2020, the American Heart Association (AHA) announced that approximately 80,000 pregnancies in 2018 were complicated by high blood pressure, nearly twice the amount of pregnancies affected in 2007. AHA noted that preventative care must start before pregnancy.

Pre‐pregnancy hypertension and hypertensive disorders of pregnancy (HDP; preeclampsia, eclampsia, gestational hypertension) are major health risks for women, both during pregnancy and in the postpartum.

High blood pressure can lead to many possible complications during pregnancy and is one of the first indicators of preeclampsia.

According to the ACC, risk factors for hypertensive disorders of pregnancy include maternal age >35 years, prior preeclampsia, chronic hypertension, pre-pregnancy diabetes and/or obesity, polycystic ovarian syndrome, prior stillbirth, multiple pregnancy, first pregnancy, chronic kidney disease, systemic lupus erythematosus, antiphospholipid antibody syndrome, and conception by assisted reproductive techniques.

Late Postpartum

A study published in May 2021 examined long‐term cardiovascular outcomes after pregnancy in 1,014 women with heart disease and a matched group of 2,028 women without heart disease. The study found that women with heart disease are at higher risk of late postpregnancy cardiovascular complications and new hypertension/diabetes compared with women without heart disease.

A very large retrospective cohort study published in February 2021 included women aged 12 to 49 years with a live, singleton birth between 2004 to 2016. The study found that pre‐pregnancy hypertension with superimposed ("on top of") HDP was associated with a 3.79‐fold increase in incident coronary heart disease, a 3.10‐fold increase in incident stroke, and a 2.21‐fold increase in all‐cause mortality within 5 years of delivery.

The presence of HDP alone or pre‐pregnancy hypertension alone was also associated with future cardiovascular outcomes up to 5 years post‐delivery.

The authors noted that extension of health care beyond the usual 6‐week postpartum period in women with a history of HDP or pre‐pregnancy hypertension is necessary for the prevention of future cardiac problems. It was also noted that women should tell their providers (even up to 5 years after delivery) if they had a history of HDP either before or during pregnancy.

Action

Women should learn their family history of cardiac disease prior to pregnancy, and share that information with their HCP during their first appointment.

Women with pre-existing cardiovascular disease may need special counseling prior to pregnancy, and require specialist care from a cardiologist during their pregnancy, together with their obstetrician/midwife.

Cardiologist: a specialist physician with specific training in the management, treatment, and prevention of disease related to the heart and blood vessels

Emergent Signs/Symptoms:

Women who experience shortness of breath, extreme fatigue, dizzy spells, changes in their vision, or feeling faint need to call their HCP right away or seek emergency care, especially if symptoms are accompanied by chest pain (Pieper PG, 2008).

Adequate hydration and physical exercise (even light to moderate) have been shown to help the body adjust to early cardiac changes, and even combat fatigue. However, the amount of hydration ideal during pregnancy is not known, as women have different fluid needs.

Women should make sure to tell their non-obstetric providers (even up to 5 years after delivery) if they had a history of HDP either before or during pregnancy.

Resources

Infographic: Pregnancy Complications & Heart Disease Risk (American College of Cardiologists)

Practice Bulletin 212: Pregnancy and Heart Disease (American College of Obstetricians and Gynecologists; May 2019)

Women and Heart Disease (U.S. Centers for Disease Control and Prevention; January 2020)

References

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