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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Although some cardiac output changes can be reversed as early as two weeks postpartum, most 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 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.
High Blood Pressure
On November 9, 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.
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.
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:
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
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.
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").
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.
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
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.
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)