Resting Heart Rate Dropping During Pregnancy: What It Means
Learn how circulatory and hormonal changes during pregnancy can influence resting heart rate and what a lower rate may indicate for overall health.
Tracking heart rate during pregnancy offers insight into cardiovascular changes. While an increased resting heart rate is common, some individuals notice a decrease, raising questions about its significance.
Understanding why this occurs requires examining the physiological changes of pregnancy.
Circulatory Adaptations
The cardiovascular system undergoes extensive modifications to support the developing fetus. Blood volume increases by 30% to 50% by the third trimester (Purdy et al., 2019, American Journal of Physiology), ensuring adequate oxygen and nutrient supply. To accommodate this, the heart adjusts its workload, affecting heart rate, stroke volume, and vascular resistance. While most experience a gradual rise in resting heart rate, some see a temporary or sustained decrease due to specific circulatory adaptations.
A key factor is the reduction in systemic vascular resistance. Progesterone and other vasodilatory hormones relax blood vessel walls, lowering peripheral resistance (Meah et al., 2020, Hypertension). This facilitates blood flow, reducing the heart’s effort and sometimes leading to a transient drop in resting heart rate, particularly in the first trimester before cardiac output fully compensates. Additionally, blood flow redistribution prioritizes the uterus, kidneys, and skin, influencing cardiovascular dynamics.
Cardiac output, the total volume of blood the heart pumps per minute, also affects heart rate. While it typically rises by 30% to 50%, this increase results from both a higher heart rate and greater stroke volume—the amount of blood ejected per beat (Sanghavi & Rutherford, 2014, Circulation). In individuals with an efficient stroke volume response, the heart may not need to beat as frequently, leading to a lower resting heart rate. This is more common in those with high baseline cardiovascular fitness, whose hearts are already conditioned to pump blood effectively.
Hormonal Shifts
Hormonal changes during pregnancy influence cardiovascular function, contributing to heart rate fluctuations. Progesterone, estrogen, and relaxin affect vascular tone, fluid balance, and cardiac output, leading to variations in heart rate.
Progesterone, which rises significantly, relaxes blood vessel walls, lowering systemic vascular resistance and, in some cases, resting heart rate before plasma volume expansion peaks. Estrogen enhances nitric oxide production, further promoting vasodilation. While its primary role is increasing blood flow to the uterus and placenta, it also contributes to reduced vascular resistance, which may affect heart rate.
The autonomic nervous system adapts to these circulatory changes. Increased progesterone levels shift autonomic balance toward greater parasympathetic (vagal) activity, which slows heart rate. This contrasts with later pregnancy, when sympathetic activity rises to support increased cardiac output. Some individuals may experience an exaggerated parasympathetic response early in pregnancy, temporarily lowering resting heart rate. Relaxin, which prepares the body for labor by softening connective tissues, also contributes to vasodilation and may play a role in transient heart rate decreases in the first trimester.
Typical Patterns of Heart Rate Variation
Resting heart rate during pregnancy does not follow a uniform pattern, as individual physiological differences shape cardiovascular responses. While an overall increase is common, fluctuations occur at different stages due to blood volume expansion, autonomic nervous system adaptations, and baseline fitness levels.
In early pregnancy, heart rate changes can be inconsistent. Some individuals experience an initial drop, likely due to early vasodilation and reduced vascular resistance before cardiac output fully adjusts. This phase is typically transient, lasting through the first trimester before the body compensates with increased stroke volume and sympathetic nervous system activity.
By the second trimester, heart rate generally rises more predictably, increasing by 10 to 20 beats per minute above pre-pregnancy levels. This corresponds with peak plasma volume expansion and heightened metabolic demands.
In the third trimester, heart rate typically reaches its highest levels due to increased uterine blood flow, higher maternal oxygen consumption, and the mechanical effects of an expanding uterus on venous return. Those with strong cardiovascular conditioning may exhibit more stable heart rates due to an efficient stroke volume response. Positional changes, such as lying supine, can momentarily lower heart rate by altering venous return dynamics.
Interpreting Lower Resting Heart Rate
A lower resting heart rate during pregnancy may raise concerns, but it is not necessarily problematic. Those with higher pre-pregnancy fitness levels often have lower baseline heart rates due to enhanced stroke volume, which can persist into pregnancy.
Parasympathetic nervous system activity, which promotes relaxation and slows heart rate, may be more pronounced in some individuals early in pregnancy before the sympathetic system compensates. Additionally, fluctuations in blood pressure and hormone levels can influence heart rate. A study in Obstetrics & Gynecology (2021) found that while most pregnant individuals experience a gradual heart rate increase, about 10% exhibited transient decreases linked to autonomic regulation shifts, with no associated adverse outcomes.
Clinical Assessment Strategies
Evaluating resting heart rate changes during pregnancy requires a comprehensive approach. While mild fluctuations are often benign, persistent or significant deviations may warrant closer monitoring. Clinicians assess heart rate trends alongside blood pressure, oxygen saturation, and cardiac output to determine whether changes align with normal gestational adjustments or indicate a potential concern.
Electrocardiography (ECG) and ambulatory heart rate monitoring can track heart rate patterns over time. If a lower resting heart rate is accompanied by symptoms of bradycardia, further evaluation through echocardiography or autonomic function testing may be necessary. Blood tests assessing thyroid function and electrolyte balance can help identify metabolic contributors.
In most cases, a lower resting heart rate without symptoms does not require intervention, but ongoing monitoring ensures early detection of abnormalities. Clinicians may recommend adjustments in physical activity, hydration, or posture to optimize cardiovascular stability throughout pregnancy.