Pregnancy requires many body systems -- including your cardiovascular system -- to make major changes, including elevation of your heart rate. However, elevated heart rate can also indicate an underlying problem. See your doctor right away if you experience a sudden increase in heart rate, light-headedness, anxiety or shortness of breath.
Heart Function in Pregnancy
Cardiovascular adaptations serve a number of purposes during pregnancy, including protection of your normal physiologic functioning, meeting the increased metabolic demands of the pregnancy and to facilitate the growth and development of the fetus. In addition to elevated heart rate, your heart will likely pump more blood to the rest of your body with each heart contraction.
In a normal, uncomplicated pregnancy, your resting heart rate will elevate by 15 to 20 beats per minute. As is the case with all of the major organ systems, cardiovascular changes that take place during pregnancy are believed to be triggered by hormonal signals, primarily increases in progesterone and estrogen, which alert the body to the presence of a fetus. Heart rate changes may begin in the first trimester of pregnancy.
During the second trimester there is a generalized relaxation of smooth muscle along with dilation of the arteries. This change accommodates the increase in circulating blood volume while maintaining your normal blood pressure. This leads to a progressive elevation of your resting heart rate, which peaks in the second trimester. By the fourth month of pregnancy, cardiac output, or the quantity of blood your heart pumps, increases by 30 percent to 50 percent. This is the point in pregnancy when problems might present if you have any previously undiscovered heart irregularities.
The period of maximum cardiac output occurs between week 28 and 35 of the third trimester. By the end of pregnancy, at about 40 weeks, your circulating blood volume will have increased to 40 percent to 90 percent above pre-pregnancy levels, causing an increase in heart rate. The heart depends on an ample blood supply to ensure adequate delivery of oxygen and nutrients to both mother and baby. In the third trimester, these changes are critical to meet the increasing demands of a rapidly growing fetus and also for preparation for labor. During labor, the heart rate elevates during contractions and in response to pain.
Normally, the healthy heart can withstand the cardiovascular changes that occur over the course of pregnancy. However, previously unknown heart problems might become apparent as the pregnancy progresses and the workload demands on the heart increase. Some common warning signs can include a racing or pounding pulse, shortness of breath even when at rest, lightheadedness, fainting spells, a persistent productive cough and palpitations, or skipped heartbeats.