Pregnancy places an incredible demand on a woman's body, requiring the cardiovascular system to undergo massive remodeling to support the growing fetus. These changes begin very early, often within the first trimester, driven primarily by rising hormone levels like progesterone and estrogen. The heart must adapt to handle a significantly larger volume of blood and deliver adequate oxygen to the uteroplacental unit while protecting maternal organs. Midwives must understand these normal adaptations to distinguish them from underlying heart disease during routine antenatal visits.
By the end of the third trimester, maternal blood volume increases by about 30 to 50 percent above non-pregnant levels. Interestingly, the liquid portion of the blood (plasma) increases much more than the red blood cells. This unequal increase results in physiological hemodilution, commonly referred to as the anemia of pregnancy. A normal hemoglobin level drops slightly, but this dilution actually protects the mother by reducing blood thickness, making it easier for the heart to pump and reducing risks if bleeding occurs during childbirth.
Cardiac output—the amount of blood the heart pumps every minute—increases by 30 to 50 percent during pregnancy. This rise is achieved through two main factors:
Because of this extra workload and shifting organs, the heart muscle undergoes mild hypertrophy and changes position, moving upward and to the left inside the chest cavity.
Despite the massive increase in blood volume and cardiac output, a pregnant woman's blood pressure usually drops during the first and second trimesters. This happens because systemic vascular resistance (SVR) decreases sharply due to the relaxing effects of progesterone and local vasodilators like prostaglandins on blood vessels. Blood pressure reaches its lowest point around mid-pregnancy before slowly returning toward pre-pregnancy levels by term. Midwives must recognize this normal dip so they do not misdiagnose normal vascular adaptation as pathology.
In the later trimesters, the heavy gravid uterus can compress the large maternal blood vessels—specifically the inferior vena cava and the descending aorta—when the woman lies flat on her back. This compression reduces venous return to the heart, causing a sharp drop in cardiac output and blood pressure. Known as supine hypotensive syndrome, it leads to dizziness, pallor, and fainting. Midwives should always advise pregnant women to rest and sleep in a lateral position, preferably on the left side, to relieve vessel compression.
Physical examination of the cardiovascular system during pregnancy often reveals findings that would be abnormal in a non-pregnant person. Due to increased blood flow turbulence and heart displacement, up to 80 percent of pregnant women develop a soft systolic murmur. A third heart sound (S3) can also be heard quite normally. However, midwives must know that diastolic murmurs, loud systolic murmurs, or murmurs accompanied by chest pain, severe breathlessness, or cyanosis are never normal and require immediate cardiology referral.