Video narration
Total blood volume rises substantially, but the red-cell mass does not increase by the same proportion. The blood becomes diluted, but red-cell mass increases. Progesterone and placental lactogen stimulates erythropoietin production indirectly through pregnancy-related changes, and maternal erythropoietin supports increased red-cell production. Red-cell mass typically rises by about twenty to thirty percent, although the increase varies with iron availability and other factors. Compare that with the forty-to-fifty-percent increase in plasma volume. Because plasma expands more than red-cell mass, haemoglobin concentration and haematocrit fall. This is physiological dilutional anaemia of pregnancy. The total oxygen-carrying red-cell mass has increased, but each unit of blood contains a lower concentration of red cells. Now connect the increased blood volume to the heart. More circulating volume, together with changes in venous capacitance and blood flow, increases venous return when the mother is appropriately positioned. Ventricular end-diastolic volume rises. By the Frank-Starling mechanism, greater myocardial fibre stretch produces a stronger contraction, increasing stroke volume. Stroke volume commonly rises by around ten to thirty percent, particularly in early and mid-pregnancy. The heart also undergoes physiological remodelling. Cardiac chambers enlarge mildly, and left ventricular mass increases, commonly by approximately five to ten percent. This is physiological remodelling, not automatically pathological hypertrophy. The changes are generally balanced and largely reversible after pregnancy. Heart rate and cardiac output increase. Cardiac output equals heart rate multiplied by stroke volume. In early pregnancy, the rise in cardiac output is driven substantially by increased stroke volume. As pregnancy progresses, heart rate becomes increasingly important. Maternal heart rate commonly rises by about ten to twenty beats per minute, approximately fifteen to twenty-five percent in many women. Overall, cardiac output rises by around thirty to fifty percent, sometimes approaching the upper end of that range by mid-to-late pregnancy. The circulation is now delivering more blood each minute to the uterus, placenta, kidneys, skin and other maternal tissues.