Physiological Changes Associated with Pregnancy

Blood volume Blood volume increases progressively from 6-8 weeks gestation (pregnancy) and reaches a maximum at approximately 32-34 weeks with little change thereafter. Most of the added volume of blood is accounted for by an increased capacity of the uterine, breast, renal, striated muscle and cutaneous vascular systems, with no evidence of circulatory overload in the healthy pregnant woman. The increase in plasma volume (40-50%) is relatively greater than that of red cell mass (20-30%) resulting in haemodilution and a decrease in haemoglobin concentration. Intake of supplemental iron and folic acid is necessary to restore haemoglobin levels to normal (12g.dl-1). The increased blood volume serves two purposes. First, it facilitates maternal and fetal exchanges of respiratory gases, nutrients and metabolites. Second, it reduces the impact of maternal blood loss at delivery. Typical losses of 300-500ml for vaginal births and 750-1000ml for caesarean sections are thus compensated with the so-called ‘autotransfusion’ of blood from the contracting uterus.