Maternal adaptation to pregnancy

This chapter discusses several physiological changes that accompany pregnancy in women. Many adaptations of pregnancy are imperceptible even to the woman carrying a child, yet are essential for a successful pregnancy. An incomplete cardiovascular adaptation to pregnancy can lead to improper fetal development or even fetal demise. The average weight gain by women during pregnancy is approximately 12.5 kg and includes the increased weights of the fetus, amniotic fluid, uterine tissue, breast tissue, and blood, as well as extravascular and intracellular water. The maternal basal metabolic rate (BMR) also increases during pregnancy. BMR increases from approximately 1,300 kcals/day to approximately 1,700 kcal/day over gestation in women of average pre-pregnancy body mass index (BMI). Blood volume increases continuously from 6-8 weeks of gestation, reaches a maximum at approximately 32 to 34 weeks, and changes little thereafter. The increase in plasma volume (40%-50%) is relatively greater than that of red cell mass (20%-30%), resulting in a decrease in hemoglobin concentration. Moreover, whole-body remodeling of the maternal circulation begins within a few weeks of conception. Circulatory changes continue through parturition, when most of the changes are reversed.