Pregnancy Basics

Pregnancy induces profound and systematic physiological adaptations across every organ system to support fetal growth while maintaining maternal homeostasis. Cardiovascular changes begin by 6 weeks gestation: plasma volume expands 40-50% (peaking at 32 weeks), cardiac output increases 30-50% through both increased heart rate (~15-20 bpm above baseline) and increased stroke volume, and systemic vascular resistance decreases due to progesterone-mediated smooth muscle relaxation and the low-resistance uteroplacental circulation — this produces the characteristic second-trimester blood pressure nadir. The expanded plasma volume exceeds the rise in red blood cell mass (which increases only 20-30%), creating physiological dilutional anemia of pregnancy (hemoglobin normally drops to ~11 g/dL). Respiratory changes include increased tidal volume by 30-40% (driven by progesterone stimulating the medullary respiratory center), with unchanged respiratory rate, producing a compensated respiratory alkalosis (PaCO2 ~30 mmHg, pH ~7.44) that facilitates CO2 transfer from fetal to maternal circulation. The enlarging uterus elevates the diaphragm by ~4 cm but this is compensated by increased chest wall diameter, maintaining vital capacity. Renal changes include 50% increase in GFR and renal plasma flow, resulting in lower serum creatinine...