Clinical meaning
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 (0.4-0.8 mg/dL, versus 0.6-1.2 non-pregnant) and lower BUN — a seemingly normal creatinine of 1.0 mg/dL may actually indicate renal impairment in pregnancy. Glucosuria is common due to increased filtered glucose load exceeding tubular reabsorption capacity. The fetoplacental unit functions as an endocrine organ: the syncytiotrophoblast produces human chorionic gonadotropin (hCG, which maintains the corpus luteum and its progesterone production until placental takeover at 8-10 weeks), human placental lactogen (hPL, which creates insulin resistance to shunt glucose to the fetus and mobilizes maternal fatty acids as alternative fuel), progesterone (maintains uterine quiescence, prevents immune rejection of the fetus), and estrogen (stimulates uterine growth and breast development). Gestational age is most accurately determined by first-trimester crown-rump length measurement (±5 days accuracy at 8-13 weeks).