What are the physiologic changes that occur during pregnancy?

Physiologic Changes in Pregnancy

Pregnancy induces numerous physiological adaptations across multiple organ systems to support fetal development and prepare the mother for labor and delivery. These changes are essential for maintaining maternal health and ensuring optimal fetal growth throughout gestation 1.

Cardiovascular Changes

• Blood volume and cardiac output:

  • Cardiac output increases 30-50% above baseline, reaching maximum by 32 weeks 1
  • Initially driven by increased stroke volume, later by heart rate (increases 15-20 bpm) 1
  • Blood volume expands by approximately 40% (plasma volume increases 50%, red cell mass 30%) 2
  • Uteroplacental blood flow increases from 50 to nearly 1000 mL/min, receiving up to 20% of maternal cardiac output at term 1

• Blood pressure and vascular resistance:

  • Systemic vascular resistance decreases due to endogenous vasodilators (progesterone, estrogen, nitric oxide) 1
  • Blood pressure typically falls early in gestation, with diastolic BP about 10 mmHg below baseline in second trimester 1
  • Blood pressure gradually returns to normal by term 1

• Structural cardiac changes:

  • Heart size can increase by up to 30%, partially due to dilatation 1
  • Left lateral position significantly improves cardiac function compared to supine position (at 32 weeks: 11% increase in ejection fraction, 35% increase in stroke volume, 24% increase in cardiac output) 1

Respiratory Changes

• Ventilation:

  • Functional residual capacity decreases 10-25% as the enlarging uterus elevates the diaphragm 1
  • Tidal volume and minute ventilation increase 20-40% above baseline by term, mediated by elevated progesterone levels 1
  • Respiratory alkalosis develops with compensatory renal bicarbonate excretion (arterial CO₂ pressure 28-32 mmHg, plasma bicarbonate 18-21 mEq/L) 1
  • Oxygen consumption increases 20-33% above baseline by third trimester 1

• Airway changes:

  • Upper airway edema and friability occur due to hormonal effects, potentially complicating airway management 1
  • Reduced functional residual capacity and increased oxygen consumption lead to rapid development of hypoxemia during hypoventilation or apnea 1

Renal and Fluid Balance Changes

• Glomerular function:

  • Glomerular hyperfiltration occurs with 40% increase in renal blood flow 1
  • Altered tubular function prevents wasting of glucose, amino acids, and proteins 1

• Fluid dynamics:

  • Narrowing of oncotic pressure-wedge pressure gradient increases tendency for pulmonary edema 1
  • Expanded intravascular volume and decreased uterine vascular resistance facilitate uterine placental blood flow 1

Gastrointestinal Changes

• Motility:

  • Progesterone relaxes gastroesophageal sphincters and prolongs intestinal transit time during second and third trimesters 1
  • Increased risk of gastric content aspiration 1, 3

• Gallbladder:

  • Decreased gallbladder motility with increased risk of gallstone formation 1

Hematologic Changes

• Coagulation:

  • Hypercoagulable state develops with increased concentration of coagulation factors, fibrinogen, and platelet adhesiveness 1
  • Diminished fibrinolysis further increases thromboembolism risk 1, 2
  • Venous stasis from enlarging uterus compounds thromboembolism risk 1
  • Highest risk for pulmonary embolism occurs immediately postpartum (0.1-0.67 per 1000 pregnancies) 1

• Blood composition:

  • Physiologic anemia of pregnancy due to disproportionate increase in plasma volume compared to red cell mass 2
  • Oxyhemoglobin dissociation curve shifts right in the mother (P50 increases from 27 to 30 mmHg) 1

Metabolic and Endocrine Changes

• Glucose metabolism:

  • Altered maternal glucose homeostasis to meet fetal-maternal needs 1

• Lipid metabolism:

  • Cholesterol levels increase up to five times during pregnancy 1

• Hormone levels:

  • Increased progesterone, estrogen, human chorionic gonadotropin (HCG), and human placental lactogen (HPL) 2
  • Estrogens may increase capillary fragility 2
  • Progesterone responsible for venous system enlargement 2

Labor and Postpartum Changes

• Hemodynamic changes during labor:
  • Systolic and diastolic BP increase 15-25% and 10-15% respectively during uterine contractions 1
  • Cardiac output increases progressively: 15% in early labor, 25% during stage 1,50% during expulsive efforts 1
  • Cardiac output increases up to 80% early postpartum due to autotransfusion from uterine involution 1
  • Uterine contractions cause autotransfusion of 300-500 mL blood back to circulation 1

Clinical Implications

• Normal maternal vital sign ranges differ from non-pregnant population 1

• Laboratory values that remain normal throughout pregnancy:

  • Oxygen saturation
  • Temperature
  • B-type natriuretic peptide (BNP)
  • Troponin (may be elevated in conditions like pre-eclampsia, pulmonary embolism, myocarditis)
  • Bilirubin, prothrombin time, gamma-glutamyltransferase, and total bile acid levels 1

• ECG changes in normal pregnancy:

  • Sinus tachycardia
  • 15° left axis deviation due to diaphragmatic elevation
  • T wave inversion in lead III and aVF
  • Non-specific ST changes, small Q waves
  • Atrial and ventricular ectopic beats 1

Evolutionary Perspective

The physiological adaptations in pregnancy favor fetal development, sometimes at the expense of maternal health 1. These changes include:

• Cardiovascular adaptations (spiral arteriole remodeling, reduced peripheral resistance)
• Immune system modifications
• Endocrine and hormonal changes
• Ligament laxity (facilitates passage of fetal head but increases risk for pelvic organ prolapse) 1

Human brain evolution has resulted in a tight fetopelvic fit, leading to more difficult labors due to fetal head-pelvic disproportion compared to other primates 1.

Human Physiology in Pregnancy

Human Physiology in Pregnancy

Human Physiology in Pregnancy