Changes in Internal Organs During Pregnancy
Pregnancy causes significant physiological changes in multiple organ systems to support fetal development and prepare the mother for labor and delivery, including increased cardiac output by 30-50%, decreased functional residual capacity by 10-25%, and increased renal blood flow by 40%. 1, 2
Cardiovascular System Changes
- Heart rate: Increases by 10-20 beats per minute, beginning at approximately 20 weeks gestation and continuing until 32 weeks 2
- Cardiac output: Increases by 30-50% due to increased stroke volume and heart rate 1, 2
- Blood pressure: Decreases by 10-15 mmHg by 20 weeks gestation, then returns to pre-pregnancy levels by term 2
- Systemic vascular resistance: Decreases significantly due to vasodilation mediated by progesterone, estrogen, and nitric oxide 1, 2
- Uteroplacental blood flow: Increases from 50 to nearly 1000 mL/min during pregnancy, receiving up to 20% of maternal cardiac output at term 1
- Positional effects: The enlarging uterus can compress the aorta and inferior vena cava starting at approximately 12 weeks, significantly affecting hemodynamics when supine 1
- At 32 weeks, left lateral positioning increases ejection fraction by 11%, end-diastolic volume by 21%, stroke volume by 35%, and cardiac output by 24% compared to supine positioning 1
Respiratory System Changes
- Functional residual capacity: Decreases by 10-25% as the uterus enlarges and elevates the diaphragm 1, 2
- Tidal volume: Increases by 20-40% above baseline by term 1, 2
- Minute ventilation: Increases by 20-40% above pre-pregnancy levels 1, 2
- Oxygen consumption: Increases by 20-33% above baseline by the third trimester 1, 2
- Acid-base balance: Mild compensated respiratory alkalosis occurs, with:
- Airway changes: Upper airway edema and friability occur due to hormonal effects, potentially reducing visualization during laryngoscopy and increasing bleeding risk 1
Renal System Changes
- Renal blood flow: Increases by 40% to accommodate maternal role in fetal detoxification 1
- Glomerular filtration: Hyperfiltration occurs to manage increased circulatory volume 1
- Tubular function: Altered to prevent wasting of glucose, amino acids, and proteins required by maternal and fetal metabolism 1
Gastrointestinal System Changes
- Gastroesophageal sphincters: Relaxed by progesterone 1
- Transit times: Prolonged throughout the intestinal tract during second and third trimesters 1
- Aspiration risk: Increased due to relaxed sphincters and prolonged transit times 1, 2
Metabolic and Biochemical Changes
- Drug metabolism: Altered through multiple mechanisms 1
- Gastrointestinal absorption and transit affect bioavailability
- Protein binding changes alter free fraction of drugs
- Steroid-induced acceleration of hepatic P450 metabolism
- Increased renal clearance lowers circulating drug levels
- Fluid dynamics: Starling forces favor narrowing of oncotic pressure-wedge pressure gradient, increasing tendency for pulmonary edema 1
Clinical Implications
- Rapid hypoxemia development: The reduced functional residual capacity combined with increased oxygen consumption leads to rapid development of hypoxemia during hypoventilation or apnea 1
- Oxyhemoglobin dissociation: The curve is shifted to the right in the mother during pregnancy, requiring higher partial pressure of oxygen to achieve the same saturation 1
- Aortocaval compression: When supine, particularly in the third trimester, can significantly reduce cardiac output and cause hypotension 1, 2
Important Considerations for Medical Management
- Positioning: Left lateral tilt is crucial to prevent aortocaval compression, especially after 20 weeks gestation 1, 2
- Oxygenation: Pregnant women desaturate more quickly during periods of apnea or hypoventilation, requiring prompt oxygen supplementation to maintain SpO₂ >94% 1, 2
- Medication dosing: Requires adjustment based on altered pharmacokinetics throughout pregnancy 2
Understanding these physiological adaptations is essential for distinguishing between normal pregnancy changes and pathological conditions, ensuring appropriate management of pregnant patients in any clinical scenario.