How are vital signs affected throughout pregnancy?

Physiological Changes in Vital Signs During Pregnancy

Normal vital signs undergo significant physiological changes throughout pregnancy to accommodate the increased metabolic demands of both mother and fetus. 1, 2 Understanding these changes is crucial for accurate clinical assessment and avoiding misdiagnosis of pathological conditions.

Cardiovascular Changes

Heart Rate

• Increases by 10-20 beats per minute (bpm), particularly in the third trimester 1
• Begins rising at approximately 20 weeks gestation and continues increasing until 32 weeks 1
• Remains elevated for 2-5 days after delivery 1
• Overall 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 1
• Systolic and diastolic blood pressure typically fall early in gestation 1
• Diastolic blood pressure reaches its lowest point in the second trimester (approximately 10 mmHg below baseline) 1
• Blood pressure gradually increases in the third trimester, returning to non-pregnant values by term 1
• This pattern is due to active vasodilation from local mediators like prostacyclin and nitric oxide 1

Systemic Vascular Resistance

• Decreases significantly due to vasodilation mediated by progesterone, estrogen, and nitric oxide 1, 2
• This decrease helps accommodate the expanded blood volume

Respiratory Changes

Respiratory Rate

• Remains unchanged throughout normal pregnancy 1, 2
• If respiratory rate exceeds 20 breaths per minute, consider a pathological cause 1

Oxygen Saturation

• Remains unchanged throughout pregnancy 1, 2
• Despite subjective dyspnea, oxygen saturation should remain normal 2

Respiratory Physiology

• 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
• Mild respiratory alkalosis develops with arterial CO₂ pressure of 28-32 mmHg and plasma bicarbonate of 18-21 mEq/L 1, 2

Temperature

• Body temperature remains unchanged throughout pregnancy 1
• Normal temperature regulation is maintained despite other physiological changes

Laboratory Values Related to Vital Signs

• BNP (B-type natriuretic peptide): Not elevated during normal pregnancy 1
• Troponin: Not elevated during normal pregnancy 1
• D-dimer: Not recommended for use in pregnancy due to physiological elevation 1
• Cholesterol: Can be elevated up to five times during pregnancy 1
• Arterial blood gas: Mild, fully compensated respiratory alkalosis is normal 1

ECG Changes

• Sinus tachycardia 1
• 15° left axis deviation due to diaphragmatic elevation 1
• T wave inversion in lead III and aVF 1
• Non-specific ST changes (e.g., depression, small Q waves) 1
• Atrial and ventricular ectopic beats may be present 1

Clinical Implications

• The physiological adaptations to pregnancy significantly influence the evaluation and interpretation of cardiac function and clinical status 1
• Normal ranges for maternal vital signs differ from the general population 1
• Physiological monitoring of pregnant women requires tailored assessment tools such as modified early obstetric warning scores 1
• Physiological changes should not be overestimated as pathology might be missed 1
• Rapid oxygen depletion can occur during periods of apnea or hypoventilation due to decreased functional residual capacity and increased oxygen consumption 2

Understanding these normal physiological changes is essential for proper assessment of pregnant women and distinguishing between normal adaptations and pathological conditions requiring intervention.