Why does blood pressure (BP) drop during the second trimester of pregnancy?

Why Blood Pressure Drops During the Second Trimester

Blood pressure drops during the second trimester primarily due to a reduction in systemic vascular resistance caused by active vasodilation through local mediators (prostacyclin and nitric oxide) and hormonal alterations mediated by progesterone and relaxin. 1

Mechanism of Blood Pressure Decline

The physiological drop in blood pressure during the second trimester results from multiple interconnected mechanisms:

Vascular Resistance Changes

• Systemic vascular resistance decreases substantially, representing the primary driver of blood pressure reduction 1
• Active vasodilation occurs through local mediators, specifically prostacyclin and nitric oxide, which directly relax vascular smooth muscle 1
• Development of the low-resistance uteroplacental circulation contributes significantly to the overall decrease in vascular resistance 1, 2

Hormonal Mechanisms

• Progesterone and relaxin mediate vasodilatory effects throughout the cardiovascular system 1, 2
• Relaxin attenuates the pressor response to angiotensin II, reducing the normal vasoconstrictive response seen in non-pregnant states 1
• Brain natriuretic peptide (BNP) and NT-proBNP concentrations increase in early pregnancy, functioning as potent vasodilators and representing an adaptive response to physiologic volume expansion 1, 2

Magnitude and Timing of Blood Pressure Changes

Expected Blood Pressure Pattern

• Diastolic blood pressure typically falls approximately 10 mmHg below baseline during the second trimester 1
• Blood pressure reaches approximately 15 mmHg lower than pre-pregnancy values during the second trimester nadir 1
• Median blood pressure values in low-risk women are 113/69 mmHg in the second trimester, compared to 114/70 mmHg in the first trimester 1, 2
• Blood pressure progressively increases through the third trimester, returning to or exceeding pre-pregnancy levels by term (approximately 121/78 mmHg at 40 weeks) 1, 2

Clinical Implications and Pitfalls

Diagnostic Considerations

• The 20-week gestational age cutoff serves as a critical diagnostic marker for classifying hypertensive disorders of pregnancy, based on the expected blood pressure decline during the first 20 weeks 1, 2
• Undiagnosed chronic hypertension may be masked by the physiological blood pressure fall in early pregnancy, leading to misclassification as gestational hypertension when elevated pressures are detected later 2
• Women who develop pregnancy-induced hypertension often demonstrate a blunted decline or increase in systolic blood pressure before 20 weeks, rather than the expected physiological drop 1, 2

Common Pitfalls to Avoid

• Do not assume normal blood pressure in early pregnancy excludes chronic hypertension, as the physiological drop can mask pre-existing disease 2
• Blood pressure at 20 weeks appears useful for risk stratification, identifying otherwise low-risk individuals at heightened risk of developing pregnancy-induced hypertension in a dose-dependent fashion 1, 2
• Women with high first-trimester blood pressure tend to exhibit a fall in blood pressure during pregnancy, whereas women with low initial blood pressure tend to exhibit a large increase, creating potential diagnostic confusion 3

Cardiovascular Adaptations Supporting Blood Pressure Changes

• Cardiac output increases by 30-50% during normal pregnancy, with early pregnancy increases primarily related to rising stroke volume 1, 4
• Plasma volume reaches a maximum of 40% above baseline at 24 weeks gestation, contributing to the hemodynamic changes 1
• Heart rate starts to rise at 20 weeks and increases until 32 weeks, becoming the major factor in maintaining cardiac output in late pregnancy 1