Does blood pressure decrease during the second trimester of pregnancy?

Blood Pressure Changes in the Second Trimester of Pregnancy

Yes, blood pressure does drop during the second trimester of pregnancy, reaching its lowest point (nadir) during this period before gradually rising again toward term. 1

Physiological Blood Pressure Pattern Across Pregnancy

Blood pressure initially declines in early pregnancy, with the lowest values occurring in the second trimester, followed by a progressive increase through the third trimester. 1

Specific Blood Pressure Values and Timing

• In a large international cohort of low-risk pregnant women, median blood pressure was 114/70 mmHg in the first trimester and 113/69 mmHg in the second trimester, demonstrating the characteristic decline. 1
• By 40 weeks of gestation, blood pressure increases to approximately 121/78 mmHg, representing an average rise of approximately 8 mmHg in both systolic and diastolic pressures from the nadir. 1
• Diastolic blood pressure typically falls approximately 10 mmHg below baseline during the second trimester before returning to or exceeding pre-pregnancy levels by term. 1, 2

Mechanism of Blood Pressure Decline

The physiological decrease in blood pressure during the second trimester results from several key mechanisms:

• Reduction in systemic vascular resistance is the primary driver, caused by active vasodilation through local mediators such as prostacyclin and nitric oxide. 1
• Development of the low-resistance uteroplacental circulation contributes significantly to decreased vascular resistance. 1
• Hormonal alterations mediated by progesterone and relaxin promote vasodilation. Relaxin, produced initially by the corpus luteum and subsequently by the placenta, attenuates the pressor response to angiotensin II. 1
• Elevated concentrations of brain natriuretic peptide (BNP) and NT-proBNP in early pregnancy function as potent vasodilators, representing an adaptive response to physiologic volume expansion. 1

Clinical Implications

Diagnostic Considerations

The expected blood pressure decline during the first 20 weeks of gestation serves as a critical diagnostic marker for classifying hypertensive disorders of pregnancy. 1

• Any hypertension documented before 20 weeks of gestation is classified as chronic hypertension. 1
• New-onset hypertension after 20 weeks is classified as either gestational hypertension or preeclampsia (if accompanied by proteinuria or target organ involvement). 1

Important Clinical Pitfall

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 later detected. 1

• Women with pre-existing hypertension may appear normotensive in early pregnancy due to the physiological decline commencing in the first trimester. 1
• This masking effect can result in chronic hypertension being incorrectly interpreted as gestational hypertension when blood pressure rises later in pregnancy. 1

Abnormal Blood Pressure Patterns

Women who develop pregnancy-induced hypertension (gestational hypertension or preeclampsia) often demonstrate a blunted decline or even an increase in systolic blood pressure before 20 weeks, rather than the expected physiological drop. 1

• A smaller decrease in second trimester blood pressure, particularly in systolic blood pressure and mean arterial pressure, is associated with increased risk of developing gestational hypertensive disease. 3
• Blood pressure at 20 weeks appears useful for identifying otherwise low-risk individuals at heightened risk of developing pregnancy-induced hypertension in a dose-dependent fashion. 1