How does systemic blood pressure normally change during pregnancy and what thresholds define gestational hypertension?

Normal Blood Pressure Changes During Pregnancy and Gestational Hypertension Thresholds

Blood pressure normally decreases during the second trimester of pregnancy by approximately 15 mmHg, reaching its lowest point around 20 weeks of gestation, then gradually returns to or exceeds pre-pregnancy levels by the third trimester and delivery. 1

Physiologic Blood Pressure Pattern Throughout Pregnancy

First and Second Trimester Changes

• BP steadily decreases from early pregnancy until approximately 20 weeks of gestation, with the nadir occurring in mid-pregnancy 2
• The decrease is approximately 15 mmHg below pre-pregnancy baseline values 1
• This physiologic drop occurs in both normotensive women and those with pre-existing hypertension 1

Third Trimester and Delivery

• After 20 weeks, BP gradually increases throughout the third trimester, returning to or exceeding pre-pregnancy levels by delivery 2
• Normotensive women experience an average 8% increase in BP between mid-gestation and delivery 2
• Heart rate increases progressively until the end of the second trimester, then slightly decreases thereafter 2

Circadian Patterns

• A highly significant 24-hour circadian rhythm persists throughout all trimesters of pregnancy, with both 24-hour and 12-hour components 3
• This circadian pattern is maintained in both healthy and complicated pregnancies 3

Diagnostic Thresholds for Gestational Hypertension

Standard Definition

Gestational hypertension is defined as new-onset BP ≥140/90 mmHg (systolic ≥140 mmHg OR diastolic ≥90 mmHg) developing after 20 weeks of gestation in a previously normotensive woman, without significant proteinuria. 4, 5

Key Diagnostic Criteria

• Requires at least two elevated BP readings on separate occasions for diagnosis 5
• Develops specifically after 20 weeks of gestation (before 20 weeks suggests chronic hypertension) 4, 5
• Typically resolves within 42 days postpartum (persistence beyond this timeframe reclassifies it as chronic hypertension) 4

Severity Classification

• Mild to moderate gestational hypertension: 140-159/90-109 mmHg 5
• Severe hypertension: ≥160/110 mmHg, requiring urgent treatment within 15 minutes to prevent maternal stroke 5

Measurement Recommendations

• 24-hour ambulatory BP monitoring is superior to conventional office measurements for predicting proteinuria, preterm delivery, birth weight, and overall pregnancy outcomes 1, 5
• For home monitoring, the threshold is ≥135/85 mmHg 5
• Diastolic BP should be measured at Korotkoff phase V (disappearance of sound) rather than phase IV, except when sounds persist near 0 mmHg 1

Clinical Implications and Early Warning Signs

Emerging Evidence for Lower Thresholds

• New-onset BP elevations of 130-139/80-89 mmHg after 20 weeks carry a 2.4-fold increased risk of developing hypertensive disorders and nearly 3-fold increased risk of preeclampsia with severe features 6
• These modest elevations may represent early disease indicators, though current diagnostic thresholds remain at 140/90 mmHg 6

Predictive Value of Early Changes

• Women who later develop gestational hypertension or preeclampsia show significantly different BP patterns as early as the first trimester, even when absolute values remain within normal ranges 3
• Both the circadian mean and amplitude (extent of daily change) differ significantly between women who develop complications versus those with uncomplicated pregnancies 3

Distinguishing Gestational Hypertension from Preeclampsia

• In the first trimester, women who develop preeclampsia versus gestational hypertension show similar circadian characteristics 3
• By the second trimester, systolic BP begins to diverge between these groups 3
• In the third trimester, women with preeclampsia demonstrate a significantly steeper slope of BP increase compared to those with gestational hypertension alone 2

Common Pitfalls to Avoid

• Do not assume gestational hypertension is benign: approximately 20% progress to preeclampsia and another 20% develop persistent gestational hypertension 4
• White-coat hypertension affects up to 25% of patients with elevated clinic readings; confirm with home or ambulatory monitoring before diagnosing true hypertension 5
• Do not discontinue postpartum monitoring at hospital discharge: 10% of maternal deaths from hypertensive disorders occur postpartum, and complications can worsen or initially present after delivery 4
• All women with gestational hypertension require close BP monitoring through 42 days postpartum 4