Mid-trimester blood pressure drop in normal pregnancy: myth or reality?

Background: Current dogma states that there is a mid-trimester fall in blood pressure (BP) in uncomplicated pregnancy. In the early stages of a longitudinal study of microcirculatory changes in pregnancy, we noted an absence of this mid-trimester fall.

Method: We prospectively studied this phenomenon in all our subsequent recruits. From a total of 326 women, 255 primigravid white women normotensive at booking and after delivery were studied. Serial BP measurements were taken under controlled conditions through to 38 weeks gestation. BP measurements by midwives were extracted from the case notes of 51 women within this cohort and analysed to validate the results. SBP progressively increased from the first trimester through to 38 weeks gestation.

Results: The increase from baseline at 13 weeks was significant when compared with measurements at 22 weeks [mean difference: 2.8 mmHg; 95% (confidence interval) CI 1.9-3.7], 28 weeks (mean difference: 5.0 mmHg; 95% CI 3.5-6.5) and 36 weeks (mean difference: 7.7 mmHg; 95% CI 6.2-9.1). DBP showed a nonsignificant dip at 22 weeks (mean difference: -0.12 mmHg; 95% CI -0.92 to 0.68), a nonsignificant increase at 28 weeks (mean difference: 2.0 mmHg; 95% CI 0.80-3.2) and a significant increase at 36 weeks (mean difference: 6.0; 95% CI 4.6-7.3). In the validation cohort, the SBP (P=0.0001) and DBP showed an increasing trend (P=0.0001).

Conclusion: BP measured under controlled conditions showed a progressive rise in pregnancy, with no significant mid-trimester drop. The findings were replicated in the routine antenatal clinic measurements.