Reference thresholds for 24-h, diurnal, and nocturnal ambulatory blood pressure mean values in pregnancy.

Objectives: Several studies have indicated that the use of the 24-h mean of blood pressure, mainly using reference thresholds derived from the general non-pregnancy practice, does not provide a proper test for diagnosis of hypertension in pregnancy. This prospective study examines previously derived reference thresholds for the 24-h, diurnal, and nocturnal mean of blood pressure as potential screening tests for the diagnosis of hypertension in pregnancy.

Methods: We studied 235 normotensive and 168 hypertensive pregnant women, who provided 2430 blood pressure series sampled every 20 min during the day and every 30 min at night for 48 consecutive hours once every 4 weeks from the first obstetric visit until delivery. Sensitivity and specificity for each parameter are based on the comparison of the distributions of mean blood pressure values with reference thresholds previously established from an independent population of 113 pregnant women also evaluated monthly by 48-h ambulatory monitoring throughout gestation.

Results: Sensitivity of mean blood pressure values, above 70% at all stages of pregnancy, was higher than that obtained from clinic blood pressure measurements, which were always below 14%. The poorest results were consistently obtained for the nocturnal mean. Sensitivity was similar for the 24-h and the diurnal mean, with values ranging from 71% for diastolic blood pressure in the first trimester of pregnancy to 93% for systolic blood pressure in the third trimester. Systolic blood pressure consistently provided better sensitivity than diastolic blood pressure at all gestational ages.

Conclusions: This prospective study on women systematically studied by 48-h ambulatory monitoring throughout gestation indicates that mean ambulatory blood pressure values provide higher sensitivity and specificity than conventional measurements. Moreover, results indicate that diagnosis of hypertension in pregnancy based on ambulatory blood pressure should be established from thresholds much lower than those currently used in clinical practice.