Several studies have indicated that the use of the 24 h mean blood pressure, mainly using reference thresholds derived from general nonpregnancy practice, does not provide an effective test for an individualized early diagnosis of hypertension in pregnancy, thus concluding that ambulatory blood pressure monitoring is not a valid approach in pregnancy. With the use of ambulatory blood pressure monitoring, epidemiologic studies have reported gender differences in the circadian variability of blood pressure and heart rate. Typically, men exhibit a lower heart rate and higher blood pressure than women, the differences being larger for systolic than for diastolic blood pressure. Moreover, normotensive and hypertensive pregnant women are characterized by differing but predictable patterns of blood pressure variability throughout gestation. However, the diminished blood pressure in nongravid women as compared with men, the added decrease in blood pressure during the second trimester of gestation in normotensive but not in hypertensive pregnant women and the large amplitude of the circadian pattern that characterizes the blood pressure of healthy pregnant women at all gestational ages, have not been taken into account when establishing reference thresholds for the diagnosis of hypertension in pregnancy. This review will describe these issues, summarize previous results from independent groups on the prognostic value of ambulatory blood pressure monitoring in pregnancy, propose answers as to an accurate reference threshold for blood pressure at different stages of gestation and suggest how this information should be used in order to identify those women at a higher risk of hypertension, who will also be more suitable for prophylactic and/or therapeutic intervention in the early stages of pregnancy.
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