Abstract-Previous studies on ambulatory blood pressure monitoring as a potential screening test for hypertension in pregnancy have not carefully considered sampling requirements. We have examined the impact of duration and frequency of blood pressure sampling in the reproducibility of mean values in pregnancy. We analyzed 2430 blood pressure series sampled every 20 minutes during the day and every 30 minutes at night for 48 hours every 4 weeks from the first obstetric visit until delivery in 235 normotensive and 168 hypertensive pregnant women. Blood pressure series were decimated to generate shorter series with data sampled every 1, 2, 3, or 4 hours for 48 hours, as well as at the original rate for the first day. Reproducibility of mean blood pressure as well as sensitivity and specificity in the diagnosis of hypertension were compared between the original and the decimated series. Sensitivity and specificity of the 24-hour blood pressure mean are similar for the values calculated from the original series and for those obtained from shorter profiles up to data sampled every 3 hours but reduced by 5% to 12% when diagnosis is based on data sampled at 20-to 30-minute intervals for the first 24 hours. Results also indicate that the 24-hour blood pressure mean is better reproduced with data sampled at 3-hour intervals for 48 hours than by data sampled at 20-to 30-minute intervals for 1 day only. This study demonstrates that reproducibility of mean blood pressure values is more dependent on duration of sampling than on sampling rate. Key Words: blood pressure Ⅲ pregnancy Ⅲ hypertension, pregnancy Ⅲ preeclampsia Ⅲ blood pressure monitoring, ambulatory P regnancies complicated by hypertension contribute markedly to perinatal morbidity and mortality rates. 1 Because an elevated blood pressure (BP) after 20 weeks of gestation in a previously normotensive woman is common to the definition of both gestational hypertension and preeclampsia, 2,3 the issue of whether the development of these complications may be predicted on the basis of BP obtained during conventional antenatal visits has been addressed in several retrospective as well as prospective studies. 4 -7 Recent studies have tried to overcome the poor results from isolated conventional BP measurements in detecting hypertensive complications in pregnancy by relying on ambulatory BP monitoring (ABPM). 8 -17 By the use of ABPM, differences between healthy and complicated pregnancies in the circadian pattern of BP, previously documented for the second trimester of pregnancy, 9 can be observed as early as in the first trimester of pregnancy, quite before the actual clinical diagnosis of gestational hypertension or preeclampsia takes place for most pregnant women investigated. 15,17 As in the general nonpregnancy practice, the most common approach has been to rely for diagnosis on the arithmetic mean of all values determined by ABPM. However, the use of the 24-hour mean of BP does not provide a proper approach for an individualized early diagnosis of hypertension in pr...