Abstract-To recognize the highly statistically significant circadian variability of blood pressure in pregnancy is to admit that the diagnosis of gestational hypertension or preeclampsia should be based not just on whether a casual blood pressure value is too high or too low, but rather on more pertinent questions: How long is blood pressure elevated above a given time-varying threshold? What is the excess blood pressure? When does most of the excess occur? Answers to these questions may be obtained by establishing (1) an adequate reference threshold for blood pressure and (2) a proper measurement of blood pressure elevation. Accordingly, we derived time-specified reference standards for blood pressure as a function of gestational age. We analyzed 1408 blood pressure series systematically sampled by ambulatory monitoring for 48 consecutive hours every 4 weeks from the first obstetric visit (usually within the first trimester of pregnancy) until delivery in 235 women with uncomplicated pregnancies. Data from each blood pressure series were synchronized according to the rest-activity cycle of each individual to avoid differences among women in actual times of daily activity. Data were then used to compute 90% circadian tolerance intervals for each trimester of pregnancy, in keeping with the trends in blood pressure along gestation previously documented. The method, derived on the basis of bootstrap techniques, does not need to assume normality or symmetry in the data, and therefore, it is highly appropriate to describe the circadian pattern of blood pressure variability. Results not only reflect expected changes in the tolerance limits as a function of gestational age, but also upper limits markedly below the thresholds currently used for diagnosing hypertension in pregnancy. The use of these time-qualified tolerance limits for the computation of a hyperbaric index as a measure of BP excess has already been show to provide high sensitivity and specificity in the early identification of gestational hypertension and preeclampsia. Key Words: blood pressure Ⅲ heart rate Ⅲ pregnancy Ⅲ circadian Ⅲ blood pressure monitoring, ambulatory T he construction of time-specified reference limits requires systematic sampling in clinical health, in particular for those variables that are characterized by circadian rhythms of large amplitude, as is the case for blood pressure (BP). To date, BP determined casually in the examiner's office has been commonly used to diagnose hypertension and evaluate treatment efficacy. 1 These conventional timeunspecified single measurements may be misleading because BP and heart rate (HR) vary according to a spectrum of rhythms with several frequencies (the circadian in particular) and because measurements may be influenced by a host of internal factors, such as autonomic nervous system tone, vasoactive hormones, and hematologic and renal variables, plus a variety of external factors, including ambient temperature/humidity, physical activity, emotional state, alcohol/ caffeine consumption, meal comp...