What is the best measure of maternal complications of term pregnancy: Ongoing pregnancies or pregnancies delivered?

OBJECTIVE: The purpose of this study was to determine whether rates of hypertensive disorders of pregnancy increase beyond 37 weeks of gestation and to address how best to analyze these rates. STUDY DESIGN: This was a retrospective cohort study of all women delivered beyond 37 weeks' gestational age from 1995 to 1999 at all Kaiser Permanente Medical Care Program delivery hospitals in Northern California. Rates of gestational hypertension, preeclampsia, and eclampsia were calculated by use of both pregnancy delivered (PD) and ongoing pregnancy (OP) as the denominator. Bivariate and multivariate analyses were conducted with use of P < .05 to indicate statistical significance. RESULTS: Among the 135,560 women in this cohort, the rates of gestational hypertension, preeclampsia, and eclampsia were the same or decreased from 37 to 43 weeks' gestation using PD, but all three increased when calculated according to OP (P < .01). CONCLUSION: We found that among complications of pregnancy that are diagnosed ante partum, use of a different denominator led to contradictory conclusions. When hypertensive disorders of pregnancy are analyzed, ongoing pregnancies should be used as the denominator.