Background: Pregnancy hypertension rates have declined in Europe, Canada and Australia, despite increasing rates of obesity, nulliparity and maternal age. Increasing rates of early planned delivery have been hypothesised as the reason for the decline. Therefore, the aim of this study was to explore whether increases in planned deliveries explain the decline.
Method: Data were obtained from linked birth and hospital records for ~1.08M deliveries in New South Wales, 2001-2012. Pregnancy hypertension included gestational hypertension, preeclampsia and eclampsia, and planned delivery included labour induction and prelabour caesarean section. Annual gestational age-specific pregnancy hypertension rates were determined among pregnancies-at-risk and assessed using chi-square for trend (P<0.001). Multivariable predictive logistic regression models for pregnancy hypertension were developed using data from 2001-2 and data from other years applied to the regression equation to produce predicted trends that accounted for the actual changes in maternal factors.
Results: The overall pregnancy hypertension rate declined from 9.9% in 2001 to 7.7% in 2012 (trend P<0.001). Contemporaneously, planned deliveries increased from 38% to 46% of all deliveries and the proportion occurring at 37-39 weeks increased from 47% to 59%. Maternal risk factors that increased (P<0.01) over the study period included nulliparity, age ≥35 years, diabetes, overweight/obesity, and use of assisted reproductive technologies; while risk factors that decreased included multi-fetal pregnancies, age <20 years, autoimmune diseases and previous pregnancy hypertension. Among women who did not develop pregnancy hypertension, the established risk factors for pregnancy hypertension were positively associated with planned births at 37-39 weeks. Gestational age-specific pregnancy hypertension rates declined from 38 weeks onwards, most steeply at ≥41 weeks. Based on changes in the prevalence of maternal risk factors, the pregnancy hypertension rate was predicted to increase to 10.5%. At least 36% of the decline in pregnancy hypertension was explained by early planned births.
Conclusions: An overall increase in early planned delivery explained part of the observed decline in pregnancy hypertension rates in New South Wales.