Association between interpregnancy interval and hypertensive disorders of pregnancy: Effect modification by maternal age.

BACKGROUND Short and long interpregnancy intervals (IPI) are associated with increased risk of hypertensive disorders of pregnancy, yet whether this association is modified by maternal age remains unclear. OBJECTIVES To examine if the association between IPI and hypertensive disorders of pregnancy varies by maternal age at birth prior to IPI. METHODS We conducted a population-based cohort study of all mothers who had their first two (n = 169 896) consecutive births in Western Australia (WA) between 1980 and 2015. We estimated the risk of preeclampsia and gestational hypertension for 6 to 60 months of IPI according to maternal age at birth prior to IPI (<20 years, 20-24, 25-29, 30-34 and ≥35 years). We modelled IPI using restricted cubic splines and reported adjusted relative risk (RRs) with 95% CI at 6, 12, 24, 36, 48 and 60 months, with 18 months as reference. RESULTS The risk of preeclampsia was increased at longer IPIs (60 months) compared to 18 months for mothers 35 years or older (RR 2.19, 95% confidence interval (CI) 1.14, 4.18) and to a lesser extent for mothers 30- to 34 years old (RR 1.43, 95% CI 1.10, 1.84). Compared to 18 months, the risk of preeclampsia was lower at 12 months of IPI for mothers younger than 20 years (RR 0.74, 95% CI 0.57, 0.96), but not for mothers 35 years or older (RR 0.62, 95% CI 0.36, 1.07). There was insufficient evidence for increased risk of hypertensive disorders of pregnancy at shorter IPIs of <18 months for mothers of all ages. CONCLUSIONS Our findings challenge the "one size fits all" recommendation for an optimal IPI, and a more tailored approach to family planning counselling may be required to improve health.