Relating patterns of blood pressure change during pregnancy to offspring birth size and gestational age at delivery in a UK birth cohort

INTRODUCTION:  In epidemiological research it is often of interest to relate trajectories of change to a later health outcome. We investigated the associations of blood pressure (BP) changes during pregnancy with offspring size at birth and duration of gestation. BP decreases until mid-pregnancy and then increases until delivery. Hypertension in the second half of pregnancy is associated with intrauterine growth restriction and preterm birth. However, it is unclear which patterns of BP change are associated with offspring risk.

METHODS:  We studied antenatal BP measurements of 9,697 women in the Avon Longitudinal Study of Parents and Children (median (interquartile range) 10 (9, 11) measurements per woman). Bivariate multilevel linear spline models with knots at 18, 30 and 36 weeks gestation were used to relate BP changes in four periods of pregnancy to perinatal outcomes. Regression coefficients adjusted for earlier change were derived from the variances and covariances of random effects, and confidence intervals calculated by simulating from their joint distribution.

RESULTS:  A greater increase in systolic BP (SBP) and diastolic BP (DBP) between 18 and 36 weeks gestation was associated with lighter and smaller-for-gestational age offspring. The confounder-adjusted mean difference (95% CI) in birthweight for a 1 mmHg/week greater increase in SBP between 18-30 weeks was -71g (-134, -14) and between 30-36 weeks was -175g (-208, -145). A smaller decrease in SBP and DBP prior to 18 weeks and a greater increase between 18 and 36 weeks was associated with a shorter gestation. Associations remained strong when restricting to normotensive pregnancies.

CONCLUSIONS:  The use of bivariate multilevel models enabled us to relate complex patterns of change to continuous outcome variables whilst adjusting for baseline values and earlier change. We found that a greater rise in BP from 18 weeks gestation onwards is associated with reduced fetal growth and an earlier delivery.