Exploring the impact of parity and its interaction with history of preterm delivery on gestational duration

Delivering preterm is the leading cause of death in neonates and children under five years of age. Both genetics and environmental factors play a role in timing of delivery, and these influences can be unique to a single pregnancy or shared across pregnancies of the same mother. The aim of this study was to understand how gestational duration is affected by parity and how parity modifies the association between history of preterm delivery and gestational duration. To investigate this, we analysed 1 118 318 spontaneous deliveries (1990 - 2012) from the Swedish Medical Birth Register, with access to pedigrees, using linear regressions and linear mixed models. We found that parity has a modest effect on the mean and a large effect on the variance of gestational duration. Interactions with a womans clinical and family history of preterm delivery revealed both pregnancy-specific and shared factors. For instance, the effect of a previous preterm delivery on gestational duration is present across pregnancies, but the magnitude of its effect is pregnancy specific. The access to pedigrees made it possible to apply linear mixed models, thus including all womans pregnancies in the model and accounting for unobserved mother-specific covariates. The linear mixed models highlighted a group effect bias when using linear regression to estimate the association between parity and gestational duration, likely caused by socioeconomic factors. Our study shed light on how parity affects gestational duration and modifies the effect of well-known risk factors of preterm delivery.