In perinatal epidemiology, the basic unit of analysis has traditionally been the individual pregnancy. In this study, we sought to explore the idea of a ‘reproductive life’-based approach to modelling the effects of reproductive exposures and outcomes, where the basic unit of analysis is a woman's entire reproductive experience. Our objective was to explore whether a first pregnancy risk factor, excess gestational weight gain, has a direct effect on the birthweight outcomes of a subsequent pregnancy, independent of the weight gain and other risk factors of the second pregnancy. A study population was created by linking the obstetric records of 1220 women who delivered their first and second offspring at a McGill University teaching hospital in Montreal, Canada. Multivariable linear and logistic regression analyses were used to model the effects of gestational weight gain above recommendation on the birthweight Z-score and risk of large-for-gestational age (LGA) subsequent offspring.
After adjusting for the risk factors of the second pregnancy, an independent effect from the first pregnancy was seen on the birthweight Z-score, (effect size OR 0.17 [95% CI 0.05, 0.28] but not risk of LGA of the second pregnancy 1.30 [95% CI 0.89, 1.89]). We concluded that a pregnancy-centred approach to research that conceptualises pregnancies as self-contained and interchangeable events may not always be appropriate, and propose that analytical methods for some perinatal research questions may need to consider a given pregnancy in the context of a woman's past reproductive experiences.