The impact of past pregnancy experience on subsequent perinatal outcomes.

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 conceptualizes 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.