Abstract
Cross-sectional studies of total gestational weight gain (GWG) and perinatal outcomes have used different approaches to operationalize GWG and adjust for gestational duration. Using birth records from California (2007-2017), Nevada (2010-2017), and Oregon (2008-2017) we compared three commonly used approaches to estimate associations between GWG and cesarean delivery (C-section), small for gestational age (SGA), and low birth weight (LBW)]: (1) the Institute of Medicine r ecommended GWG ranges at a given gestational week, (2) total weight gain categories directly adjusting for gestational age as a covariate, and (3) weight-gain-for-gestational-age z-scores derived from an external longitudinal reference population. Among 5,461,130 births, the three methods yielded similar conclusions for C-section and SGA. However, for LBW, some associations based on z-scores were in the opposite direction of methods 1 and 2, paradoxically suggesting higher GWG increases risk of LBW. This was due to a greater proportion of preterm births among those with high z-scores, and controlling for gestational age in the z-score model brought the results in line with the other methods. We conclude that the use of externally-derived GWG z-scores based on ongoing pregnancies can yield associations confounded by duration of pregnancy when the outcome is strongly associated with gestational age at delivery.
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