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Inadequate or excessive total gestational weight gain is associated with increased risks of small- and large-for-gestational-age births, respectively, but evidence is sparse regarding overall and trimester-specific patterns of gestational weight gain in relation to these risks. Characterizing the interrelationship between patterns of gestational weight gain across trimesters can reveal whether the trajectory of gestational weight gain in the first trimester sets the path for gestational weight gain in subsequent trimesters, thereby serving as an early marker for at-risk pregnancies.We sought to describe overall trajectories of gestational weight gain across gestation and assess the risk of adverse birthweight outcomes associated with the overall trajectory and whether the timing of gestational weight gain (first vs second/third trimester) is differentially associated with adverse outcomes.We conducted a secondary analysis of a prospective cohort of 2802 singleton pregnancies from 12 US prenatal centers (2009 through 2013). Small and large for gestational age were calculated using sex-specific birthweight references <5th, <10th, or ≥90th percentiles, respectively. At each of the research visits, women’s weight was measured following a standardized anthropometric protocol. Maternal weight at antenatal clinical visits was also abstracted from the prenatal records. Semiparametric, group-based, latent class, trajectory models estimated overall gestational weight gain and separate first- and second-/third-trimester trajectories to assess tracking. Robust Poisson regression was used to estimate the relative risk of small- and large-for-gestational-age outcomes by the probability of trajectory membership. We tested whether relationships were modified by prepregnancy body mass index.There were 2779 women with a mean of 15 (SD 5) weights measured across gestation. Four distinct gestational weight gain trajectories were identified based on the lowest Bayesian information criterion value, classifying 10.0%, 41.8%, 39.2%, and 9.0% of the population from lowest to highest weight gain trajectories, with an inflection at 14 weeks. The average rate in each trajectory group from lowest to highest for 0-<14 weeks was –0.20, 0.04, 0.21, and 0.52 kg/wk and for 14-39 weeks was 0.29, 0.48, 0.63, and 0.79 kg/wk, respectively; the second lowest gaining trajectory resembled the Institute of Medicine recommendations and was designated as the reference with the other trajectories classified as low, moderate-high, or high. Accuracy of assignment was assessed and found to be high (median posterior probability 0.99, interquartile range 0.99-1.00). Compared with the referent trajectory, a low overall trajectory, but not other trajectories, was associated with a 1.55-fold (95% confidence interval, 1.06–2.25) and 1.58-fold (95% confidence interval, 0.88–2.82) increased risk of small-for-gestational-age <10th and <5th, respectively, while a moderate-high and high trajectory were associated with a 1.78-fold (95% confidence interval, 1.31–2.41) and 2.45-fold (95% confidence interval, 1.66–3.61) increased risk of large for gestational age, respectively. In a separate analysis investigating whether early (<14 weeks) gestational weight gain tracked with later (≥14 weeks) gestational weight gain, only 49% (n = 127) of women in the low first-trimester trajectory group continued as low in the second/third trimester, and had a 1.59-fold increased risk of small for gestational age; for the other 51% (n = 129) of women without a subsequently low second-/third-trimester gestational weight gain trajectory, there was no increased risk of small for gestational age (relative risk, 0.75; 95% confidence interval, 0.47–1.38). Prepregnancy body mass index did not modify the association between gestational weight gain trajectory and small for gestational age (P = 0.52) or large for gestational age (P = .69).Our findings are reassuring for women who experience weight loss or excessive weight gain in the first trimester; however, the risk of small or large for gestational age is significantly increased if women gain weight below or above the reference trajectory in the second/third trimester.