Introduction: The average birth weight in Japan has gradually decreased and the number of low birth weight infant has increased over the last 30 years. Japanese pregnant women are characterized with lower prevalence of obesity than those in other developed countries, and maternal underweight before pregnancy is independently associated with low birth weight. However, the association between maternal inadequate gestational weight gain (GWG) and the risk of small for gestational age (SGA) in Japanese uncomplicated pregnant women has not been fully understood. This study aimed to examine the effect of maternal inadequate GWG on the risk of SGA in Japanese uncomplicated pregnancies. Methods: We retrospectively analyzed uncomplicated Japanese singleton pregnancies that delivered at term gestation in our institution from 2006 to 2016. The association between GWG and birth weight was analyzed by multiple linear regression. Potential confounding factors included maternal age, parity, prepregnancy BMI, and neonatal sex. The association between inadequate GWG and SGA was also examined by logistic regression. Results: A total of 3837 mother–neonate dyads were analyzed. Maternal GWG was 10.1 ± 3.7 kg (mean ± SD), and 2529 (66%) had inadequate GWG. After adjusting for confounding factors, GWG significantly correlated with birth weight (standardized β = 0.199, p <.001). Inadequate GWG increased the risk of delivering SGA neonate (adjusted odds ratio (aOR) = 1.97 [1.45–2.68], p <.001). This association was particularly pronounced in underweight (aOR = 2.95 [1.38–6.29], p =.005) and normal weight mothers (aOR = 1.79 [1.27–2.52], p =.001), and not in overweight/obese mothers (p =.115). Conclusion: Maternal GWG is associated with birth weight in Japanese women with uncomplicated singleton pregnancies. Inadequate GWG is an important risk factor of SGA, particularly in non-obese women. The present finding would potentially provide further evidence that promoting adequate gestational weight gain in Japanese underweight and normal weight mothers would reduce SGA, as well as metabolic dysfunction in later life.
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