Background: Small-for-gestational-age in infancy is a known risk factor not only for short-term prognosis but also for several long-term outcomes, such as neurological and metabolic disorders in adulthood. Previous research has shown that severe nausea and vomiting in early pregnancy (NVP) and hyperemesis gravidarum, which is an extreme form of NVP, represent risk factors for small-for-gestational-age birth. However, there is no clear consensus on this association. Thus, in the present study, we investigated the correlation between hyperemesis gravidarum and NVP on the one hand, and infant birth weight on the other, using data from the Japan Environment and Children's Study (JECS). Methods: The data utilized in the present study were obtained from the JECS, an ongoing cohort study that began in January 2011. Our sample size was 8635 parent-child pairs. The presence or absence of severe NVP, hyperemesis gravidarum, and potential confounding factors were noted. A multivariable regression analysis was used to estimate risks for small-for-gestational-age birth, and the results were expressed as risk ratios and 95 % confidence intervals. Results: The risk ratios of small-for-gestational-age birth (95 % confidence interval) for mothers with severe NVP and those with hyperemesis gravidarum were 0.86 (0.62-1.19) and 0.81 (0.39-1.66), respectively, which represents a non-significant result. Conclusions: In our analysis of JECS data, neither severe NVP nor hyperemesis gravidarum was associated with increased risk for small-for-gestational-age birth.
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