Background: We aimed to investigate whether there was a high incidence of young Japanese mothers being underweight (pregravid body mass index [BMI] < 18.5 kg/m2) and if they delivered infants who were small for gestational age (SGA).Materials and methods: Our study was carried out from 2013 to 2017 and included adolescent women (maternal age at delivery: <20 years old; n = 94), women in their early twenties (maternal age at delivery: 20-24 years; n = 479), and controls (maternal age at delivery: 25-34 years; n = 2061).Results: In both the adolescent and early twenties groups, the incidence of being underweight, based on prepregnancy BMI, was significantly higher than controls (p < .0001 and p < .05, respectively). In both groups, gestational weight gain (GWG) was significantly higher than controls (p < .0001). There were no notable group differences in the incidences of SGA.Conclusion: The incidence of being underweight was higher in younger mothers than in controls. However, the risk of SGA did not increase in younger mothers, possibly because their GWG overcame the influence of their pre-pregnancy body composition.
Keywords: Adolescent; Japanese; gestational weight gain; small for gestational age; underweight.