Abstract
BackgroundThe extremes of maternal pre-pregnancy body mass index (BMI) are known to be risk factors associated with obstetric and adverse perinatal outcomes. Among Japanese women aged 20 years or older, the prevalence of underweight (BMI < 18.5 kg/m2) was 11.5% in 2019. Maternal thinness is a health problem caused by the desire to become slim. This study aimed to investigate the association between the severity of maternal low pre-pregnancy BMI and adverse perinatal outcomes, including preterm birth (PTB), low birth weight (LBW), and small-for-gestational age (SGA).MethodsWe conducted a prospective cohort study using data from the Japan Environment and Children’s Study, which recruited pregnant individuals between 2011 and 2014. Pre-pregnancy BMI was categorized as severe-moderate underweight (BMI < 16.9 kg/m2), mild underweight (BMI, 17.0–18.4 kg/m2), low-normal weight (BMI, 18.5–19.9 kg/m2), high-normal weight (BMI, 20.0–22.9 kg/m2), overweight (BMI, 23.0–24.9 kg/m2), and obese (BMI ≥ 25.0 kg/m2). The high-normal weight group was used as the reference for statistical analyses. Adjusted logistic regression was performed to evaluate the association between pre-pregnancy BMI and PTB, LBW, and SGA.ResultsOf 92,260 singleton pregnant individuals, the prevalence was 2.7% for severe-moderate underweight, 12.9% for mild underweight, and 24.5% for low-normal weight. The prevalence of adverse outcomes was 4.6% for PTB, 8.1% for LBW, and 7.6% for SGA. The adjusted odds ratios (aORs) for PTB were 1.72 (95% confidence interval [CI], 1.46–2.03) for severe-moderate underweight and 1.26 (95% CI, 1.14–1.39) for mild underweight. The aORs of LBW were 2.55 (95% CI, 2.27–2.86) for severe-moderate underweight, 1.64 (95% CI, 1.53–1.76) for mild underweight, and 1.23 (95% CI, 1.16–1.31) for low-normal weight. The aORs of SGA were 2.53 (95% CI, 2.25–2.84) for severe-moderate underweight, 1.66 (95% CI, 1.55–1.79) for mild underweight, and 1.29 (95% CI, 1.21–1.38) for low-normal weight.ConclusionsA dose-response relationship was found between the severity of low pre-pregnancy BMI and PTB, LBW, and SGA. Even low-normal BMI (18.5–19.9 kg/m2) increased the risk of LBW and SGA. This study provides useful information for pre-conception counseling in lean individuals.
Highlights
The extremes of maternal pre-pregnancy body mass index (BMI) are known to be risk factors associ‐ ated with obstetric and adverse perinatal outcomes
Of the participants who had both sets of data, 65.4% reported the same weight in both sets
Yes Missing Autoimmune disease No Yes Missing Thyroid disease No Yes Missing Gestational age at delivery Birth weight Neonatal sex Male Female Missing Preterm birth (< 37 weeks’ gestation) Very preterm birth (< 34 weeks’ gestation) Low birth weight (< 2500 g) Very low birth weight (< 1500 g) Extremely low birth weight (< 1000 g) Small-for-gestational age No Yes Missing a Values are presented as n (%) or median was 17.4% (2515/14,448) for severe-moderate underweight (BMI < 16.9 kg/m2) and 82.6% (11,933/14,448) for mild underweight (BMI, 17.0–18.4 kg/m2)
Summary
The extremes of maternal pre-pregnancy body mass index (BMI) are known to be risk factors associ‐ ated with obstetric and adverse perinatal outcomes. This study aimed to investigate the association between the severity of maternal low pre-pregnancy BMI and adverse perinatal outcomes, including preterm birth (PTB), low birth weight (LBW), and small-for-gestational age (SGA). The extremes of maternal pre-pregnancy body mass index (BMI) are known to be risk factors associated with obstetric and adverse perinatal outcomes [1,2,3,4,5,6,7]. Several studies reported that low maternal pre-pregnancy BMI was significantly associated with adverse perinatal outcomes, such as PTB, LBW, and SGA [1, 4, 5, 7,8,9,10,11, 15]. Two large cohort studies in Japan reported the association between pre-pregnancy underweight and PTB and SGA, the severity of maternal pre-pregnancy underweight was not evaluated [3, 16]
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