Trends in maternal prepregnancy body mass index (BMI) and its association with birth and maternal outcomes in California, 2007–2016: A retrospective cohort study

Objective
To determine recent trends in maternal prepregnancy body mass index (BMI) and to quantify its association with birth and maternal outcomes.
Methods
A population-based retrospective cohort study included resident women with singleton births in the California Birth Statistical Master Files (BSMF) database from 2007 to 2016. There were 4,621,082 women included out of 5,054,968 women registered in the database. 433,886 (8.6%) women were excluded due to invalid or missing information for BMI. Exposures were underweight (BMI < 18.5 kg/m2), normal weight (18.5–24.9 kg/m2), overweight (25.0–29.9 kg/m2), and obese (≥ 30 kg/m2) at the onset of pregnancy. Obesity was subcategorized into class I (30.0–34.9 kg/m2), class II (35.0–39.9 kg/m2), and class III (≥ 40 kg/m2), while adverse outcomes examined were low birth weight (LBW), very low birth weight (VLBW), macrosomic births, preterm birth (PTB), very preterm birth (VPTB), small-for-gestational-age birth (SGA), large-for-gestational-age birth (LGA), and cesarean delivery (CD). Descriptive analysis, simple linear regression, and multivariate logistic regression were performed, and adjusted odds ratios (AORs) with 95% confidence intervals (CIs) for associations were estimated.
Results
Over the ten-year study period, the prevalence of underweight and normal weight women at time of birth declined by 10.6% and 9.7%, respectively, while the prevalence of overweight and obese increased by 4.3% and 22.9%, respectively. VLBW increased significantly with increasing BMI, by 24% in overweight women and by 76% in women with class III obesity from 2007 to 2016. Women with class III obesity also had a significant increase in macrosomic birth (170%) and were more likely to deliver PTB (33%), VPTB (66%), LGA (231%), and CD (208%) than women with a normal BMI. However, obese women were less likely to have SGA infants; underweight women were 51% more likely to have SGA infants than women with a normal BMI.
Conclusions
In California from 2007 to 2016, there was a declining trend in women with prepregnancy normal weight, and a rising trend in overweight and obese women, particularly obesity class III. Both extremes of prepregnancy BMI were associated with an increased incidence of adverse neonatal outcomes; however, the worse outcomes were prominent in those women classified as obese.

研究目标：明确育龄女性孕前体重指数（body mass index, BMI）的最新变化趋势，并量化其与分娩结局及母体结局的关联。

研究方法：本研究为基于人群的回顾性队列研究，纳入2007年至2016年间加州出生统计主文件（California Birth Statistical Master Files, BSMF）数据库中记录的单胎分娩常住女性。原始注册数据库中共纳入5,054,968名女性，最终纳入分析的女性共4,621,082名；另有433,886名（占比8.6%）女性因体重指数（BMI）信息无效或缺失被排除。本研究的暴露因素为妊娠启动时的体重状态：体重不足（BMI < 18.5 kg/m²）、体重正常（18.5~24.9 kg/m²）、超重（25.0~29.9 kg/m²）及肥胖（≥30 kg/m²）。其中肥胖进一步细分为I级肥胖（30.0~34.9 kg/m²）、II级肥胖（35.0~39.9 kg/m²）及III级肥胖（≥40 kg/m²）。本研究关注的不良结局包括：低出生体重（low birth weight, LBW）、极重度低出生体重（very low birth weight, VLBW）、巨大儿分娩、早产（preterm birth, PTB）、极早产（very preterm birth, VPTB）、小于胎龄儿分娩（small-for-gestational-age birth, SGA）、大于胎龄儿分娩（large-for-gestational-age birth, LGA）及剖宫产（cesarean delivery, CD）。本研究采用描述性分析、简单线性回归及多因素logistic回归进行分析，并估算了关联的校正比值比（adjusted odds ratios, AORs）及95%置信区间（confidence intervals, CIs）。

研究结果：在为期10年的研究周期内，分娩时体重不足及体重正常女性的占比分别下降了10.6%与9.7%，而超重及肥胖女性的占比分别上升了4.3%与22.9%。2007年至2016年间，极重度低出生体重（VLBW）的发生率随孕前BMI升高呈显著上升趋势：超重女性的VLBW发生率升高24%，III级肥胖女性则升高76%。与体重正常女性相比，III级肥胖女性的巨大儿分娩发生率显著升高170%，早产（PTB）、极早产（VPTB）、大于胎龄儿分娩（LGA）及剖宫产（CD）的发生风险分别升高33%、66%、231%与208%。但肥胖女性诞下小于胎龄儿（SGA）的风险更低；而体重不足女性诞下小于胎龄儿的风险较体重正常女性高出51%。

研究结论：2007年至2016年加州地区的育龄女性中，孕前体重正常的占比呈下降趋势，而超重及肥胖女性的占比持续上升，其中III级肥胖的增长趋势尤为显著。孕前体重指数处于两个极端（体重不足与肥胖）均与不良新生儿结局发生率升高相关，但肥胖女性的不良结局风险升高更为显著。