Baseline characteristics of overall population.

The March of Dimes Global action report indicated that preterm birth (PTB) rates are increasing in most countries. It is the most important cause of neonatal deaths and the second leading cause of death in children under age 5. Literature reporting the relationship between maternal pre-pregnancy body mass index (BMI) and PTB has previously yielded inconsistent conclusions. Our objective is to fill in the knowledge gap by evaluating the interaction of socio-economic status (SES) and BMI and its relationship to the rate of PTB. This is a case control study using the Natality Data of the National Vital Statistics System from the years 2020–2022. BMI was a significant factor in PTB for lower socioeconomic status (LSES) women. For every increase in BMI, there was a decrease in the probability of PTB (OR = 0.923, 95% CI 0.915–0.931, P < 0.001). Those who were LSES also had a curved relationship with PTB indicating that the as BMI increases, the odds of PTB decreases up until a BMI value, then the PTB rate increases. This relationship was not found in higher economic status women. Our study had two significant findings. We first found an obesity paradox in PTB for those mothers who are LSES. We also found that the relationship between BMI and PTB was not linear but curvilinear, bridging the gap in the conclusions of other studies. This study fills in the knowledge gap of BMI and PTB by adding the consideration of social class and by creating a polynomial BMI term.

《美国三月母婴健康基金会（March of Dimes）全球行动报告》显示，多数国家的早产（preterm birth, PTB）率正逐步上升。早产是新生儿死亡的首要病因，也是5岁以下儿童死亡的第二大诱因。既往针对孕前母亲体重指数（body mass index, BMI）与早产关联的研究，结论始终存在分歧。本研究旨在填补这一认知空白，通过评估社会经济地位（socio-economic status, SES）与体重指数的交互作用及其与早产率的关联展开分析。本研究采用2020-2022年国家生命统计系统（National Vital Statistics System）的出生登记数据开展病例对照研究。分析结果表明，体重指数对低社会经济地位（lower socioeconomic status, LSES）女性的早产发生风险存在显著影响：体重指数每升高1个单位，早产发生概率均显著降低（比值比OR=0.923，95%置信区间CI：0.915~0.931，P<0.001）。此外，低社会经济地位人群中，体重指数与早产的关联呈曲线特征：随着体重指数升高，早产发生风险先逐步降低，达到某一临界值后，早产率反而开始上升；而这一关联在高社会经济地位人群中并未观测到。本研究获得两项重要发现：其一，在低社会经济地位的孕产妇群体中，早产存在肥胖悖论现象；其二，体重指数与早产的关联并非线性关系，而是呈曲线型，这一结果弥合了既往相关研究结论的分歧。本研究通过纳入社会阶层因素并构建多项式BMI项，填补了体重指数与早产关联研究的认知空白。