Logistic regression for LSES 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.

三月胎基金会（March of Dimes）发布的全球行动报告显示，多数国家的早产（preterm birth, PTB）率呈上升趋势。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。此前关于孕妇孕前体重指数（body mass index, BMI）与早产相关性的研究结论并不一致。本研究旨在评估社会经济地位（socio-economic status, SES）与体重指数的交互作用及其与早产率的关联，以填补该领域的研究空白。
本研究采用2020-2022年国家生命统计系统（National Vital Statistics System）的活产儿数据开展病例对照研究。分析结果显示，对于社会经济地位较低（lower socioeconomic status, LSES）的女性，体重指数是影响早产的显著因素：体重指数每增加1个单位，早产发生风险随之降低（优势比（odds ratio, OR）=0.923，95%置信区间（confidence interval, CI）=0.915–0.931，P<0.001）。社会经济地位较低的群体还表现出体重指数与早产的曲线关联：即随着体重指数升高，早产发生优势比逐渐降低至某一临界值后，早产率反而开始上升。这一关联在社会经济地位较高的女性群体中并未观察到。
本研究得到两项重要发现：其一，在社会经济地位较低的产妇群体中，早产存在肥胖悖论（obesity paradox）现象；其二，体重指数与早产的关联并非线性，而是呈曲线关系，这一结果弥合了既往研究结论间的分歧。本研究通过引入社会阶层维度并构建BMI多项式项（polynomial BMI term），填补了体重指数与早产关联研究的知识空白。