# An obesity paradox in preterm birth: A case control study

## TL;DR

This study finds that higher BMI in low socioeconomic status women is linked to lower preterm birth rates, but only up to a point, revealing a complex relationship.

## Contribution

The study introduces a curvilinear relationship between BMI and preterm birth, mediated by socioeconomic status, resolving prior inconsistencies.

## Key findings

- Higher BMI in low socioeconomic status women is associated with lower odds of preterm birth up to a certain BMI threshold.
- The relationship between BMI and preterm birth is curvilinear, not linear, for low socioeconomic status women.
- Socioeconomic status moderates the effect of BMI on preterm birth rates.

## Abstract

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.