Association between pre-pregnancy BMI and preterm birth in Chinese women: a retrospective study

To explore the association between pre-pregnancy BMI (PPBMI) and preterm birth (PTB) across the Chinese population.

A total of 72,827 singleton pregnancies were included in this retrospective cohort study. Women were divided into 4 groups according to PPBMI (kg/m²): underweight (<18.5), normal weight (18.5-23.9), overweight (24-27.9) and obesity (≥28); Underweight was further categorized into severe (<16.5), moderate (16.5-17.4) and mild (17.5-18.4) underweight. PTB was grouped as spontaneous and medically indicated PTB based on clinical presentation, or grouped as 24-27, 28-31 and 32-36 weeks based on gestational age. Binary logistic regression was applied to estimate odds ratios for PTB, adjusted for potential confounders. Restricted cubic spline was used to assess potential nonlinear relationships between PPBMI and PTB.

The overall PTB rate was 4.8% (n = 3, 478). Compared with the normal weight group, the risks of PTB were similar across underweight categories: aOR = 1.10(95% CI 0.97-1.24) in mild, aOR = 0.96 (95% CI 0.79-1.18) in moderate and aOR = 1.16 (95% CI 0.85-1.60) in the severe underweight group; The risks of PTB were higher both in the overweight (aOR, 1.32; 95% CI, 1.20-1.45) and obesity (aOR, 1.47; 95% CI, 1.24-1.74) group; the trends associated with PPBMI in the risks for medically indicated PTB, spontaneous PTB and PTB at different weeks of gestation were generally consistent with those observed in overall PTB. The association between PPBMI and PTB in the study population exhibited a J-shaped curve, and the nadir of PTB risk corresponded to a PPBMI of approximately 18.5-21.5 kg/m², with a potential risk threshold around 24.5 kg/m².

In Chinese population, pre-pregnancy underweight and severity of underweight were not significantly associated with the risk of PTB; while overweight and obesity increased the risk of PTB. The nadir of PTB risk corresponded to a PPBMI of approximately 18.5-21.5 kg/m², with a potential risk threshold around 24.5 kg/m².