Maternal early pregnancy body mass index and risk of preterm birth

To determine the association between maternal body mass index (BMI) in early pregnancy and the risk of preterm birth (PTB) in Chinese women.

Data were obtained from a population-based perinatal care program in China during 1993-2005. Women whose height and weight information was recorded at the first prenatal visit in the first trimester of pregnancy and delivered a singleton live infant were selected. Women with multiple gestations, stillbirths, delivery before 28 weeks or after 44 weeks of gestation, and infants affected by major external birth defects were excluded. BMI was categorized as underweight (less than 18.5 kg/m(2)), normal weight (18.5-23.9 kg/m(2)), overweight (24-27.9 kg/m(2)), and obese (≥28 kg/m(2)) based on BMI classification criteria for Chinese. Logistic regression analysis was conducted to adjust for potential confounders, such as maternal age, education, occupation, city or county, gender of infant, and year of delivery.

A total of 353,477 women were selected. The incidence of preterm birth in women who were underweight, normal weight, overweight, obese was 3.69% (3.61-3.76%), 3.59% (3.55-3.62%), 3.83% (3.71-3.96%), 4.90% (4.37-5.43%), respectively. The incidence of elective preterm birth, overweight, and obesity increased remarkably during 2000-2005 compared with that during 1993-1996. After having adjusted for potential confounders including maternal age, maternal occupation, education, city or county, gender of the infant and year of birth, the risk of PTB increased significantly with BMI (P < 0.05). Among nulliparae, the risk of elective preterm birth increased with increasing BMI. Nulliparae who were underweight were less likely to deliver elective preterm births (OR = 0.89, 95% CI 0.80-0.98). Nulliparae who were overweight and obese in early pregnancy were at a greater risk of elective PTB than normal weight nulliparae (for the overweight OR = 1.36, 95% CI 1.18-1.56, for the obese OR = 2.94, 95% CI 2.04-4.25).

In this study, indigenous Chinese cohort women who are overweight, obese, and nulliparous are at an increased risk of elective preterm birth.