Prospective data were from 83,544 pregnancies in the Norwegian Mother and Child Cohort Study. PTD was divided into early PTD (22 + 0 to 31 + 6 weeks' gestation) and late PTD (32 + 0 to 36 + 6 weeks' gestation).
The overall prevalence of PTD was 5.1 % . Increased body mass index was associated with an increased risk of PTD; adjusted odds ratio (aOR) ranged from 1.11 (95 % confidence interval [CI], 1.03-1.20) for preobesity to 2.00 (95 % CI, 1.48-2.71) for grade-III obesity in the group that included all PTD subgroups. Grade-III obese women had an increased risk of both early and late PTD: aOR, 3.24 (95 % CI, 1.71-6.14) and 1.81 (95 % CI, 1.29-2.54), respectively.
Prepregnancy maternal overweight increases the risk of both early and late PTD.