Background Post-term births and preterm births (PTBs) are associated with increased risks for both mother and baby, and have resource implications for NHS maternity services. Increased body mass index (BMI) is associated with both PTB and post-term birth. Extremely obese women (BMI ≥ 50 kg/m²) and their babies have the greatest obesity-related risks in pregnancy. However, there is a lack of research in this population on gestational age at delivery. The aim of this study was to investigate the association between maternal extreme obesity and PTB and post-term birth.

Methods We included women who delivered singleton live births in 16 UK maternity units between 1990–2007. We categorised gestational age at delivery as extreme PTB (20–27 weeks), very PTB (28–31 weeks), moderately PTB (32–36 weeks), early-term birth (37–38 weeks), full-term birth (39–40 weeks), late-term birth (41 weeks) and post-term birth (≥42 weeks). Women with missing gestational age at delivery (n = 53,514, 10.1%) were excluded. Maternal BMI was categorised as underweight (<18.5 kg/m²), recommended weight (18.5–24.9 kg/m²), overweight (25–29.9 kg/m²), moderately obese (30–34.9 kg/m²), severely obese (35–39.9 kg/m²), morbidly obese (40–49.9 kg/m²) and extremely obese). The risk of birth occurring in each gestational age category (compared to full-term birth), according to maternal BMI was estimated using Multinomial logistic regression. Models were adjusted for parity, gestational age at booking, maternal socioeconomic status, employment, ethnicity and maternal age. Missing data was imputed using multiple imputation.

Results Of 477,777 women with data on gestational age at delivery, 2.7% were underweight, 46.0% were recommended weight, 21.5% were overweight, 8.1% were moderately obese, 2.9% were severely obese, 1.2% were morbidly obese and 0.1% were extremely obese. The odds of extreme PTB, very PTB, moderately PTB and post-term birth were increased in all categories of obesity, but the effect sizes were consistently highest in extremely obese women. For example, compared to recommended weight women, extremely obese women were at 2.7 times the odds of extreme PTB (95% confidence interval (CI): 1.2–6.2; p = 0.002), 1.7 times the odds of very PTB (95% CI: 0.7–4.1; p = 0.246), 2.2 times the odds of moderately PTB (95% CI: 1.6–3.0; p < 0.001), 1.8 times the odds of early term birth (95% CI: 1.4–2.3; p < 0.001), 1.3 times the odds of late term birth (95% CI: 1.0–1.6; p = 0.080) and 2.3 times the odds of post-term birth (95% CI: 1.6–3.1; p < 0.001).

Conclusion While all obese women were at increased risk of PTB and post-term birth, extremely obese women were at the greatest risk, almost three times more likely to have an extreme PTB, which poses the greatest risk to the baby.