Objective To investigate the effect of maternal body mass index (BMI) on postdates pregnancy, length of gestation and likelihood of spontaneous onset of labour at term.
Design Retrospective cohort study.
Setting Swedish Medical Birth Register.
Population A total of 186 087 primiparous women (of whom 143 519 had spontaneous onset of labour at term) who gave birth between 1998 and 2002.
Methods Mann–Whitney test, one-way analysis of variance, linear regression and single variable logistic regression.
Main outcome measures Postdates pregnancy (≥294 days or 42+0 weeks), length of gestation and likelihood of spontaneous onset of labour at term.
Results About 6.8% of pregnancies delivered postdates. Higher maternal BMI (kg/m2) during the first trimester was associated with longer gestation (P < 0.001) as was a greater change in BMI between the first and third trimesters (BMI measured on admission prior to delivery) with mean (SD) gestation at delivery of 280.7 (8.6) and 283.2 (8.6) days for increases in BMI of <2 and ≥10 kg/m2, respectively. Higher BMI during the first trimester was associated with a lower chance of spontaneous onset of labour at term. Compared with BMI 20 to <25 kg/m2, the odds ratios (95% CI) for spontaneous onset of labour at term were 1.21 (1.15–1.27) for BMI of <20 kg/m2, 0.71 (0.69–0.74) for BMI of 25 to <30 kg/m2, 0.57 (0.54–0.60) for BMI of 30 to <35 kg/m2 and 0.43 (0.40–0.47) for BMI of ≥35 kg/m2. Higher BMI during the first trimester (BMI of ≥35 kg/m2 compared with BMI of 20 to <25 kg/m2) was also associated with an increased risk of complications including stillbirth (OR 3.90, 95% CI 2.44–6.22), gestational diabetes (OR 5.61, 95% CI 4.61–6.83) and caesarean section (OR 2.39; 95% CI 2.20–2.59).
Conclusions Higher maternal BMI in the first trimester and a greater change in BMI during pregnancy were associated with longer gestation and an increased risk of postdates pregnancy. Higher maternal BMI during the first trimester was also associated with decreased likelihood of spontaneous onset of labour at term and increased likelihood of complications.