Maternal obesity is associated with an increased risk of late pregnancy complications such as stillbirth, gestational diabetes, and cesarean delivery. A low body mass index (BMI), however, has been reported to correlate with an increased risk of preterm delivery, and in some studies a BMI above the “normal” range is associated with decreased rate of spontaneous preterm birth. This study, based on the Scottish Morbidity Record, examined the association between maternal BMI and the risk of preterm delivery in 187,290 women. Preterm delivery was defined as childbirth before 37 weeks’ gestation.
With increasing BMI, the risk of spontaneous preterm delivery decreased while the risk of elective preterm delivery increased. Elective preterm delivery was associated with a reduced risk of neonatal death (relative risk [RR], 0.72; 95% confidence interval [CI], 0.55–0.94), and an increased risk of delivering an ELBW infant who survived for 12 months (RR, 1.92; 95% CI, 1.49–2.47). However, the net effect of BMI on these adverse outcomes varied by parity. Nulliparous women with a BMI ≥35 had an increased risk of overall preterm birth (OR, 1.34; 95% CI, 1.15–1.56) and elective preterm birth (OR, 2.13; 95% CI, 1.75–2.58), of which 40% were due to preeclampsia. They were also at increased risk of neonatal death (OR, 2.77; 95% CI, 1.54–4.99) and an ELBW infant still alive at one year (OR, 3.31; 95% CI, 2.13–5.14). In contrast, multiparous women with a BMI ≥35 were not at increased risk of overall preterm birth, neonatal death, or ELBW infant alive at one year. They were at increased risk of elective preterm birth, although to a lesser degree than nulliparas (OR, 1.45; 95% CI, 1.21–1.75) and only 18% of their elective preterm deliveries were due to preeclampsia.
These findings show that maternal obesity is associated with an increased risk of preterm delivery. Nulliparous obese women are at highest risk of elective preterm delivery, probably reflecting their increased risk of preeclampsia, as well as increased perinatal mortality and long-term disability in surviving offspring.