Maternal Obesity in Early Pregnancy and Risk of Spontaneous and Elective Preterm Delivery: A Retrospective Cohort Study

Gordon C.S. Smith, MD, PhD; Imran Shah, MSc; Jill P. Pell, MD; Jennifer A. Crossley, PhD; Richard Dobbie, BSc


Am J Public Health. 2007;97(1):157-162. 

In This Article

Abstract and Introduction


Objectives: We sought to determine the association between maternal body mass index and risk of preterm delivery.
Methods: We assessed 187290 women in Scotland and estimated adjusted odds ratios for spontaneous and elective preterm deliveries among overweight, obese, and morbidly obese women relative to normal-weight women.
Results: Among nulliparous women, the risk of requiring an elective preterm delivery increased with increasing BMI, whereas the risk of spontaneous preterm labor decreased. Morbidly obese nulliparous women were at increased risk of all-cause preterm deliveries, neonatal death, and delivery of an infant weighing less than 1000 g who survived to 1 year of age (a proxy for severe long-term disability). By contrast, obesity and elective preterm delivery were only weakly associated among multiparous women.
Conclusions: Obese nulliparous women are at increased risk of elective preterm deliveries. This in turn leads to an increased risk of perinatal mortality and is likely to lead to increased risks of long-term disability among surviving offspring.


The association between maternal obesity and pregnancy outcomes is complex. Maternal obesity is known to be associated with increased rates of complications in late pregnancy such as stillbirth, cesarean delivery, gestational diabetes, and shoulder dystocia.[1–3] However, a low body mass index (BMI) is associated with an increased risk of preterm delivery, and some studies have shown that BMIs above the "normal" range are protective against spontaneous preterm births.[4–6] A large-scale retrospective cohort study demonstrated an interaction between BMI and parity: obese nulliparous women were at increased risk of extreme preterm deliveries and neonatal death, whereas obese multiparous women were not at increased risk of these outcomes.[7] The reasons for these complex patterns of association are unclear.

Preterm deliveries can occur as a result of preterm labor or can be elective procedures. Preeclampsia is the reason for elective preterm deliveries in more than 40% of cases.[4] It is well recognized that obese women are at increased risk of preeclampsia[8] and that nulliparous women are at higher risk of preeclampsia than multiparous women. We hypothesized that the higher background risk of preeclampsia among nulliparous women might lead to a stronger association between obesity and elective preterm deliveries and might therefore explain the association between obesity and extreme preterm deliveries among these women.

Our aim was to determine the association between maternal obesity in early pregnancy and risk of preterm delivery, with attention given to type of delivery (spontaneous vs elective), parity (nulliparous vs multiparous), and the most important negative consequences of prematurity. In assessing consequences of prematurity, we examined both neonatal death and long-term survival of extremely low-birthweight (ELBW) infants. Because ELBW infants have a 40% to 45% risk of severe neurodevelopmental delays in childhood,[9] we used ELBW as a proxy measure of severe long-term morbidity.


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