The effects of maternal body mass index on pregnancy outcome

  • A. S. Khashan
  • L. C. KennyEmail author
Perinatal Epidemiology


The increasing prevalence of obesity is presenting a critical challenge to healthcare services. We examined the effect of Body Mass Index in early pregnancy on adverse pregnancy outcome. We performed a population register-based cohort study using data from the North Western Perinatal survey (N = 99,403 babies born during 2004–2006), based at The University of Manchester, UK. The main outcome measures were Caesarean section delivery, preterm birth, neonatal death, stillbirth, Macrosomia, small for gestational age and large for gestational age. The risk of preterm birth was reduced by almost 10% in overweight (RR = 0.89, [95% CI: 0.83, 0.95]) and obese women (RR = 0.90, [95% CI: 0.84, 0.97]) and was increased in underweight women (RR = 1.33, [95% CI: 1.16, 1.53]). Overweight (RR = 1.17, [95% CI: 1.09, 1.25]), obese (RR = 1.35, [95% CI: 1.25, 1.45]) and morbidly obese (RR = 1.24, [95% CI: 1.02, 1.52]) women had an elevated risk of post-term birth compared to normal women. The risk of fetal macrosomia and operative delivery increased with BMI such that morbidly obese women were at greatest risk of both (RR of macrosomia = 4.78 [95% CI: 3.86, 5.92] and RR of Caesarean section = 1.66 [95% CI: 1.61, 1.71] and a RR of emergency Caesarean section = 1.59 [95% CI: 1.45, 1.75]). Excessive leanness and obesity are associated with different adverse pregnancy outcomes with major maternal and fetal complications. Overweight and obese women have a higher risk of macrosomia and Caesarean delivery and lower risk of preterm delivery. The mechanism underlying this association is unclear and is worthy of further investigation.


Obesity Body mass index Preterm birth Birthweight Pregnancy outcome 



The authors would like to thank Ms Cathie Forest from the North Western Perinatal Survey (University of Manchester) directed by Professor Philip Baker for their assistance in providing the data. Funding: This study was funded by Tommy’s the Baby Charity and the Heath Research Board, Ireland.


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Copyright information

© Springer Science+Business Media B.V. 2009

Authors and Affiliations

  1. 1.The Anu Research Centre, Department of Obstetrics and Gynaecology, Cork University Maternity HospitalUniversity College CorkCorkIreland
  2. 2.The Maternal and Fetal Health Research Centre, St. Mary’s HospitalUniversity of ManchesterManchesterUK

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