A 2006 study of 187,290 women in Scotland, published in the American Journal of Public Health, found that maternal obesity in early pregnancy is associated with an increased risk of elective preterm deliveries.  This association was much stronger in first-time moms (nulliparous women).  “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).”  What is interesting is that the risk of spontaneous preterm delivery is actually decreased – it is elective preterm delivery (induced labor) that is increased in this study.

Why would this be?  My guess is that it is partly because obesity increases the risk of other morbidity (diabetes, hypertension) which indicates early delivery, and partly because obstetric providers worry about big babies and obstetric trauma (shoulder dystocia) if obese women are allowed to go to term, so they induce them early.  In fact, if I recall correctly, obese women do not have larger babies or more true shoulder dystocia.  They are more likely to have “soft tissue dystocia,” that is, less room for the baby to get out because of increased fat in the birth canal.  This can be minimized if they can move around, squat, etc. during the pushing phase – a problem if they are epiduralized. 

This study brings up more questions than answers!  One really interesting question is why obese women are less likely to go into spontaneous preterm labor than normal weight women; another is why these women are induced.  Lastly, the $64,000 question is how to get overweight women to lose weight before becoming pregnant, as the article suggests – no easy answers to that one.   

Gordon C.S. Smith, Imran Shah, Jill P. Pell, Jennifer A. Crossley, and Richard Dobbie.  Maternal Obesity in Early Pregnancy and Risk of Spontaneous and Elective Preterm Deliveries: A Retrospective Cohort Study. American Journal of Public Health: January 2007, Vol. 97, No. 1, pp. 157-162.
doi: 10.2105/AJPH.2005.074294