Categories: For Midwives For Parents Reproductive Health

A new study published in the online advance issue of the American Journal of Public Health states that obesity raises preterm delivery risk in nulliparous women (first-time moms).  What is interesting is that the risk of spontaneous preterm delivery is actually decreased - it is elective preterm delivery (induced labor) that is increased. 

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 it 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.   

Source: American Journal of Public Health 2006; 96: Advance online publication:
Investigating the associations between body mass index and spontaneous and
elective preterm delivery.

Obese women who become pregnant for the first time are more likely than other women to undergo elective preterm delivery, increasing the risk of perinatal mortality and long-term disability among surviving offspring,conclude UK researchers.

To assess the link between maternal body mass index (BMI) and the risk of preterm delivery, Gordon Smith, from Cambridge University, and colleagues studied data on 187,290 women, calculating adjusted odds ratios for spontaneous and elective preterm deliveries.

The results showed that, irrespective of weight, nulliparous women had an increased risk of adverse pregnancy outcomes in comparison with multiparouswomen. While the risk of spontaneous preterm delivery decreased withincreasing BMI for nulliparous women, the risk of elective preterm delivery increased.

Nulliparous women who were morbidly obese were found to be at increased risk of all-cause preterm delivery, neonatal death, and delivery of a very low birth weight infant, which increases the likelihood of severe long-term disability. There was only a weak association between obesity and elective preterm delivery in multiparous women.

The team concludes: "These data indicate that morbidly obese women who are planning to conceive should be encouraged to lose weight before their first birth."