Re: Narchi H, Skinner A. 2010. Overweight and obesity in pregnancy do not adversely affect neonatal outcomes: new evidence. Journal of Obstetrics and Gynaecology 30:679–686

Dear Sir,

In their recent study, Narchi and Skinner (2010) found no evidence for an association between maternal body mass index (BMI) and several adverse neonatal outcomes. We would like to make some comments on the results and conclusions. First, there were no underweight women, while in our recent study from a similar population, almost 3% of the mothers were underweight (Khashan and Kenny 2009). It is also surprising that only 2.3% of the babies were classified as small for gestational age.

Second, the authors explained the lack of evidence of an association between maternal BMI and most neonatal outcomes as a consequence of adjustment for several potential confounders. Looking at Table IV, it appears that the combined confounding effect of those variables was almost negligible, i.e. the crude and adjusted OR estimates were similar. In few cases the crude OR was significant, while the adjusted was no longer significant (e.g. SGA), however, the lack of significance is more likely to be explained by lack of statistical power than by confounding adjustment, as the ORs remained almost unchanged.

Third, the statistics presented in Table III appear to be consistent with our recent results, with overweight and obese women having a higher risk of large for gestational age but lower risk of SGA, at very (30–34 weeks) and severely (<30 weeks) pre-term birth.

It is surprising that the authors have presented the ORs of adverse pregnancy outcomes (Table IV) in relation to overall BMI, rather than for each category as in Table III. This may have diluted the effect of specific BMI groups when they were combined with other groups. Overall, we would have expected to see the crude and adjusted ORs of neonatal outcomes in relation to overweight, moderate obesity and severe obesity compared to normal BMI. Based on the fact that the variables adjusted for in Table IV had little confounding effect and the data presented in Table III, we would conclude that maternal BMI is associated with a higher risk of some pregnancy outcomes (LGA and macrosomia) and a lower risk of other outcomes (SGA and pre-term birth).

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.