Is birth weight the major confounding factor in the study of gestational weight gain?: an observational cohort study

Much interest has been focussed on both maternal obesity and gestational weight gain (GWG), particularly on their role in influencing birth weight (BW). Several large reviews have reported that excessive GWG is associated with an increase in BW^. However recent large, well-designed, randomized controlled trials studying interventions aimed at reducing GWG have all consistently failed to show a reduction in BW despite achieving a reduction in GWG. The aim of this longitudinal prospective study was to examine the relationship between GWG and birth weight in women where GWG and Body Mass Index (BMI) were measured accurately in a strictly standardized way.

Women were enrolled at their convenience before 18 weeks gestation. Height and weight were measured accurately at the first antenatal visit and BMI calculated. Maternal weight was measured again after 37 weeks gestation. The weight of the baby was measured at birth. Relationships were tested using linear regression analysis, chi-squared tests and t-tests as appropriate.

Of the 522 women studied, the mean BMI was 25.3 kg/m² and 15.7% were obese. The mean BW at term was 3576 g (2160-5120) and 2.7% (n = 14) weighed ≥4500 g. The mean GWG overall was 12.3 kg (4.6 to 28.4) and GWG decreased as BMI increased. The mean GWG was less in obese women, at 8.7 kg (- 4.6 to 23.4), compared to non-obese,13.0 kg (0.6-28.4) (p < 0.001). Mean BW in obese women was 3630 g vs 3565 g in non-obese (p = 0.27). The total GWG correlated positively with BW (p < 0.001). When BW was subtracted from total GWG, GWG no longer correlated with BW (p = 0.12).

The positive correlation between GWG in pregnancy and BW can be accounted for by the contribution of fetal weight to GWG antenatally without a contribution from increased maternal adiposity. There was a wide range of BW irrespective of the degree of GWG and obese women had a lower GWG than non-obese women. These findings help explain why Randomized Controlled Trials (RCTs) designed to reduce GWG have failed to decrease BW and suggest there is no causative link between excessive GWG and increased BW.