The bias in current measures of gestational weight gain

Abstract

Conventional measures of gestational weight gain (GWG), such as average rate of weight gain, are likely to be correlated with gestational duration. Such a correlation could introduce bias to epidemiological studies of GWG and adverse perinatal outcomes because many perinatal outcomes are also correlated with gestational duration. This study aimed to quantify the extent to which currently used GWG measures may bias the apparent relationship between maternal weight gain and risk of preterm birth. For each woman in a provincial perinatal database registry (British Columbia, Canada, 2000-2009), a total GWG was simulated such that it was uncorrelated with risk of preterm birth. The simulation was based on serial antenatal GWG measurements from a sample of term pregnancies. Simulated GWGs were classified using three approaches: total weight gain (kg), average rate of weight gain (kg/week) or adequacy of GWG in relation to Institute of Medicine recommendations. Their association with preterm birth ≤32 weeks was explored using logistic regression. All measures of GWG induced an apparent association between GWG and preterm birth ≤32 weeks even when, by design, none existed. Odds ratios in the lowest fifths of each GWG measure compared with the middle fifths ranged from 4.4 [95% confidence interval (CI) 3.6, 5.4] (total weight gain) to 1.6 [95% CI 1.3, 2.0] (Institute of Medicine adequacy ratio). Conventional measures of GWG introduce serious bias to the study of maternal weight gain and preterm birth. A new measure of GWG that is uncorrelated with gestational duration is needed.

Figure 1

Relation between gestational age at delivery and average rate of gestational weight gain, assuming a 2 kg weight gain during the first trimester and a steady weight gain of 0.42 kg per week in the second and third trimester as recommended by United States Institute of Medicine guidelines (solid black line).

Figure 2

Observed associations between conventional measures of gestational weight gain and risk of preterm birth≤32 weeks in 128 371 women of normal-weight pre-pregnancy BMI in British Columbia, Canada (2000–2009), where gestational weight gain has been simulated to be independent of risk of preterm birth (see text for details). Gestational weight gain is expressed as a) total weight gain (kg) b)average rate of gestational weight gain (kg/week), and c) Institute of Medicine (IOM) adequacy ratio (total gestational weight gain/weight gain for gestational age recommended by 2009 IOM guidelines).

Figure 3

Serial gestational weight gain measurements in a cohort of 818 women delivering a singleton term birth at Magee-Womens Hospital in Pittsburgh, United States (open circles) with gestational weight gain expected by US Institute of Medicine guidelines) overlaid (solid line).