Rethinking causal assumptions about maternal BMI, gestational weight gain, and adverse pregnancy outcomes

Jodie M Dodd; Jennie Louise; Andrea R Deussen; Megan Mitchell; Lucilla Poston

doi:10.1186/s12916-024-03410-2

Rethinking causal assumptions about maternal BMI, gestational weight gain, and adverse pregnancy outcomes

BMC Med. 2024 May 15;22(1):197. doi: 10.1186/s12916-024-03410-2.

Authors

Jodie M Dodd^{1

2}, Jennie Louise^{3

4}, Andrea R Deussen⁵, Megan Mitchell⁵, Lucilla Poston⁶

Affiliations

¹ Department of Obstetrics and Gynaecology, The Robinson Research Institute, The University of Adelaide, Adelaide, South Australia, Australia. jodie.dodd@adelaide.edu.au.
² Department of Obstetrics and Gynaecology, Women's and Babies Division, The Women's and Children's Hospital, 72 King William Road, North Adelaide, South Australia, 5006, Australia. jodie.dodd@adelaide.edu.au.
³ SAHMRI Women and Kids, South Australian Health and Medical Research Institute, Adelaide, South Australia, Australia.
⁴ Women's and Children's Research Centre, Women's and Children's Hospital Research Network, Adelaide, South Australia, Australia.
⁵ Department of Obstetrics and Gynaecology, The Robinson Research Institute, The University of Adelaide, Adelaide, South Australia, Australia.
⁶ Women and Children's Health and School of Life Course Sciences, King's College London, London, UK.

Abstract

Background: The aim of this study was to evaluate commonly assumed causal relationships between body mass index (BMI), gestational weight gain (GWG), and adverse pregnancy outcomes, which have formed the basis of guidelines and interventions aimed at limiting GWG in women with overweight or obesity. We explored relationships between maternal BMI, total GWG (as a continuous variable and as 'excessive' GWG), and pregnancy outcomes (including infant birthweight measures and caesarean birth).

Methods: Analysis of individual participant data (IPD) from the i-WIP (International Weight Management in Pregnancy) Collaboration, from randomised trials of diet and/or physical activity interventions during pregnancy reporting GWG and maternal and neonatal outcomes. Women randomised to the control arm of 20 eligible randomised trials (4370 of 8908 participants) from the i-WIP dataset of 36 randomised trials (total 12,240 women). The main research questions were to characterise the relationship between maternal BMI and (a) total GWG, (b) the risk of 'excessive' GWG (using the Institute of Medicine's guidelines), and (c) adverse pregnancy outcomes as mediated via GWG versus other pathways to determine the extent to which the observed effect of maternal BMI on pregnancy outcomes is mediated via GWG. We utilised generalised linear models and regression-based mediation analyses within an IPD meta-analysis framework.

Results: Mean GWG decreased linearly as maternal BMI increased; however, the risk of 'excessive' GWG increased markedly at BMI category thresholds (i.e. between the normal and overweight BMI category threshold and between the overweight and obese BMI category threshold). Increasing maternal BMI was associated with increased risk of all pregnancy outcomes assessed; however, there was no evidence that this effect was mediated via effects on GWG.

Conclusions: There is evidence of a meaningful relationship between maternal BMI and GWG and between maternal BMI and adverse pregnancy outcomes. There is no evidence that the effect of maternal BMI on outcomes is via an effect on GWG. Our analyses also cast doubt on the existence of a relationship between 'excessive' GWG and adverse pregnancy outcomes. Our findings challenge the practice of actively managing GWG throughout pregnancy.

Keywords: Body mass index; Causal pathways; Dietary and lifestyle intervention; Gestational weight gain; Obesity; Pregnancy.

MeSH terms

Adult
Body Mass Index*
Female
Gestational Weight Gain* / physiology
Humans
Obesity / complications
Obesity / physiopathology
Overweight
Pregnancy
Pregnancy Complications
Pregnancy Outcome*
Randomized Controlled Trials as Topic

Grants and funding

1196133/National Health and Medical Research Council