Oral Presentation Australian and New Zealand Obesity Society Annual Scientific Conference 2024

Rethinking the causal assumptions about maternal BMI, gestational weight gain and adverse pregnancy outcomes (#102)

Jodie M Dodd 1 2 , Jennie Louise 3 4 , Andrea R Deussen 1 , Megan Mitchell 1 , Lucilla Poston 5
  1. Robinson Research Institute, University of Adelaide, Adelaide, South Australia, Australia
  2. Department of Obstetrics and Gynaecology, Women’s and Babies Division, The Women’s and Children’s Hospital, Adelaide, South Australia, Australia
  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. Women and Children’s Health and School of Life Course Science, King's College London, London, United Kingdom

Weight management during pregnancy is guided by the Institute of Medicine (IOM) recommendations for gestational weight gain (GWG) (1, 2) for body mass index (BMI) category (‘normal’, ‘overweight’ and ‘obese’) aiming to reduce the risk of adverse pregnancy outcomes. We aimed to evaluate assumed causal relationships between BMI, GWG and adverse pregnancy outcomes, including infant birthweight, birthweight z-score, large for gestational age infant, and caesarean birth.

Individual participant data (IPD) from the International Weight Management in Pregnancy (i-WIP) Collaboration (3), from the control groups of randomised trials of diet and/or physical activity interventions during pregnancy, were analysed. Twenty of 36 eligible trials (4370 of 8908 participants) from the i-WIP dataset were included. We characterised the relationship between maternal BMI and (a) total GWG, (b) ‘excessive’ GWG (per IOM guidelines), and (c) adverse pregnancy outcomes to determine the extent to which the observed effect of maternal BMI on pregnancy outcomes is mediated via GWG. Generalised linear models and regression-based mediation analyses within an IPD meta-analysis framework were utilised.

Mean GWG decreased linearly as maternal BMI increased; however, the number of women with ‘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.

There is evidence of a 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.

  1. Institute of Medicine (US) Committee on Nutritional Status During Pregnancy and Lactation. Nutrition during pregnancy: Part I: weight gain, Part II: nutrient supplements. 1990.
  2. Institute of Medicine (US) and National Research Council, (US) Committee to Reexamine IOM Pregnancy Weight Guidelines. Weight gain during pregnancy: reexamining the guidelines. 2009.
  3. International Weight Management in Pregnancy (i-WIP) Collaborative Group. Effect of diet and physical activity based interventions in pregnancy on gestational weight gain and pregnancy outcomes: meta-analysis of individual participant data from randomised trials. BMJ. 2017;358:j3119.