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Epidemiology and Population Health

Pre-pregnancy body mass index and adverse maternal and perinatal outcomes in twins: A population retrospective cohort study



To examine the association between pre-pregnancy BMI and severe maternal morbidity (SMM), perinatal death and severe neonatal morbidity in twin pregnancies.


All twin births at ≥ 20 weeks gestation in British Columbia, Canada, from 2000 to 2017 were included. We estimated rates of SMM, a perinatal composite of death and severe morbidity, and its components per 10,000 pregnancies. Confounder-adjusted rate ratios (aRR) between pre-pregnancy BMI and outcomes were estimated using robust Poisson regression.


Overall, 7770 (368 underweight, 1704 overweight, and 1016 obese) women with twin pregnancy were included. The rates of SMM were: 271.1, 320.4, 270.0, and 225.9 in underweight, normal BMI, overweight and obese women, respectively. There was little association between obesity and any of the primary outcomes (e.g., aRR = 1.09, 95% CI = 0.85, 1.38 for composite perinatal outcome). Underweight women had higher rates of the composite perinatal adverse outcome (aRR = 1.79, 95% CI = 1.32–2.43), driven by increased rates of severe respiratory distress syndrome, and neonatal death.


There was no evidence of elevated risk of adverse outcomes among twin pregnancies of women who were overweight or obese. Risk was higher in underweight women, who may require specific care when carrying twins.

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Fig. 1: Adjusted risk of adverse outcomes by pre-pregnancy body-mass-index.

Data availability

Access to data provided by the Data Steward(s) is subject to approval but can be requested for research projects through the Data Steward(s) or their designated service providers. All inferences, opinions, and conclusions drawn in this publication are those of the author(s), and do not reflect the opinions or policies of the Data Steward(s).


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The authors would like to thank Dr. Cande Anath for his thoughtful feedback on a previous version of this manuscript.


This study was supported by funding from the Sick Kids Foundations (grant number SKF-154852). JB is supported by a four-year PhD fellowship award from the University of British Columbia (award number 6456). KSJ is supported by an Investigator award from the BC Children’s Hospital Research Institute. SL is supported by a scholar award from the Michael Smith Foundation for Health Research. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. All inferences, opinions and conclusions drawn in this paper are those of the authors and do not reflect the opinions or policies of the Data Stewards.

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SL and JB designed the study and formulated the research question. JB analyzed the data and wrote the first draft of the manuscript with supervision from SL. KS, LAM, LW, and CM read and reviewed the manuscript and provided critical feedback and suggested additional analyses. All authors approved the final version.

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Correspondence to Jeffrey N. Bone.

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The authors declare no competing interests.

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Ethics approval for this study was obtained from the University of British Columbia - Children’s and Women’s Hospital and Health Centre of British Columbia Research Ethics Board (H18–03154).

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Bone, J.N., Joseph, K.S., Magee, L.A. et al. Pre-pregnancy body mass index and adverse maternal and perinatal outcomes in twins: A population retrospective cohort study. Int J Obes (2023).

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