Introduction: Gestational diabetes is up to three times more common in twin versus singleton pregnancies, yet the reason for this increased risk is unclear. Although gestational weight gain is a known modifiable cause of gestational diabetes, and weight gain is higher among twins, the extent to which increased weight gain explains the relationship between plurality and gestational diabetes is unknown. We evaluated the extent to which increased risk of gestational diabetes in twin pregnancies is mediated by increased gestational weight gain.
Methods: We leveraged previously-collected serial weights and glucose screening/diagnostic data abstracted from medical charts for 1397 twin and 2622 singleton pregnancies with normal or overweight pre-pregnancy body mass index (BMI) delivered between 1998 and 2013 at Magee Women’s Hospital in Pittsburgh, Pennsylvania. We used causal mediation analyses to estimate natural indirect and direct effects, or those mediated and not mediated by weight gain, respectively.
Results: We found that although odds of gestational diabetes were higher among twin pregnancies [marginal total effect = 2.83 (95% CI 1.55; 5.17) for normal weight and 2.10 (95% CI 1.17; 3.75 for overweight pre-pregnancy BMI], there is limited evidence that this relationship is mediated by weight gain [natural indirect effect = 1.21 (95% CI 0.95; 1.54) for normal weight and 1.06 (95% CI 0.90; 1.24 for overweight pre-pregnancy BMI], and more evidence of mediation via other mechanisms [natural direct effect = 2.34 (95% CI 1.25; 4.39) for normal weight and 1.99 (95% CI 1.10; 3.61) for overweight pre-pregnancy BMI].
Conclusion: We conclude that, while twin pregnancies experience nearly 200% increased risk of gestational diabetes relative to singletons, only approximately 10% of this is mediated by gestational weight gain. We recommend research that investigates alternative mechanisms for the observed relationship between plurality and gestational diabetes, as well as the role of weight gain in causing gestational diabetes among twins.
Jay Kaufman– Professor, McGill University
Lisa Bodnar– Professor, University of Pittsburgh
Robert Platt– Full Professor, McGill University
Katherine Himes– Assistant Professor, University of Pittsburgh
Jennifer Hutcheon– Associate Professor, University Of British Columbia