Obesity increases the odds of multiple pregnancies: A nationwide register-based cohort study in Finland

Objective: To investigate the effects of increased pre-pregnancy body mass index (BMI, calculated as weight in kilograms divided by the square of height in meters) on the risk of having spontaneous multiple pregnancies using a nationwide register-based study sample. Methods: Data from the National Medical Birth Register (MBR) (2004–2018) were used to evaluate the effects of a higher pre-pregnancy BMI on the risk of multiple pregnancies. Lower and higher pre-pregnancy BMI classes, using the WHO classification, were compared with a normal weight class. A logistic regression model was used to assess the primary outcomes. Adjusted odds ratios (aOR) with 95% confidence intervals (CI) between the groups were compared. The model was adjusted by maternal age, maternal height, and maternal smoking status. Results: The odds of multiple pregnancies were higher among women in the overweight group (aOR 1.07, 95% CI 1.02–1.12), obesity class I group (aOR 1.11, 95% CI 1.04–1.18), and obesity class II group (aOR 1.15, 95% CI 1.03–1.28) compared with women in the non-overweight BMI class. Women in the underweight group had lower odds for multiple pregnancies (aOR 0.82, 95% CI 0.73–0.93). Conclusion: The odds of multiple pregnancies slightly increase with pre-pregnancy obesity, and this should be acknowledged as a minor risk factor for multiple pregnancies.