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

Abstract 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.