BACKGROUND: The proportion of overweight or obese pregnant women is increasing in many countries and babies born to a mother who is overweight or obese are at higher risk for complications. Our primary objective was to describe sociodemographic and obstetric factors across Body Mass Index (BMI) classifications, with secondary objective to investigate stillbirth and other pregnancy outcomes in relation to BMI classifications and gestational week.

METHODS: This population-based cohort study with data partly based on a cluster-randomized controlled trial includes 64,632 women with singleton pregnancy, giving birth from 28 weeks' gestation. The time period was January 2016 to 30 June 2018 (2.5 years). Women were divided into five groups according to BMI: below 18.5 underweight, 18.5-24.9 normal weight, 25.0-29.9 overweight, 30.0-34.9 obesity, 35.0 and above, severe obesity.

RESULTS: Data was obtained for 61,800 women. Women who were overweight/obese/severely obese had lower educational levels, were to a lesser extent employed, were more often multiparas, tobacco users and had maternal diseases to a higher extent than women with normal weight. From 40 weeks' gestation, overweight women had a double risk of stillbirth compared to women of normal weight (RR 2.06, CI 1.01-4.21); the risk increased to almost four times higher for obese women (RR 3.97, CI 1.6-9.7). Women who were obese or severely obese had a higher risk of almost all pregnancy outcomes, compared to women of normal weight, such as Apgar score < 7 at 5 min (RR1.54, CI 1.24-1.90), stillbirth (RR 2.16, CI 1.31-3.55), transfer to neonatal care (RR 1.38, CI 1.26-1.50), and instrumental delivery (RR 1.26, CI 1.21-1.31).

CONCLUSIONS: Women who were obese or severely obese had a higher risk of almost all adverse pregnancy outcomes and from gestational week 40, the risk of stillbirth was doubled. The findings indicate a need for national guidelines and individualized care to prevent and reduce negative pregnancy outcomes in overweight/obese women. Preventive methods including preconception care and public health policies are needed to reduce the number of women being overweight/obese when entering pregnancy.