Methods and materials
The study population consisted of 988,988 women with singleton term (≥37+0) vaginal deliveries in Sweden 2005-2016. The data was extracted from the Swedish Medical Birth Registry (MBR), which was founded in 1973 and includes data on almost all deliveries (98-99%) in Sweden. All health care providers are obliged to report to the registry and the system is identical throughout the country. It contains information collected from medical records registered at antenatal visits, delivery units and initial paediatric examination of the infant. A detailed quality control and validation of the registry was published in 2003.
At the first antenatal visit the women are interviewed by their midwife regarding their pre-pregnancy health, including smoking habits and use of medical drugs. Also, the height and their booking weight are registered. Maternal Body Mass Index (BMI) was calculated through weight (kg)/(height (m))2 , and was classified according to WHO‘s classification into underweight (BMI <18.5), normal weight (BMI 18.5 – 24.9), overweight (BMI 25-29.9), and obesity (BMI ≥ 30). Information regarding maternal educational level and maternal country of birth was obtained through linkage with data from Statistics Sweden. Cases of OASIS were identified through checkboxes, or ICD-code O702 and O703 reported to the MBR or the National Patient Registry (NPR) within six weeks after delivery.