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.