Cotinine validation of self-reported smoking during pregnancy in the Swedish Medical Birth Register.
Self-reported data on smoking during pregnancy from the Medical Birth Register of Sweden (MBR) are widely used. However, underreporting of such behaviour may occur, leading to biases. It is of importance to validate the smoking data in the MBR. The main objective was to investigate the agreement between self-reported smoking data from the MBR and cotinine levels in maternal serum among women from