The objectives of this prospective study were to compare physical activity in 70 normal-weight women with a body mass index (BMI) 20-25 kg/m(2), and 70 obese with a BMI ≥ 30 kg/m(2), before and during pregnancy, and to compare compliance using the pedometer. Physical activity before pregnancy was assessed by questionnaires and during pregnancy by a pedometer worn on 7 consecutive days every 4th week. Obese women were less physically active than normal-weight women both before (p <0.05) and during pregnancy (p <0.0012). Both the compliance and the physical activity gradually declined during gestation. The change in physical activity could be described by a significant interaction between BMI group, gestational age (p <0.007) and the day of the week (p <0.001) when using ANOVA and interaction analysis. Maternal weight gain was larger in the normal-weight than in the obese women, but lower in the non-compliant obese women compared with the compliant (p <0.05).