Maternal obesity is a growing concern worldwide. We aimed to determine trends of obesity in women of reproductive age over a 15-year period, the relationship between prepregnancy weight status and pregnancy-related outcomes, and possible changes over a 15-year period. Data was derived from two identical, cross-sectional National Perinatal Surveys (NPSs). Analysis was restricted to the mothers of healthy, full-term, singleton infants. Overall, the study population consisted of 18,752 mother-infant pairs (7,208 from the 1st NPS and 11,544 from the 2nd NPS). The prevalence of overweight and obese women prior to pregnancy increased significantly between the two surveys, from 12 to 15.3% and 2.1 to 4.7%, respectively. In both study periods, prepregnancy weight status was positively associated with pre-eclampsia (P < 0.001) and method of delivery (P < 0.001). After adjusting for potential confounders, overweight and obese women had higher odds for caesarian section (adjusted (odd ratio) OR = 1.87) and pre-eclampsia (adjusted OR = 2.59) than normal-weight women. These associations did not change between the study periods. In conclusion, prepregnancy maternal obesity is steadily increasing over time. The magnitude of the problem underlies the need for public health programs to focus on obesity prevention, particularly among women of reproductive age, and re-evaluate the guidelines for various perinatal practices.