Background/Aims: Obesity along with high prepregnancy body mass index (PP-BMI) is known to cause many adverse pregnancy outcomes. In Thailand, there is not much study showing both the prevalence and complications of these conditions. The objectives of this study were to estimate the prevalence of prepregnancy overweight and obesity and their impacts on adverse pregnancy outcomes. Methods: This study was a cross sectional study. Data were collected retrospectively from hospital electronic database along with manual retrieval from medical charts and labor records. Data of all delivery women from 1^st February 2011 to 31^st August 2012 were collected. When excluded cases with incomplete data and those without PP-BMI, 5420 cases were into analysis. Descriptive and inferential data analyses were used with both univariate and multivariate methods. Results: The proportion of pregnant women with overweight and obesity were 11.1% and 3.9%. After multiple logistic regression analysis was done, women in obesity group were correlated with having 1, 2 and 3 complications. They were also correlated with preeclampsia, gestational diabetes, cesarean section, higher birth weight group and long neonatal length. Conclusions: This Thai prevalence of obesity in pregnancyshould alarm health care providers to be more prepared, for a future health problem of the country. Many complications that come with obese pregnant women that were reported in western countries also happen in Thai population. Decreasing body weight before conception, giving correct health education, well planned pregnancy; antenatal lifestyle intervention and even gestational weight gain restricttion could help avoiding these uneventful morbidities.