We retrospectively detected overt diabetes during pregnancy (ODMP) using a modified IADPSG definition and assessed whether ODMP increases the risk of developing maternal and neonatal complications and postpartum diabetes in Korean pregnant women. According to the definition of IADPSG, ODMP pregnant women were defined and 71 ODMP, 1781 gestational diabetes mellitus (GDM) and and 463 non-GDM pregnant women were included in a hospital-based study. Their blood glucose levels were tightly regulated by modifying lifestyles and insulin treatment. The pregnancy outcomes and postpartum glucose tolerances were determined among the non-GDM, GDM and ODMP groups. The ODMP women had higher plasma glucose levels after overnight-fasting and at 2 h after 100 g OGTT challenge as well as higher overnight-fasted plasma insulin and HbA_1c levels than GDM women. HbA_1c levels at delivery were close to the normal range in both GDM and ODMP groups. Most pregnancy outcomes such as Apgar score and the rate of preterm delivery were not significantly different among three groups. Only the rate of large for gestational age (LGA) was greater in the ODMP group than other groups. However, about 73% of ODMP women remained diabetic at 6-8 week postpartum as compared to 4.3% of GDM. The development of postpartum diabetes was also associated with postpartum waist circumferences and duration of breast feeding. In conclusion, ODMP women in this study maintained tight control of glucose homeostasis and did not experience serious adverse outcomes except for LGA infants; however most ODMP women still had postpartum glucose dysregulation.