To determine if the placental weight to birthweight ratio (placental ratio) is increased in pregnancies complicated by diet-treated gestational impaired glucose tolerance (IGT) and diabetic (GDM) pregnancies, a retrospective case control study was performed on 478 singleton IGT/GDM pregnancies delivered in 1994 and compared with a control group that consisted of the next case with normal OGTT performed on the same day as each index case. The placental weight and ratio, and the placental ratio alone, were found to be significantly increased in the IGT and GDM groups respectively compared with the control group. Significant correlation was demonstrated between the placental ratio with OGTT 2-h value and maternal body mass index. Multiple regression analysis indicated that the OGTT 2-h value was the significant determinant for placental ratio. Our results showed that increased placental size and ratio were found in IGT/GDM pregnancies that required diet treatment only, and this increase was related to the degree of glucose intolerance as shown in the OGTT 2-h value.