Unexplained intra-uterine fetal death is still a problem in diabetic pregnancies, especially in those with an LGA-infant. We hypothesized that in these pregnancies impaired placental function, in terms of abnormal placental weight and/or abnormal placental histology, may account for this phenomenon. To test this hypothesis, we assessed the relative placental weight and scored several histological abnormalities in 34 AGA- and 24 LGA-placentae of type 1 diabetic women and in 22 AGA- and 16 LGA-placentae of control women. Relative placental weight was comparable in the LGA-diabetic cases and in the control groups, but was significantly higher in the AGA-diabetic subgroup. Histological abnormalities such as the presence of nucleated fetal red blood cells, fibrinoid necrosis, villous immaturity and chorangiosis were observed more often in the diabetic placentae compared with the control placentae. These differences in histology were particularly observed when we compared both AGA-groups. LGA-control placentae showed a high incidence of histological abnormalities, almost comparable to the diabetic placentae. Only fibrinoid necrosis was significantly more common in the LGA-diabetic placentae. Three of the four cases of perinatal death/asphyxia in the diabetic group concerned an LGA-infant with a relative low placental weight. In conclusion, placentae of women with type 1 diabetes showed several abnormalities that can be associated with impaired functioning. The difference between AGA- and LGA-diabetic placentae was related to relative placental weight and our data suggest that an increase in relative weight may protect the fetus from asphyxia. Placentae from LGA-non-diabetic women showed several similarities to those of women with diabetes.