Objective(s): To evaluate the relation between gestational diabetes mellitus (GDM) and maternal and/or fetal DNA integrity. Method: 59 pregnant women were classified into two groups on the basis of 75 g oral glucose tolerance test (OGTT) and glycemic profile (GP): Control group (OGTT and GP normal, n = 29) and GDM group (abnormal 75 g OGTT, n = 30). The umbilical cord blood and placental samples obtained from the maternal side were collected at the time of delivery. Alkaline comet assay was performed for the determination of DNA damage. The trial was approved with the protocol number 72867572.050.01.04-299082. Result(s): Body mass index (BMI), weight gain during pregnancy, glycemic means and fetal weight were increased in GDM group compared control group (p = .01, .0001, .04, and .01, respectively). In the GDM group, the number of large-for-gestational-age (LGA) infants was significantly higher compared to the nondiabetic group (p = .04). Tail DNA percentages in placental samples were higher in the GDM group compared to controls (p = .01); however, DNA integrity in umbilical cord leukocytes was similar between the groups (p = 0.1). In contrast to umbilical cord DNA damage, placental DNA damage showed positive correlation with maternal glycemia in the whole group and within each group. The positive association of placental DNA damage and GDM remained after adjusting for age, BMI, smoking, glycemia, gestational age at delivery, fetal weight at delivery, and delivery type (p = .01). Conclusion(s): Placental DNA damage is associated with GDM and placental cells seem to be more vulnerable to DNA damage compared to fetal blood cells.