Rapid growth of human fetal tissues requires insulin or insulin-like growth factors. A high rate of human fetal growth occurs between implantation and about 14 weeks of gestation. Fetal pancreatic insulin secretion begins much later. Since maternal insulin does not cross the blood/placental barrier, other sources of insulin or insulin-like growth factors may be provided for fetal development. We report here that placental polyadenylylated RNAs from the first and third trimester of normal pregnancy as well as from term pregnancies of diabetic mothers hybridize to a 32P-labeled cloned cDNA of an insulin-related sequence expressed in fetal pancreas. Moreover, placentas from diabetic women express much more of these sequences. These results suggest that insulin-related genes are expressed in placental tissue during fetal development and may be a source of growth-promoting hormones for the human fetus. Fetuses developing in diabetic women receive a large influx of glucose. This in turn may stimulate the expression of insulin-related sequences, which may result in higher utilization of glucose, thus bringing about the macrosomia and high incidence of malformation and still-births known to result from pregnancies in diabetics.