An indirect method, based on placental morphology, was used to establish the incidence of heteroploidy in early human pregnancy and to determine the possible effect of the IUD on its occurrence. The non‐IUD‐wearing control population, comprised of 5,779 healthy fertile women undergoing therapeutic abortion for socioeconomic reasons, yielded an incidence of heteroploidy of 0.83% as compared with an overall incidence of 46.7% in our group of spontaneous aborters. In contrast heteroploidy was never detected pathologically in IUD‐associated spontaneous and therapeutic abortions. It was concluded that the IUD significantly prevented the development of heteroploid conceptuses. The biological and clinical significance of these findings was discussed. Copyright © 1979 Wiley‐Liss, Inc., A Wiley Company