One form of gestational trophoblastic disease, a molar pregnancy, results from abnormal fertilization of the ovum. A complete hydatidiform mole, shown here, occurs when the ovum is lacking a maternal complement of chromosomes and is fertilized by a haploid sperm, usually containing an X chromosome. Duplication of this chromosome set typically yields a 46, XX karyotype that is paternally derived. No fetus develops, but there is an abnormal placenta consisting of a mass of tissue with grape-like, swollen chorionic villi.