Abstract： Objective To investigate the parental origin for a rare case of complete hydatidiform mole and coexisting fetus and to discuss its diagnosis and differential diagnosis.Methods Tissues from the fetus,mole and placenta were collected and pathology analysis and chromosome analysis were done.The DNA from the fetus,mole and parents' peripheral blood leukocytes was amplified with five short tandem repeat (STR) markers (D4S2460,D18S488,D21S2039,DXS1205 and DYS219) at the same time to confirm the parental source of the hydatidiform.Results (1) Casereport:A 27-year-old woman,gravida 1,para 0,was found high risk for neural tube defects at 20 weeks of gestation.At 24+5 weeks of gestation,ultrasound examination demonstrated a normal fetus,a normal placenta and a huge mass with a multicystic appearance attached to the placenta with an obvious demarcation.The fetus died at 26 weeks of gestation.Serum human chorionic gonadotropin-β(β -hCG) level decreased obviously during the first two weeks after artificial induction,but elevated at the third week,and β-hCG titers fell to normal after 2 courses of chemotherapy.Fetus autopsy showed no structure abnormality.Histopathologic examination of the hydatidiform showed swelling of chorionic villi with hyperplasia of the trophoblast and formation of central cisterns suggesting of a twin pregnancy consisting of a complete hydatidiform mole and coexisting fetus.(2) Genetic analysis:The karyotype analysis of the normal placental villi was 46,XY; the cell cultures of fetal cartilage tissue and hydatidiform were failed.STR analysis showed that the fetus was diploid from biparental source;the mole was androgenetic source.And the mole had locus both from Y and X chromosome of the father,so it was heterozygous.It was suggested that this case was derived from one single oocyte fertilized with three spermatozoas.Conclusions STR analysis could be used to confirm the diagnosis of complete hydatidiform mole and coexisting fetus and to find the pathogenetic rnechanism.