Furuya S, et al. - This study was performed to compare the frequencies of chromosomal abnormality of ectopic pregnancy (EP) conceptus with those of intrauterine pregnancies, to identify factors that may influence the incidence of EP, and to evaluate their roles on the etiology of EP. Embryos with abnormal karyotype and multipara might possess some defensive mechanisms against EP. Maternal age, ART pregnancy, and sex of conceptus did not influence the occurrence of EP. Most commonly known risk factors for EP were affirmed to be a more plausible explanation for the etiology of EP than embryo-related factors.