The preoperative diagnosis of primary ovarian pregnancy (POP) remains elusive and the final diagnosis relies heavily on histologic findings. The diagnostic criteria for POP, established in 1878 by Spiegelberg, are based primarily on the identification of an embryonic sac within the ovary and the localization of conception products therein. However, these diagnostic criteria may be overly strict, which may not only significantly underestimate the prevalence of POP, but also potentially mislead patient management. In this series, we present 7 cases that showed no embryonic sac within the ovary (thus not meeting the Spiegelberg criteria for POP), but were nonetheless classified by the authors as POP based on the unequivocal presence of chorionic villi and implantation sites within the ovary. Immmunohistochemical studies for beta-human chorionic gonadotropin, human placental lactogen, and inhibin highlighted the trophoblastic populations. These findings indicate that POP may occur even if no embryonic sac is pathologically demonstrable. Accordingly, we propose the following modified diagnostic criteria for POP: (1) no pathologic evidence of ipsilateral fallopian tube involvement is present; and (2) evidences of gestation, including presence of chorionic villi and/or implantation site are present within the ovary. If both criteria are met, the diagnosis of POP should be rendered. These proposed diagnostic criteria should lead to more accurate diagnoses of POP, provide more contemporary insights into its true prevalence, heighten clinical awareness of the disease, and ultimately, optimize its clinical management.