Recent changes of the incidence of complete and partial mole in Chiba prefecture.

Aims: To study the changes of the incidence of complete mole (CM) and partial mole (PM) by 10-year age groups in Chiba Prefecture.

Methods: All women registered as CM and PMs in Chiba Prefecture during these 18 years were included in this study. The diagnosis of CM and PM was based on the macroscopic and/or microscopic findings. The annual numbers of pregnancy were obtained from the Division of Statistics in Chiba Prefecture Government.

Results: The incidence of CM at the upper and lower extremes of maternal age is higher than that of PM. The incidence of CM has decreased constantly at all maternal ages and significantly decreased in women of middle reproductive age (20-39 years old) since 1991, while that of PM has stayed constant during these 18 years.

Conclusions: The incidence of CM and PM in Chiba Prefecture has become as low as that in Europe or the USA. These recent changes suggest that Japanese women may have lost the increased risk to ovulate a nuclear or inactive oocytes, or the differential diagnosis between CM and PM may be obscured with the macroscopic and/or microscopic findings.