Changes in the incidence of molar pregnancies. A population-based study in Chiba Prefecture and Japan between 1974...

The aim of this population-based retrospective study was to evaluate the trend in the incidence of molar pregnancy.

Questionnaires about the occurrence of hydatidiform mole were mailed to all clinics and hospitals in Chiba Prefecture between 1974 and 2000. The diagnosis of hydatidiform mole was based on the post-operative morphological and/or pathological findings. Data of live births were collected from the annual reports of maternal and child health statistics. A total of 3778 women with hydatidiform mole and 1 661 579 women with live births were analysed.

The incidence of hydatidiform mole per 1000 live births was constant from 1974 to 1990, while it decreased significantly after 1991 (chi(2)-test for trend, P < 0.0001). The incidence of complete mole also decreased after 1991 (chi(2)-test for trend, P = 0.0001), though the incidence of partial mole remained unchanged.

The incidence of hydatidiform mole in Chiba Prefecture has become as low as that in Europe or the USA, and it is attributed to the decreasing incidence of complete mole. A number of environmental factors that prevent the ovulation of 'empty' oocytes might have contributed to the decreasing incidence of complete mole.