Decreasing incidence of registered hydatidiform moles in Denmark 1999-2014

Incidences of hydatidiform mole (HM) registered in European countries varies from 0.98/1000 to 2.17/1000 deliveries, while higher incidences have been reported in other parts of the world. We calculated the incidence by selecting data on HMs classified as "first", "second" and "third" from 01.01.1999 to 31.12.2014 registered in the Danish Pathology Registry, which we previously showed to be the most complete data source on the number of HMs in Denmark. In the study period, 1976 first HMs were registered; 1080 (55%) were classified as PHMs (partial HMs) and 896 (45%) as NPHMs (HMs not registered as PHMs). The average incidence of HM was 1.98/1000 deliveries. The incidence of PHM was 1.08/1000 deliveries and the incidence of NPHM was 0.90/1000 deliveries. Forty HMs were registered as second HMs; 85% (34/40) were of the same histopathological type as the first HM. The registered incidence of HM decreased from 2.55/1000 deliveries in 1999 to 1.61/1000 deliveries in 2014 (p < 0.005). The decrease in the incidence of HM was identical with a decrease in the incidence of PHM. New medical practices such as medical abortion and only forwarding selected pregnancy products for histopathologic examination may cause a declining number of HMs registered.