Partial hydatidiform mole: a common but underdiagnosed condition. A 3-year retrospective clinicopathological and DNA flow cytometric analysis.

This retrospective study reports the incidence of hydatidiform mole in a population of 19,457 pregnancies over a 3-year period. During the study period all "products of conception" from first and second trimester abortions were referred for pathological examination. Coded histologic sections were reviewed using the published histologic criteria for hydatidiform mole. Ploidy was estimated by DNA flow cytometry. Clinical data were retrieved from maternal case notes. Thirty-eight cases were confirmed as hydatidiform mole, 10 (26%) as complete mole, and 28 (74%) as partial mole. Twenty-three cases of partial mole (88%) were triploid, and nine of 10 complete moles were diploid. The incidence of hydatidiform mole was 1:512 pregnancies, (complete mole, 1:1,945; partial mole, 1:695). Only one case (3.5%) of partial mole was suspected clinically. One case of persistent gestational trophoblastic disease occurred following a complete mole. No sequelae were encountered following partial mole. We conclude that hydatidiform mole is a common condition and the majority of cases are partial moles. Quantitatively imprecise morphologic criteria contribute to the inaccuracy in reporting of partial mole; analysis of ploidy is useful in the evaluation of problem cases. Follow-up of partial mole is warranted because its true biological potential is as yet unclear.