Answer: Atypical villous morphology associated with fetal trisomy

Histology: The sections show some irregularly shaped villi with variation of size, but two discrete populations are not clearly evident. Focal trophoblastic hyperplasia is also noted but no inclusions are found. Fetal parts are present.

Discussion: Most complete hydatidiform moles with typical features can be recognized on gross and microscopic examination. However, the recognition of partial hydatidiform moles and their distinction from non-molar abnormal villous morphology remain problematic, particularly when ploidy studies are not available. Ideally, partial hydatidiform mole should be diagnosed based on a combination of histologic features and the presence of triploidy. When ploidy data are not available, stringent application of morphologic criteria is necessary to improve diagnostic accuracy. To diagnose a parital hydatidiform mole, one has to observe a constellation of morphological features, including (i) two discrete populations of villi, (ii) circumferential trophoblastic hyperplasia, (iii) scalloping of villi, (iv) trophoblastic inclusions, and (v) cisterns. It has been shown that using at least three of these five features is highly correlated with triploidy confirmed by FISH analysis. However, abnormal villous morphology with focal trophoblastic hyperplasia has also been reported in non-molar conceptuses, including trisomy 7, 18 and 21, and monosomy X. Trophoblastic hyperplasia has been observed in 22% of trisomy 21 cases. In this case, trisomy 21 was diagnosed by chorionic villous sampling prior to abortion. Therefore, abnormal villous morphology in this case is associated with trisomy 21 and does not reflect a partial hydatidiform mole.