Prevalence of Partial Hydatidiform Mole in Products of Conception From Gestations With Fetal Triploidy Merits Reflex Genotype Testing Independent of the Morphologic Appearance of the Chorionic Villi.

Diagnosis of first-trimester partial mole is challenging as the key morphologic features may not be well-developed and may overlap with those of a nonmolar gestation harboring a cytogenetic disorder or degenerative changes. Genotype testing has emerged as the reference tool to distinguish partial mole (diandric triploid genotype) from its nonmolar mimics. However, observer variation in defining the minimum threshold of how much morphologic alteration is required to trigger genotype testing may result in a subset of partial moles that go undetected. We hypothesized that the results of fetal aneuploidy testing performed for prenatal screening or evaluation of miscarriage may assist with triggering molecular testing in the evaluation of products of conception, specifically if fetal triploidy is detected. Gestations with fetal triploidy are either a partial mole (diandric triploidy) or are nonmolar (digynic triploidy). The aims of this study were to define the prevalence of partial mole in 20 products of conception specimens with known fetal triploidy by performing genotype testing and then to determine how well established morphologic criteria for partial mole correlate with the genotype results in this setting. Genotype testing demonstrated that 65% (13/20) were a partial mole and the remainder were nonmolar digynic triploid gestations. Most partial moles were under 9 weeks gestational age and, as expected, lacked classic well-developed morphologic features. Nearly a third (4/13) of the partial moles were originally interpreted as normal or nonmolar gestations with minimal abnormalities that did not merit molecular testing to exclude a partial mole. Even with the retrospective systematic morphologic review, only 23% (3/13) exhibited the combination of chorionic villous enlargement of ≥2.5 mm and cisterns, which has been previously established as the morphologic criteria with the highest predictive value for a molecularly defined partial mole. The other 77% exhibited focal, limited, variable degrees and extent of villous morphologic alterations. We conclude that, given the high prevalence of partial mole among products of conception with known fetal triploidy and the low prevalence of diagnostic morphologic findings in such specimens, reflex genotype testing should be performed in all such cases, regardless of whether or not the morphologic features are suspicious for a partial mole. This reflex testing strategy mitigates against the subjectivity of determining whether subtle villous abnormalities are significant enough to merit pursuing genotype testing. The success of this strategy depends on the clinician documenting the fetal triploidy result at the time of submitting the products of conception specimen and therefore clinician education is needed. Finally, it remains to be determined whether the risk for postmolar gestational trophoblastic disease is the same in diandric triploid gestations that exhibit classic morphologic features as in those that exhibit minimal or negligible villous morphologic abnormalities.