The incidence of complete hydatidiform mole is about 1 to 1.5 per 2000 pregnancies in the United States and other Western countries. For unknown reasons, the incidence is much higher in Asian countries. Moles are most common before the age of 20 and after the age of 40 years, and a history of the condition increases the risk for molar disease in subsequent pregnancies. Although molar disease formerly was discovered at 12 to 14 weeks of pregnancy during investigation for a gestation that was “too large for dates,” early monitoring of pregnancies by ultrasound has lowered the gestational age at detection. In both complete and partial moles, elevation of hCG in the maternal blood and absence of fetal heart sounds are typical.
MORPHOLOGYInadvancedcasestheuterinecavityisexpandedbyadelicate,friablemassofthin-walled,translucentcysticstructures(
Fig.19.19 ).Fetalpartsarerarelyseenincompletemolesbutarecommoninpartialmoles.Onmicroscopicexamination,thecompletemoleshowshydropicswellingofpoorlyvascularizedchorionicvilliwithaloose,myxomatous,edematousstroma.Thechorionicepitheliumtypicallyshowsaproliferationofbothcytotrophoblastsandsyncytiotrophoblasts(
Fig.19.20 ).HistologicgradingtopredicttheclinicaloutcomeofmoleshasbeensupplantedbymonitoringofhCGlevels.Inpartialmoles,villousedemainvolvesonlyasubsetofthevilli,andthetrophoblasticproliferationisfocalandslight.Inmostcasesofpartialmole,somefetalcellsarepresent,rangingfromfetalredcellsinplacentalvillito,inrarecases,afullyformedfetus.
Overall, 80% to 90% of moles do not recur after thorough curettage, but 10% of complete moles are invasive (described next). No more than 2% to 3% give rise to choriocarcinoma.
Invasive moles are complete moles that are locally invasive but lack the metastatic potential of choriocarcinoma. An invasive mole retains hydropic villi, which penetrate the uterine wall deeply, possibly causing rupture and sometimes life-threatening hemorrhage. On microscopic examination, the epithelium of the villi shows atypical changes, with proliferation of both trophoblastic and syncytiotrophoblast components.
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Fig.19.19Completehydatidiformmole,consistingofnumerousswollen(hydropic)villi.
Although the marked invasiveness of this lesion makes removal technically difficult, metastases do not occur. Hydropic villi may embolize to distant organs, such as lungs or brain, but these emboli do not behave like true metastases and may regress spontaneously. Because of deeper invasion into the myometrium, an invasive mole is difficult to remove completely by curettage, so if serum β-hCG remains elevated, further treatment is required. Fortunately, in most cases cure is possible with chemotherapy.