Hydatidiform Mole

A hydatidiform mole, commonly referred to as a molar pregnancy, is a rare abnormality in pregnancy classified under gestational trophoblastic diseases (GTD), which stem from the placenta and may metastasize. Hydatidiform moles are characterized by abnormal fertilization, resulting in villous hydrops and trophoblastic hyperplasia with or without embryonic development. Other forms of GTD include gestational choriocarcinoma—which can be aggressively malignant and invasive—and placental site trophoblastic tumors. Please see StatPearls' companion reference, "Gestational Trophoblastic Disease," for more information. Although described as early as Hippocrates’ era, who explained their formation through the consumption of dirty water by pregnant women, the condition was more accurately characterized in 1752 by William Smelie, who likened its appearance to a “bunch of grapes,” a hallmark feature of molar pregnancies resulting from swollen, hydropic villi.

Hydatidiform moles are categorized as complete or partial and are usually noninvasive forms of GTD. Overrepresentation of the paternal genome in sporadic hydatidiform moles is a fundamental genetic event leading to overall alteration of imprinting gene expression in the molar trophoblast, being completely androgenetic in complete hydatidiform moles and diandric triploid in partial hydatidiform moles. However, although hydatidiform moles are considered benign, they are premalignant lesions and can potentially become malignant and invasive.

Complete molar pregnancies present with symptoms such as first-trimester vaginal bleeding, severe nausea, and high β-human chorionic gonadotropin (β-hCG) levels. Partial moles may present similarly but often resemble a spontaneous abortion and may include detectable fetal heart tones. Both molar types are detectable via ultrasonography, which typically reveals distinct patterns; complete moles demonstrate anechoic cystic clusters (“grape clusters”) and partial moles demonstrate fetal parts.

Histopathological examination postevacuation confirms the diagnosis with visualization of characteristic villous changes. Treatment typically involves dilation and curettage (D&C), and hysterectomy may be considered for patients not desiring future pregnancies. Posttreatment, regular β-hCG monitoring is essential to detect potential progression to gestational trophoblastic neoplasia (GTN).