Published online by Cambridge University Press: 16 April 2025
The term “molar pregnancy” is commonly used to describe disorders of the villous anatomy which results from a lack, maldevelopment or regression of the villous vasculature that make the drainage of fluid supplied by the trophoblast impossible. When these changes are associated trophoblastic hyperplasia on histology they are described as hydatidiform moles (HM) which are classified into either complete hydatidiform moles (CHM) where the trophoblastic hyperplasia is diffuse, fetal development is absent and the morphological changes are generalised, or partial hydatidiform moles (PHM) which characteristically display trophoblastic hyperplasia with focal villous hydropic changes and evidence of fetal development. The vast majority of CHM and triploid PHM miscarry spontaneously during the first three months of pregnancy. Following uterine evacuation, around 15% and between 0.5-5.6% of patients with a CHM and PHM develop gestational trophoblastic neoplasia (GTN) requiring chemotherapy and thus it is essential that patients with HM are diagnosed early in pregnancy and provided with follow-up. The molar transformation of the placental tissue in HM is a progressive phenomenon secondary to the oedema of the villous mesenchyme and the typical cystic molar changes found on ultrasound are often not visible before 9 weeks of gestation. Most patients with HM are likely to be first seen in an early pregnancy unit.
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