Introduction

Molar pregnancies result from disorders of human fertilisation. They are the most common form of gestational trophoblastic disease, which incorporates a wide spectrum of abnormalities of trophoblastic development. Hydatidiform moles were first described by Hippocrates in ancient Greece. The etymology is probably derived from the Greek word hydatis meaning ‘a drop of water’, referring to the watery content of the cysts, and the Persian word mylon meaning a misshapen thing (false conception). The modern medical term hydatidiform mole refers to a disorder of placental development in which the villous mesenchyme, vasculature and trophoblast are all affected and there is little or no fetal development.

Molar pregnancies are characterised by villous hydrops. Trophoblastic hyperplasia is the characteristic microscopic feature of true molar pregnancies. Biochemical analysis of the fluid from molar vesicles indicates that it is derived from the diffusion of maternal plasma but also contains specific trophoblastic proteins. These biochemical findings suggest that the hydropic (hydatidiform) transformation of the villous mesenchyme is caused by maldevelopment, regression or lack of the villous vasculature, which compromises the drainage of fluid produced by the trophoblast. Mild to moderate generalised villous oedema, which is often found following the demise of an embryo or early fetus, supports this concept and highlights the fact that hydropic villous changes are not exclusive to molar pregnancies. Hydatidiform transformation of villi is a non-specific feature of vascular dysfunction and only the hyperplastic microscopic appearance of the trophoblast is decisive for the diagnosis of molar pregnancy. Hyperplasia can be identified histologically before the hydropic villous changes are visible macroscopically.