Gestational trophoblastic disease
Gestational trophoblastic disease — usually referred to as a a mole — is a very rare abnormality of pregnancy in the reproductive female that involves abnormal trophoblast proliferation. It is the result of a (purely chance) genetic error during the fertilization process that in turn causes the growth of abnormal tissue (which is not an embryo) within the uterus. The growth of this material is disproportionately rapid when compared to normal fetal growth.
The two types of hydatidiform molar pregnancy are complete and partial.
Complete moles are the most common type of mole, and are when the mass of tissue is completely made up of abnormal cells that would have become the placenta in a normal pregnancy. There is no fetus and nothing can be found at the time of the first scan. Complete moles often have a diploid karyotype 46,XX due to fertilization of an empty ovum by a single sperm followed by replication of the haploid chromosome. On ultrasound, a complete mole has a snow storm pattern, and the uterus is large for dates. Microscopically, there is edema of most villi, which gives the appearance of a large and random collection of grape-like cell clusters.
In a partial mole, the mass may contain both these abnormal cells and often a fetus that has severe defects. In this case, the fetus will be consumed by the growing abnormal mass very quickly. Partial moles have a triploid karyotype (69,XXX or 69,XXY) due to the fertilization of a single egg with two sperm. They also have a lower volume of tissue, and smaller hydropic villi (grape-like), as well as normal villi mixed in with the abnormal. Rarely, partial moles can progress to gestational choriocarcinoma.