Hydatiform mole pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]



A mole is characterized by a conceptus of hyperplastic trophoblastic tissue attached to the placenta. The conceptus does not contain the inner cell mass (the mass of cells inside the primordial embryo that will eventually give rise to the fetus).

The hydatidiform mole can be of two types: a complete mole, in which the abnormal embryonic tissue is derived from the father only; and a partial mole, in which the abnormal tissue is derived from both parents.

  • Complete moles usually occur when an empty ovum is fertilized by a sperm that then duplicates its own DNA (a process called androgenesis). This explains why most complete moles are of the 46,XX genotype. A 46, XY genotype may occur when 2 sperm (one 23, X and the other 23, Y) fertilize an empty egg. They grossly resemble a bunch of grapes ("cluster of grapes" or "honeycombed uterus" or "snow-storm"[1]). Their DNA is purely paternal in origin (since all chromosomes are derived from the sperm), and is diploid (i.e. there are two copies of every chromosome). Ninety percent are 46,XX, and 10% are 46,XY. In a complete mole, the fetus fails to develop, thus on gross examination there are no signs of fetal tissue. All of the chorionic villi are enlarged. The main complication of the complete mole is a 2% chance of progression to a cancer called choriocarcinoma.
  • Partial moles can occur if a normal haploid ovum is fertilized by two sperm, or, if fertilized by one sperm, if the paternal chromosomes become duplicated. Thus their DNA is both maternal and paternal in origin. They can be triploid (e.g. 69 XXX, 69 XXY) or even tetraploid. Fetal parts are often seen on gross examination. There is also an increased risk of choriocarcinoma, but the risk is lower than with the complete mole.


  1. Woo J, Hsu C, Fung L, Ma H (1983). "Partial hydatidiform mole: ultrasonographic features". Aust N Z J Obstet Gynaecol. 23 (2): 103–7. PMID 6578773.