A molar pregnancy is known medically as a 'Hydatidiform Mole'. The word 'hydatidiform' derives from 'hydatid', meaning 'water-filled cysts'. Other terms used to describe a molar pregnancy are 'vesicular mole' or 'trophoblastic disease'. Molar pregnancies are very rare, resulting from the abnormal development of the placenta.
A hydatidiform mole starts at the time the egg is fertilised. The woman usually misses her period and experiences the normal physical signs of pregnancy. However she may be more severely nauseated and vomit frequently. At some stage during the pregnancy (usually in the first 3 to 4 weeks), part of the placenta (known as the 'chorionic villi', or 'trophoblast') starts to produce fluid filled cysts, or vesicles that multiply rapidly. The vesicles can range in size from a pinhead to a small grape. The woman may then start to experience some vaginal bleeding (often around 12 weeks).
There are 2 types of molar pregnancies:
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A 'complete mole'. This is where a baby never develops, but the placenta implants and grows many small cysts, like sacs filled with fluid. |
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A 'partial mole'. This is where a baby starts to develop, but is unable to survive, often being absorbed into the vesicles that continue to multiply. |
We do not know why molar pregnancies occur, and the incidence varies between different ethnicities. In Australia, a molar pregnancy can occur in 1:750 pregnancies (or 0.13%), in the US and Europe it can occur in about 1:1000 (0.1%) and in South East Asia it can be as high as 1:600 (or 0.16%). The chance of a molar pregnancy recurring in the same woman is about 1:80 (or 1.25%). You can read about the physical signs and treatments in
molar pregnancy.