A molar pregnancy is somewhat uncommon but not rare. In the U.S. it occurs in about 1/1000 to 1/1500 pregnancies. It occurs predominantly in older women or the chance of recurrence increases with each affected pregnancy (risk for subsequent molar pregnancy is 1-2% and the risk of a 3rd after two is about 25%). Symptoms are uterine bleeding, rapid enlargement of the uterus, absence of fetal heart activity, nausea and vomiting.
Hydatiform Mole is one type of "gestational trophoblastic disease". In some cases, there may be an embryo in addition (partial molar pregnancy) or there may have never been an embryo or fetus at all. This conception then does not result in the orderly course of events which produce embryonic tissue organization but instead the early products of conception (the chorionic villi) are converted into a mass of clear vesicles and these continue to grow and fill up the uterus. The uterus grows quickly making the size almost always larger than dates would indicate and extreme nausea may be present. There are no fetal heart tones and often bleeding and cramping occur.
This is most always the result of the sperm penetrating the egg which then goes on to duplicate its own chromosomes. The chromosomes of the ova are either absent or inactivated. When there is a partial mole and an embryo is present also, the fetus is often non-viable with chromosomal abnormalities but some do survive.
Following evacuation of the molar tissue, weekly beta hCG levels should be drawn until the hCG titer is within normal limits for 3 weeks. The titers are then observed at monthly intervals for 6-12 months. This is to rule out the possibility of developing choriocarcinoma, a rare malignancy which can occur afterwards.
This definition is from parentsplace.com