First of all let me explain what molar pregnancy is: it is an abnormality of the placenta (afterbirth), and it is caused by a problem when the egg and sperm join together at fertilization. This problematic joint of sperm and egg results in a very complicated pregnancy, called molar pregnancy. There are two types of molar pregnancies: complete and partial.
In cases of complete molar pregnancy, there is no baby: these pregnancies only have placental parts. This type of molar pregnancy happens when sperm fertilizes an empty egg. In partial molar pregnancy 2 sperm fertilize an egg. If everything would go right and normal, twins should form, but unfortunately sometimes things go wrong, and a pregnancy with abnormal placenta and fetus happens. In cases like this baby with too many chromosomes is formed and almost always dies in uterus. At this point, it is very important to bring out that molar pregnancies are accidents of nature. They are no one s fault: molar pregnancy is not caused because you had a glass of wine at the dinner (not that I recommend drinking wine for other reasons).
However, molar pregnancies are in relation with woman's age and interestingly with location: The older the woman, the highest is the change of molar pregnancy; for example age over 40 is a risk factor for molar pregnancy. Also statistically in US the percentage of molar pregnancies is calculated in per milles, in US 1 in 1000 women will experience molar pregnancy, while in Southeast Asia the incidence is almost 10 times higher! Also interestingly, white women living in US have higher rates of molar pregnancies than black women living in US. But, the reasons for the geographic, racial and age differences are currently unknown.