It occurs when there are certain abnormalities in the fertilized egg during conception. In this case the fertilized egg doe not develop into an embryo or it tends to develop abnormally or does not survive. In case of normal pregnancies, the fertilized egg consists of twenty three chromosomes from the father and twenty three from the mother.
In case of molar pregnancies the fertilized egg contributes no maternal chromosomes and the chromosomes from the father’s sperm get duplicated so one ends up with two copies of chromosomes from the father and none from the mother. As a result there is no embryo and it leads to the formation of mass of cysts which look like a cluster of grapes in the placenta.
In case of partial molar pregnancies, the fertilized egg gets the normal complement of chromosomes from the mother but double from the father hence there are sixty nine chromosomes instead of the normal forty six.
It is really a sad feeling of losing pregnancy in such a manner. If one gets a proper treatment then it is a rare chance that it results in long term physical consequences.
One in every thousand pregnancies in the United States is a case of molar pregnancy. If one is under the age of twenty or over the age of forty or if one has had a previous molar pregnancy then the risk of molar pregnancy increases.