What causes a molar pregnancy?
A molar pregnancy occurs because at conception the embryo develops with an extra set of chromosomes from the father. Why this occurs is not known. In a normal embryo, cells contain one set of chromosomes from the mother and one set from the father. In a molar pregnancy, more than one set of chromosomes come from the father, either because the chromosomes from one sperm have been duplicated, or because two separate sperm have fertilised the mother's egg. A complete molar pregnancy occurs when the egg contains no chromosomes, whereas in a partial molar pregnancy the egg contain the normal 23 chromosomes.