A molar pregnancy is a rare and unusual complication. It happens from the moment of conception, and occurs when the dividing cells form incorrectly. A molar pregnancy is always unsustainable.
In a normal pregnancy, the fertilised egg will receive 23 chromosomes from both parents, making a total of 46. In a molar pregnancy, an incorrect amount of chromosomes pair together, and the cells that will form the placenta are also abnormal.
There are two types of molar pregnancy, complete and partial.
In a complete molar pregnancy, the fertilised egg will not receive any chromosomes from the mother. Instead, it will have two copies of the father's chromosomes. Because of this totally incorrect structure, neither an embryo nor amniotic sac will grow. However, a placenta will begin to form but will be abnormal, and will develop into a mass of cysts.
A partial molar pregnancy, the fertilised egg will usually comprise 23 chromosomes from the mother, but will have two sets of chromosomes from the father, resulting in 69 chromosomes.