Gestational Trophoblastic Disease - Molar Pregnancy

What Is a Molar Pregnancy?

Molar pregnancy, also known as hydatidiform mole, is a rare type of conception occurring about 1 in every 600 pregnancies. It is not well understood why a molar pregnancy occurs but we know that a problem occurs at the time of conception when the sperm and the egg join together. This type of conception has excessive development of the cells that form the placenta (also known as  the” afterbirth”) and a molar pregnancy has little or no fetal (baby) development. There are two types of molar pregnancy known as complete hydatidiform mole(CHM) and partial hydatidiform mole (PHM).  These two types of molar pregnancy differ in their genetic makeup and development.

What Happens In a Normal Conception? 

In a normal conception an egg from the mother has 23 chromosomes (these are strands of DNA that contain our genes). This egg is fertilised by the father’s sperm that also has 23 chromosomes which join up to create a baby (fetus) made  up of 46 chromosomes.

Complete Molar Pregnancy (CHM)

In complete molar pregnancy the genetic  information (chromosomes) has been inactivated or lost from the mother’s egg. The “empty” egg is fertilised by the father’s sperm so there are only 23 chromosomes present. In normal conception there should be 46 chromosomes in a fertilised egg but in this situation duplication of the father’s chromosomes occurs producing the normal number of chromosomes leading to 46 chromosomes – but all from the father. A baby cannot develop as there is no genetic input from the mother. However, the cells that form the placenta will grow rapidly and your pregnancy test will be positive. In addition, you may have physical symptoms associated with normal pregnancy for example breast tenderness, nausea and vomiting. This happens because the over-production of these placental cells will produce high levels of the pregnancy hormone (hCG). Because of the growth of the abnormal placenta women with CHM often present with early pregnancy bleeding and the diagnosis can usually be suspected on ultrasound scan.

Partial Molar Pregnancy (PHM)

In partial molar pregnancy two sperm fertilise a normal egg leading to one set of chromosomes from the mother and two sets from the father – a total of 69 chromosomes. Again there is an imbalance of chromosomes but it differs from CHM as there can be evidence of some fetal development. These pregnancies almost always end in miscarriage. The majority of partial moles present with early pregnancy bleeding but the ultrasound scan is not usually diagnostic and the diagnosis is only made when laboratory examination of the miscarriage tissue has taken place.