What is a partial molar pregnancy?

A partial molar pregnancy is a variation of a molar pregnancy, an abnormal pregnancy in which an embryo (the fertilized egg) either develops incompletely, or doesn’t develop at all. Instead, a cluster of grape-like cysts (known as a hydatidiform mole) grows in the uterus.

In a partial molar pregnancy, an incomplete embryo and placenta (the sac in which the fetus grows) may actually start to develop.

What causes a partial molar pregnancy?

A partial molar pregnancy is a genetic accident. In a normal pregnancy, the egg receives one set of 23 chromosomes from the father and one set of 23 chromosomes from the mother, for a total of 46 chromosomes. In a partial molar pregnancy, the egg receives two sets of chromosomes from the father, usually because two sperm have fertilized the egg. The egg now has 69 chromosomes, instead of the normal 46.

Molar pregnancies occur approximately once every 1,000 pregnancies. They are more common in women who are older than 40 years and in women who have had two or more miscarriages. Partial molar pregnancies occur less frequently.

What are the symptoms of a partial molar pregnancy?

The most prominent symptom of a molar pregnancy is heavy bleeding from the vagina early in the pregnancy. The blood may be dark brown. Symptoms of a partial molar pregnancy include severe nausea, vomiting, and hypertension (high blood pressure) early in the pregnancy, often in the first trimester.

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