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.

How is a partial molar pregnancy diagnosed?

The physician diagnoses a molar pregnancy by performing an ultrasound, which will reveal the presence of cysts in the uterus. The physician will also perform another test that measures the levels of beta human chorionic gonadotropin (HCG). Beta HCG is a hormone that pregnant women produce. It appears in the blood or urine a week after the egg has been fertilized. Beta HCG levels rise rapidly in normal pregnancies; in a partial molar pregnancy, the beta HCG levels rise at a slower rate and often to a lower level than a normal pregnancy.

What is the treatment for a partial molar pregnancy?

A partial molar pregnancy is treated by removing the embryo and placenta from the uterus. This is done with a surgical procedure known as a suction curettage, in which the cervix is dilated (opened) and the embryo and placenta are removed from the uterus.

After this procedure, the doctor will continue to measure the woman’s beta HCG levels for up to a year to determine if there are any remnants of the molar tissue in the uterus. The woman should use reliable birth control methods during this time to keep from becoming pregnant.

In a small number of partial molar pregnancy cases, there will be some molar tissue remaining in the uterus after the suction curettage. This tissue is known as gestational trophoblastic disease and may continue to grow. It can be treated with methotrexate, a medication that is also used to treat cancer. In rare cases, the gestational trophoblastic disease may develop into a cancer (known as a choriocarcinoma). This, too, can be treated with anti-cancer agents.

Can a woman become pregnant after a partial molar pregnancy?

Yes. The odds of the woman having another molar pregnancy are small (approximately 1-2%). In order to be sure that the uterus is free of molar tissue, the woman should not try to become pregnant until a year after treatment.


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This information is provided by the Cleveland Clinic and is not intended to replace the medical advice of your doctor or health care provider. Please consult your health care provider for advice about a specific medical condition. This document was last reviewed on: 12/10/2014...#12332