Molar Pregnancy - Topic Overview

What is a molar pregnancy?

A molar pregnancy is a mass of tissue (hydatidiform mole) that forms an abnormal placenta inside the uterus. It starts from two or three sets of the father's chromosomes, with none from the mother.¹ Even though it is not an embryo, a mole triggers symptoms of pregnancy. About 1 out of 1,000 women with early pregnancy symptoms has a molar pregnancy.²

There are two types of molar pregnancy: complete and partial.

Complete molar pregnancy. In place of a normal placenta and embryo, the hydatidiform mole is abnormal placental tissue that grows into a grapelike cluster that can fill the uterus.
Partial molar pregnancy. The placenta grows abnormally into molar tissue. Any fetal tissue that develops is likely to have severe defects.

In extremely rare cases, an apparent twin pregnancy is found to be one complete mole and one normal, healthy placenta and fetus.³

What kind of risks are related to a molar pregnancy?

A hydatidiform mole can cause heavy bleeding from the uterus.

Some molar pregnancies lead to abnormal cell growth called gestational trophoblastic disease.

About 15% to 20% of complete molar pregnancies develop trophoblastic disease that keeps growing after the molar pregnancy is removed. A small percentage of these may become invasive cancer.^4,⁵ Fortunately, nearly 100% of those women who develop cancer are cured with treatment.²
About 5% of partial molar pregnancies develop trophoblastic disease.⁵

In rare cases, the abnormal tissue can spread (metastasize) to other parts of the body.

What causes a molar pregnancy?

Molar pregnancy is thought to be caused by a problem with the genetic information of an egg or sperm. A molar pregnancy can develop during the earliest stage of a pregnancy when:

An abnormal egg with no genetic information is fertilized by a sperm. The sperm's chromosomes duplicate and develop into a complete mole.
A normal egg is fertilized by two sperm. This cell mass is most likely to develop into a partial mole.

Factors that may increase your risk of having a molar pregnancy include:

Age. Risk for complete molar pregnancy steadily increases after age 35.¹
History of molar pregnancy, particularly if you've had two or more.⁵
History of miscarriage.
A diet low in carotene (a form of vitamin A). Women with low carotene or vitamin A intake have a higher rate of complete molar pregnancy.¹

What are common symptoms of a molar pregnancy?

A molar pregnancy triggers the same first-trimester symptoms that a normal pregnancy does (a missed menstrual period, breast tenderness, fatigue, increased urination, morning sickness). It may be diagnosed during an early ultrasound test. In addition to normal pregnancy signs, a molar pregnancy usually causes additional symptoms, which can include:

Vaginal discharge of tissue that is shaped like grapes. This is the most characteristic symptom of a molar pregnancy.
Vaginal bleeding (light or heavy).
A uterus that is abnormally large for the length of the pregnancy.
Severe nausea and vomiting.
Signs of hyperthyroidism, such as fatigue, weight loss, increased heart rate, heat intolerance, sweating, irritability, anxiety, muscle weakness, and thyroid enlargement.
Pelvic discomfort.

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WebMD Medical Reference from Healthwise

Last Updated: October 29, 2007

This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.