What is a molar pregnancy?

When a sperm fertilizes an egg, things usually go according to plan. The sperm and egg each contribute equally to donating their chromosomes to the future developing fetus. Sometimes, however, this process can be a little mixed up and cause the growth of abnormal tissue in the uterus. This can lead to a molar pregnancy.

A molar pregnancy (or a mole) is when a normal fetus is not formed after fertilization. Two scenarios can result: a complete mole or a partial mole. A complete mole is when the sperm fertilizes an empty egg (there is no genetic material in it), and this leads to the development of only placental tissue. There is no fetus in a complete mole. However, a partial mole is usually the result of too many chromosomes present at fertilization. This leads to abnormal tissue as well as a fetus that usually has severe and often fatal defects.

From the start, these pregnancies are not normal, and they usually will lead to symptoms early on. The majority of women with a molar pregnancy will have bleeding in the first trimester. This will lead to an ultrasound that will appear abnormal or blood levels of the pregnancy hormone level hCG that do not increase normally or actually decrease.

Other signs and symptoms of a molar pregnancy include abnormally high hCG levels, hyperemesis gravidarum (from those very high hormone levels), a uterus that measures bigger than it should for how far along a woman is, and abnormal cysts seen on the ovaries. Nowadays with ultrasound being so common, the abnormal appearance of tissue in the uterus combined with hCG testing is usually how this is diagnosed.

Treatment is by dilation and curettage,or D&C, since leaving this abnormal tissue in a woman’s uterus can lead to worse bleeding and impair her health. The treatment does not end here, however. In order to ensure that all the tissue has been removed, a doctor will check blood hCG levels to make sure they go down appropriately and eventually are undetectable. This is usually done once a week, and the process may take a few weeks or months. During this time it is very important not to get pregnant again. Using abstinence or a reliable form of birth control is extremely important.

Why is it important that the hCG levels go back to normal? This is because a small percentage of molar pregnancies can actually persist and go on to cause certain types of cancer known as malignant gestational trophoblastic disease. This is less common in partial moles (less than 5 percent) than in complete moles (about 6-32 percent). Treatments for these include chemotherapy and surgery (which may include a hysterectomy), depending on the type and stage of malignant gestational trophoblastic disease.

While all of this sounds very scary, the good news is that molar pregnancies are pretty rare, occurring in only 1 out of every 1,500 pregnancies in the US. And luckily, 80 percent of those women affected by molar pregnancies will not have any issues with malignant gestational trophoblastic disease afterward. Women who’ve had a molar pregnancy before, however, have a 1-2 percent chance of it happening again. These women should be sure to establish prenatal care very early in subsequent pregnancies so an early ultrasound can be done.

Reviewed by Dr. Jen Lincoln, April 2020