Molar pregnancy

What is a molar pregnancy?

A molar pregnancy (also called a hydatidiform mole or HM) is a tumor (growth) that develops in the uterus (womb) at the beginning of pregnancy. In a molar pregnancy, the placenta doesn’t form correctly. Instead it develops into a mass of cysts (small sacs filled with fluid) and can’t supply the baby with food and oxygen like it does in a healthy pregnancy.

A molar pregnancy almost always ends in pregnancy loss. About 1 in 1,000 pregnancies (less than 1 percent) in the United States is a molar pregnancy. Most women who have a molar pregnancy can go on to have a healthy pregnancy later. The risk of having another molar pregnancy is only about 1 to 2 in 100 women (1 to 2 percent).

If not treated, a molar pregnancy can be dangerous to the woman. It sometimes can cause a rare form of cancer.

A molar pregnancy is a kind of gestational trophoblastic disease (also called GTD). This is a group of conditions that cause tumors to grow in the uterus.

What causes a molar pregnancy?

Molar pregnancy is caused when there’s a problem with an embryo’s chromosomes. Chromosomes are the structures that hold genes. A gene is part of your body’s cells that stores instructions for the way your body grows and works. Genes are passed from parents to children. For each pair of chromosomes, you get one from your mother and one from your father, so you have 46 in all. Molar pregnancy happens when there are two sets of chromosomes from the father.

There are two kinds of molar pregnancy:

1. Complete molar pregnancy. In this condition, there’s no embryo or placental tissue. It happens when chromosomes from the mother’s egg are lost or not working, and chromosomes from the father are copied, so all 46 chromosomes come from the father.
2. Partial molar pregnancy. In this condition, there’s an embryo and maybe some placental tissue. The embryo may start to grow, but it can’t survive. This condition happens when the embryo has two sets of chromosomes from the father and one from the mother, so it has 69 chromosomes instead of 46. This can happen when the father’s chromosomes are copied or when two sperm fertilize one egg.

Are you at risk for having a molar pregnancy?

You may be more likely than other women to have a molar pregnancy if you:

• Are younger than 20 or older than 35. The risk is higher if you’re older than 40.
• Have had a molar pregnancy before
• You’ve had two or more miscarriages

What are the signs and symptoms of a molar pregnancy?

A molar pregnancy may seem like a healthy pregnancy at first. But call your health care provider right away if you have any of these signs or symptoms:

• Bleeding from the vagina in the first 3 months of pregnancy
• Grape-like cysts that come from your vagina. A cyst is a sac filled with fluid.
• Intense nausea and throwing up. Nausea is feeling sick to your stomach.
• Pelvic pressure or pain. This happens rarely.

When checking your health, your provider may find other signs of a molar pregnancy, including:  

• Anemia. This is when you don’t have enough healthy red blood cells to carry oxygen to the rest of your body.
• Cysts on your ovaries. Ovaries are where eggs are stored in your body. You have two ovaries, one on each side of the uterus.  
• High blood pressure (also called hypertension). High blood pressure is when the force of blood against the walls of the blood vessels is too high. It can stress your heart and cause problems during pregnancy.
• Hypothyroidism. This is when your thyroid is too active. The thyroid is a gland in your neck that makes hormones that help your body store and use energy from food.
• Your uterus grows too much or too little.

How is a molar pregnancy diagnosed?

Your provider uses ultrasound to diagnose a molar pregnancy. An ultrasound uses sound waves and a computer screen to show a picture of your baby inside the womb. Your provider also gives you a blood test to measure the levels of a pregnancy hormone called human chorionic gonadotropin (also called hCG). A molar pregnancy can cause hCG levels to rise more slowly than in a healthy pregnancy.

How is a molar pregnancy treated?

If you have a molar pregnancy, your provider does a procedure called dilation and curettage (also called D&C) to remove the molar tissue (tissue from the tumor) from the lining of your uterus. After your procedure, your provider regularly measures your hCG levels until they’re normal. Your provider may want to check your hCG levels for 6 months to 1 year to be sure there’s no molar tissue in your uterus. Your provider may recommend that you not get pregnant during this time. Being pregnant changes your hCG levels and makes it hard to detect molar tissue.

If after a D&C molar tissue is still growing in the uterus, it can cause a condition called persistent GTD. About 1 in 5 women (20 percent) has this condition after a molar pregnancy, usually after a complete molar pregnancy. Persistent GTD can be treated with hysterectomy or chemotherapy. A hysterectomy is surgery to remove your uterus. Chemotherapy is medicine used to treat cancer. In rare cases, you may get a cancer-causing form of GTD called choriocarcinoma. This condition is treated with several kinds of cancer medicines.

If there’s a lot of molar tissue and you don’t ever want to get pregnant again, you may choose to have a hysterectomy.

More information

From hurt to healing (free booklet from the March of Dimes for grieving parents)

Share Your Story (March of Dimes online community for families to share experiences with prematurity, birth defects or loss)

Centering Corporation (general grief information and resources)

Compassionate Friends (resources for families after the death of a child)

Journey Program of Seattle Children’s Hospital (resources for families after the death of a child)

Share Pregnancy & Infant Loss Support (resources for families with pregnancy or infant loss)

Last reviewed: October, 2017