What is a molar pregnancy?

Molar pregnancy: What it is

A molar pregnancy (a hydatidiform mole or HM) is a growth in the uterus. It happens after an egg is fertilized and the cells that would normally form the placenta instead develop into a mass of cysts (small, fluid-filled sacs). The placenta is unable to support the embryo, and there is no space for the embryo to develop normally. This rare complication happens at the beginning of pregnancy and results in pregnancy loss.

There are two kinds of molar pregnancies: a complete mole and a partial mole.

How common are molar pregnancies?

Molar pregnancies are uncommon. Only about 1 in 1,000 pregnancies in the United States is a molar pregnancy.

You're more likely to have a molar pregnancy if you:

• Are under age 20 or over age 35. Your risk of complications is higher if you're older than 40. (Age is a risk factor for complete molar pregnancies but not partial molar pregnancies.)
• Have had a previous molar pregnancy. The risk of a repeat mole is about one percent after one mole (10 to 20 percent more than the risk for the general population) and 15 to 20 percent after two moles.
• Have had two or more miscarriages. The risk of a molar pregnancy increases two- to three-fold compared to women without a history of prior miscarriage. Still, most people with miscarriages – even multiple miscarriages – will never develop a mole.
• Have had problems with infertility. Difficulty with fertility is associated with double the risk of a complete mole and three times the risk of a partial mole.
• Low levels of dietary carotene (vitamin A) and animal fat. (This has been associated with complete moles but not partial moles.)

How does a molar pregnancy happen?

A molar pregnancy happens when there are certain problems with the genetic information (the chromosomes) in the fertilized egg at conception. In normal pregnancies, the fertilized egg contains 23 chromosomes from the father and 23 from the mother. In a molar pregnancy, there are two sets of chromosomes from the father.

Are there different types of molar pregnancies?

Yes, there are two types of molar pregnancies.

A complete mole usually results when the fertilized egg contains two copies of the chromosomes from the father and none from the mother. The egg develops into a growth with no embryo, amniotic sac, or any normal placental tissue. Instead, the placenta forms a mass of cysts that looks like a cluster of grapes.

A partial mole usually happens when the fertilized egg has the normal set of chromosomes from the mother and two sets from the father, so there are 69 chromosomes instead of the normal 46. (This can happen when chromosomes from the sperm are duplicated or when two sperm fertilize the same egg.)

In a partial molar pregnancy, there may be some normal placental tissue among the cluster of abnormal tissue. The embryo does begin to develop, so there may be a fetus or just some fetal tissue or an amniotic sac. But even if a fetus is present, in most cases, the fetal anatomy is so abnormal that it can't survive.

Molar pregnancy symptoms

If you do have signs of a molar pregnancy, you may have some spotting or heavier bleeding after having typical pregnancy symptoms. The bleeding might be bright red or a brown discharge, continuous or intermittent, light or heavy. This bleeding could start as early as six weeks into your pregnancy or as late as 12 weeks, or even later.

In addition to bleeding, you might also have:

• Severe nausea and vomiting
• Vaginal passage of grapelike cysts (fluid-filled sacs)
• Abdominal cramping
• Pelvic pressure or pain

Your doctor may find other signs of a molar pregnancy, such as:

• Anemia
• Ovarian cysts
• Hypertension (high blood pressure)
• Hyperthyroidism (overactive thyroid)
• Abnormal growth of the uterus (too large for gestational age)

Some women develop very early-onset preeclampsia around mid-pregnancy if they have an undiagnosed molar pregnancy. However, because ultrasound helps practitioners diagnose molar pregnancies earlier these days, it's rare to carry one long enough for this condition to develop.

Call your doctor or midwife right away if you have any spotting or bleeding or other symptoms of a molar pregnancy.

How is a molar pregnancy diagnosed?

Molar pregnancy is usually diagnosed with a combination of one or more of these:

• Blood work. Levels of the hormone hCG are typically higher than normal in a molar pregnancy.  
• Ultrasound. A complete mole is generally diagnosed in the first trimester on ultrasound because of the classic appearance of a "snowstorm" pattern and lack of a fetus. A partial mole is more difficult to diagnose early on, because the ultrasound may appear normal in the first trimester. For cases that don't end up in first trimester miscarriage, a later, anatomy ultrasound will be abnormal with severe growth restriction and multiple birth defects.
• Symptoms (see above)

Genetic testing with amniocentesis can also help diagnose a partial mole, as can NIPT testing, although NIPT testing is not as accurate for this purpose.

What to expect from a molar pregnancy ultrasound

Your doctor will probably use an ultrasound to diagnose the molar pregnancy. Sometimes a transvaginal ultrasound is used. (Transvaginal ultrasound involves inserting a wand-like device in your vagina to get the clearest possible view of your uterus.)

An ultrasound can detect a complete molar pregnancy as early as eight or nine weeks of pregnancy.

The ultrasound may show these signs of a complete molar pregnancy:

• No embryo or fetus
• No amniotic fluid
• A thick, cystic placenta. A central mass with spaces that's sometimes described as a "snowstorm" pattern. A grapelike cluster of blood vessels and tissue
• Ovarian cysts

Or the ultrasound may show these signs of a partial molar pregnancy:

• An unexpectedly small fetus for the gestational age
• Low amniotic fluid
• A placenta with one or more abnormalities
• Changes in the shape of the gestational sac
• Multiple fetal abnormalities if you are further along (in the second trimester)

Treatment options for molar pregnancy

Sometimes a molar pregnancy will miscarry on its own, but in order to reduce the risk of complications, your provider will want to treat the molar pregnancy as soon as its diagnosed. If not treated, a molar pregnancy can cause the uterus to rupture or other complications (see below).

D & C

If you're diagnosed with a molar pregnancy, the abnormal tissue must be removed. Usually a D&C (dilation and curettage) or suction curettage is performed to remove the tissue. This procedure can be done under general or regional anesthesia, or you can be sedated intravenously.

To perform a D&C, the doctor inserts a speculum into the vagina, cleans the cervix and vagina with an antiseptic solution, and dilates the cervix with narrow metal rods. They then pass a hollow plastic tube through the cervix and suction out the tissue from the uterus. Finally, they use a spoon-shaped instrument called a curette to gently scrape the rest of the tissue from the walls of the uterus.

One thing to note: If you decide you don't want another pregnancy, you might opt for a hysterectomy instead of a D&C to remove a molar pregnancy, because it lowers your risk that the abnormal cells will return. (See long-term complications, below.) Women over 40 who have complete moles are often offered this option because they're at particular risk.

Chest X-ray

If you have pulmonary symptoms, such as chest pain, you'll have a chest X-ray as well to see whether abnormal cells from the molar pregnancy have spread to your lungs. It's rare for these cells to spread to other parts of the body, but if they do, the lungs are the most common site.

HCG monitoring

Your practitioner will then want to monitor your levels of hCG once a week to make sure they're declining – an indication that no molar tissue remains. Once the levels go down to zero for a few weeks in a row, you'll have to have them checked every month or two for the next year.

Are there any long-term complications?

Occasionally, abnormal cells remain and continue to grow after the tissue is removed. This happens in up to 5 percent of women with partial moles and about 15 to 20 percent of women with complete moles, and it's called persistent gestational trophoblastic neoplasia (GTN). Two types of GTN are:

• Invasive mole. This happens when the moles grow into the muscle layer of the uterus. The risk of invasive moles increases the longer the pregnancy continues. They're usually treated with medication.
• Choriocarcinoma. A very small number of moles (1 in 20,000 to 40,000) advance to this type of cancer, which develops in the placenta and can spread to other organs. Choriocarcinoma is treatable with chemotherapy. With prompt and appropriate treatment, nearly 100 percent of cases of choriocarcinoma are curable when it hasn't spread beyond the uterus. Even in rare cases in which the abnormal cells have spread to other organs, 80 to 90 percent of cases can be cured. After you're in complete remission, you'll need to have your hCG levels monitored for a year, and possibly other regular testing.

Getting pregnant after a molar pregnancy

No matter what kind of treatment you've received, you'll need to wait a year after your hCG levels go back down to zero before trying to get pregnant again. If you got pregnant before then, your hCG levels would rise, and it would be impossible for your practitioner to tell whether abnormal tissue was growing back.

The good news is that having a molar pregnancy doesn't affect your fertility or ability to have a normal pregnancy, even if you've had chemotherapy. You're not at any increased risk for stillbirth, birth defects, preterm delivery, or other complications. And your odds of having another molar pregnancy are only 1 to 2 percent. You'll have a first-trimester ultrasound in any subsequent pregnancies to make sure all is well.