What Is a Molar Pregnancy?

Most pregnancies are healthy and progress normally. However, approximately 1 in 1,000 pregnancies are complicated by a condition called a molar pregnancy. Molar pregnancies are caused when the genetic material within the egg and sperm combine in an imbalanced way. Molar pregnancies are almost always not viable and need to be removed from the uterus.

Fortunately, molar pregnancies rarely impact future pregnancies. However, while they are generally benign (not cancerous) for the pregnant person, in rare cases, they can become malignant if left untreated. Here's what to know about molar pregnancy causes, risk factors, symptoms, and how they're typically treated.

What Is a Molar Pregnancy?

A molar pregnancy (also known as a hydatidiform mole) is a tumor that develops from the cells that help an embryo attach to the uterine lining and form the placenta after fertilization.

The phenomenon that results in a molar pregnancy happens at the very beginning of gestation, says Sasha Andrews, MD, a maternal-fetal medicine specialist at Obstetrix of Colorado, part of Pediatrix Medical Group. Molar pregnancies occur when genetic material from the egg and sperm combine abnormally, causing the cells that usually form the placenta to develop abnormally.

There are two main types of molar pregnancies:

• In a complete molar pregnancy, an abnormal placenta forms without an embryo
• In a partial molar pregnancy, an abnormal placenta forms, along with an embryo that's unable to survive (in extremely rare cases, the embryo can develop, but typically with significant genetic abnormalities)

As a molar pregnancy progresses, the placenta forms a mass, which takes over the pregnancy. This mass, or tumor, is called a hydatidiform mole, which is how molar pregnancy gets its name.

Most of the time, molar pregnancies resolve completely after treatment, but sometimes they become invasive or cancerous, which is why it's important that molar pregnancies be treated. Follow-up testing of human chorionic gonadotropin (hCG) levels is conducted to ensure no pregnancy tissue remains.

During this follow-up period, it's important to not get pregnant, as a new gestation could make it difficult to tell if a cancer is forming. It should be safe to conceive again after several months.

Causes of Molar Pregnancy

Molar pregnancies occur because of problems that happen when the sperm and egg meet. "Both partial and complete molar pregnancies are caused by abnormal fertilization, leading to an imbalance of genetic material from the egg and sperm," describes Dr. Andrews.

In complete molar pregnancies, one or two sperm fertilize an empty egg, resulting in genetic material solely from the paternal side. In a partial molar pregnancy, maternal genetic material is present, but two sperm fertilize the egg, resulting in an abnormally high number of paternal chromosomes (69 total chromosomes, instead of the typical 46).

Risk Factors for Molar Pregnancy

Anyone can have a molar pregnancy, though they're very uncommon. Still, certain people may be at higher risk of a molar pregnancy, says Dr. Andrews. For example, people under 20 and those over the age of 35 have a higher risk of molar pregnancy.

Additionally, experiencing a molar pregnancy increases your chances of having another one, notes Dr. Andrews. Research shows that the risk of having a second molar pregnancy after experiencing a complete molar pregnancy is about 1%. The risk is much lower after a partial molar pregnancy.

Symptoms of Molar Pregnancy

Early symptoms of a molar pregnancy may resemble symptoms of any pregnancy, such as a missed period or sore breasts, says Adi Katz, MD, the director of gynecology at Lenox Hill Hospital. As time progresses, you may notice other, more concerning symptoms.

These include intense nausea and vomiting, vaginal bleeding, and lower abdominal pain or pressure from your uterus, says Dr. Katz. "The uterus can also be bigger than expected," she says. Some of these molar pregnancy symptoms, such as intense nausea and vomiting or hyperemesis gravidarum, result from higher than normal amounts of the pregnancy hormone hCG.

Occasionally, people with molar pregnancies will develop hypertension or signs of preeclampsia, such as swelling of the extremities. Thyroid issues (like hyperthyroidism) can develop, and grape-like cysts (called molar pregnancy discharge) may also pass out of your vagina.

There are certain symptoms of molar pregnancy that require emergency care, warns Dr. Katz. Anemia from excess vaginal bleeding is one of them. "Another emergency can be severe nausea and vomiting with inability to eat or drink, therefore becoming severely dehydrated," she adds.

Diagnosing a Molar Pregnancy

Although your pregnancy may at first appear normal in many ways, your health care provider may suspect that you have a molar pregnancy if you're experiencing abnormal bleeding or other concerning symptoms, your hCG levels are higher than expected, or you have an abnormal first-trimester ultrasound result.

Occasionally, partial molar pregnancies are diagnosed with pathology after submitting tissue from what seems to be a normal first-trimester miscarriage.

How Molar Pregnancy Is Treated

Some people will miscarry their molar pregnancy spontaneously (without medical intervention). When this happens, you may notice bleeding, along with the molar pregnancy discharge of small, grape-like cysts. However, miscarriage of a partial molar pregnancy may be indistinguishable from a non-molar first-trimester miscarriage.

While spontaneous miscarriage is possible, more often molar pregnancies require treatment with a procedure known as a dilation and curettage (D&C), which uses a suctioning device to remove the molar pregnancy tissue from your uterus.

Taking a "wait and see" approach with molar pregnancies is not advisable, says Dr. Katz. "It is unsafe to leave it untreated," she explains. In addition to D&C treatment, your hCG levels will need to be monitored for several weeks or months, until you no longer have any pregnancy hormone left in your body, says Dr. Katz. HCG levels are monitored to ensure that no remnants of the molar pregnancy remain.

Most health care providers recommend waiting at least six to 12 months before getting pregnant again after a molar pregnancy to ensure that no abnormal tissue remains and decrease your chance of having another molar pregnancy.

Possible Complications of Molar Pregnancy

If not properly treated and promptly addressed, molar pregnancies can result in immediate and longer-term complications. "Immediate complications of molar pregnancies can include anemia from vaginal bleeding and hyperemesis gravidarum (a form of severe morning sickness)," says Dr. Andrews. "The high hCG levels can also cause abnormal thyroid function and ovarian theca lutein cysts." In rare cases, preeclampsia may occur, she adds.

In some people, some tissue from the molar pregnancy remains in the uterus even after treatment. When this happens, the abnormal tissue develops into a condition called gestational trophoblastic neoplasia (GTN). This occurs in about 15% to 20% of complete molar pregnancies and 1% to 5% of partial molar pregnancies.

With GTN, the residual molar tissue can grow and invade into surrounding tissue, and can even become cancerous. GTN is typically treated with chemotherapy drugs and/or surgery. Even more rarely, the molar tissue develops into choriocarcinoma, a type of cancer that may require chemotherapy and other cancer-fighting drugs.