What Is a Molar Pregnancy?

Molar pregnancies result from abnormal fertilization, and they can cause serious health issues if not treated properly. Here's what to know about the rare pregnancy complication.

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Most pregnancies are healthy and progress normally. Rarely, some people experience a complication called a molar pregnancy, also known as a hydatidiform mole. Molar pregnancies happen as a result of abnormal fertilization of the egg, leading to an abnormal mass in a person's uterus. They almost never amount to a viable pregnancy.

Fortunately, molar pregnancies are uncommon and they rarely impact future pregnancies. Here's what to know about molar pregnancy causes, risk factors, symptoms, and how they're typically treated.

What Is a Molar Pregnancy?

The phenomenon that results in a molar pregnancy happens at the very beginning of gestation, says Sasha Andrews, M.D., a maternal-fetal medicine specialist at Obstetrix of Colorado, part of Pediatrix Medical Group. "Molar pregnancies occur when cells that normally form the placenta develop abnormally," she explains.

There are two main types of molar pregnancies: complete and partial:

  • 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

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 benign when treated, but sometimes they become invasive or cancerous, which is why it's important that molar pregnancies be thoroughly removed from the body and follow-up treatment be conducted.

Thankfully, molar pregnancies are rare, occurring in about every 1 in 1,000 pregnancies.

What Causes a 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 a Molar Pregnancy

Anyone can have a molar pregnancy, though they're uncommon in the first place. Still, certain people may be more prone to experiencing a molar pregnancy, says Dr. Andrews. Younger women (less than 20 years) and women over the age of 35 have a higher risk. Additionally, experiencing a previous molar pregnancy increases your chances of another one, notes Dr. Andrews. The risk of having a repeat molar pregnancy is about 1 to 2 percent, according to March of Dimes.

Symptoms of a Molar Pregnancy

Early signs of a molar pregnancy may resemble symptoms of any pregnancy, such as a missed period, says Adi Katz, M.D., 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, 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 may also form 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," Dr. Katz explains.

Diagnosing a Molar Pregnancy

Although your pregnancy may 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 uterus is larger than normal, and no fetal heartbeat is heard. If a molar pregnancy is suspected, you'll need to have an ultrasound to confirm it. Your health care provider may want to perform some additional tests for a molar pregnancy diagnosis, such as a blood test to confirm hGC levels.

Molar Pregnancy Treatment

Some people will miscarry their molar pregnancy spontaneously. When this happens, you may notice bleeding, along with the discharge of small, grape-like cysts. Often, molar pregnancies need to be removed surgically, usually via a dilation and curettage (D&C) procedure, which uses a suctioning device to remove 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. This signals that no remnants of the molar pregnancy remain.

Most health care providers recommend waiting 6-12 months before getting pregnant again after a molar pregnancy. This ensures that no abnormal tissue remains, and decreases your chance of having another molar pregnancy.

Possible Complications from a Molar Pregnancy

If not properly treated and promptly addressed, molar pregnancies can result in both 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, says Dr. Andrews.

If tissue from the molar pregnancy remains in your uterus after treatment, longer term complications can appear. First, about 20% of people might develop a syndrome called persistent gestational trophoblastic neoplasia, where the molar tissue invades surrounding tissue in your body. This is often treated with chemotherapy drugs or hysterectomy. Even more rarely, the molar tissue develops into choriocarcinoma, a type of cancer that may require chemotherapy and other cancer-fighting drugs.

The Bottom Line

Hearing about molar pregnancies and their complications can be scary. But it's important to understand that the majority of people who experience molar pregnancies—especially if they are properly treated—do not end up with long-term issues, and they go on to have healthy pregnancies in the future. If you have questions about molar pregnancies or think you may be experiencing one, please contact your OB-GYN or midwife.

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