What Is Molar Pregnancy?

Pregnant woman having ultrasound scan
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What Is Molar Pregnancy?

A molar pregnancy (also known as a hydatidiform mole) is a type of gestational trophoblastic disease (GTD). This type of pregnancy loss is caused by chromosomal abnormalities during conception that result in abnormal growths of placental tissue. Essentially, this occurs when an egg is fertilized, but instead of developing into a fetus, a mass of fluid-filled cells develops.

Except in very rare cases, molar pregnancies cannot be carried to term and do not include a viable (able to live) fetus. Molar pregnancies are usually not cancerous, but in some cases the tissue becomes malignant.

Molar pregnancies can have scary health complications, requiring months of precautionary monitoring after treatment, which usually includes dilation and curettage (D&C), a procedure to remove products of conception tissue from the uterus. After treatment, most molar pregnancies resolve without further complications, but there is a potential for lingering adverse effects.

Possible Adverse Effects

About 20% of women who have had molar pregnancies will develop one of two serious problems: an invasive mole or choriocarcinoma. These conditions are more likely to occur after complete molar pregnancies than partial molar pregnancies (these two types of molar pregnancies are detailed below). Only 2% to 4% of partial moles will develop either condition.

Invasive moles are more common than choriocarcinoma and occur when moles grow into the muscle layer of the uterus.

The risk of developing invasive moles increases the longer the pregnancy continues without treatment and can develop before or after surgical treatment. Invasive moles primarily occur after molar pregnancies, but will occasionally develop after miscarriage or term delivery.

Choriocarcinoma is a type of cancer that can develop at the placenta site and spread to the body. The tumors grow from cells that normally would develop into the placenta. While serious, this fast-growing cancer is almost always treatable with chemotherapy.

Risk Factors

A few risk factors, such as a previous molar pregnancy or advanced maternal age, can increase your odds of having a molar pregnancy. As with other miscarriages, though, a molar pregnancy can occur without any risk factors.

Known risk factors for molar pregnancy include the following:

  • Maternal age younger than 20 or older than 35, with increasing risk the older the mother is at conception 
  • Poor nutrition (specifically, low consumption of dietary carotene, which is found in orange fruits and vegetables like carrots, apricots, and sweet potatoes, and animal fat)
  • Previous molar pregnancy (about one in 100 will experience a repeated molar pregnancy)
  • Previous miscarriage

Rates of occurrence vary significantly worldwide, with some countries (particularly in Asia) experiencing much higher incidences than others. In North America, hydatidiform moles occur in about 0.6 to 1.1 of every 1,000 pregnancies, whereas studies in Southeast Asia and Japan have shown rates as high as 2 in 1,000.

The differences in incidence patterns are primarily thought to be due to nutritional and other socioeconomic factors. For example, in Korea, rates of molar pregnancy dropped from a high of 4.4 per 1,000 pregnancies in the 1960s to around 1.6 per 1,000 in the 1990s. This change is attributed to concurrent improvements made in the diet and living conditions of the general population.

Symptoms

Some women with molar pregnancies exhibit symptoms, while others have no specific symptoms. Often, pregnant women won't know anything is wrong with their pregnancy until the molar pregnancy is discovered at a prenatal doctor appointment.

Diagnostic clues that point to molar pregnancy might include higher than average human chorionic gonadotropin (hCG) hormone levels (the pregnancy hormone screened for in pregnancy tests), enlarged ovaries and uterus, placental cysts (small sacs filled with fluid) that form grape-like clusters visible via ultrasound, ovarian cysts, and early preeclampsia.

Molar pregnancy symptoms include the following:

  • Anemia
  • High blood pressure
  • Hyperthyroidism
  • No fetal heartbeat or movement
  • Pelvic pressure or discomfort
  • Uterus size that's either too big or too small for gestational age
  • Vaginal bleeding
  • Vaginal passage of grape-like cysts
  • Vomiting or nausea

Vaginal bleeding and nausea occur most commonly in molar pregnancies, but these symptoms can also occur in normal pregnancies or typical miscarriages. In addition, molar pregnancies can cause swelling in the abdominal area, but women with normal pregnancies can “show” early also.

Diagnosis

Most molar pregnancies are diagnosed in the first trimester. This condition may be discovered when a heartbeat does not become detectable by 12 weeks, but this can also be true of missed miscarriages. Diagnosis is usually confirmed by ultrasound, which reveals the absence of a healthy embryo and an abnormal placenta in its place that appears like a bunch of grapes.

Causes

Molar pregnancy is caused by chromosomal abnormalities of the egg and/or sperm and resulting embryo that disrupt how it develops. A healthy embryo has one set of chromosomes (or genetic material) from each parent. In a molar pregnancy, the genetic material is imperfect or missing, which causes a tumor to grow in the uterus instead of a healthy embryo and placenta.

Types

Molar pregnancies fall into two categories: complete and partial hydatidiform moles. Abnormal tissue growth happens because either one or two sperm fertilize an egg that contains no genetic material (a complete molar pregnancy) or the egg is normal but two sperm fertilize the same egg, resulting in two sets of chromosomes from the father. In either case, a viable fetus does not develop.

Although a partial molar pregnancy may contain some fetal tissue, it is often mixed in with the abnormal tissue. Additionally, the abnormal placenta cannot sustain a pregnancy and the chromosomal abnormalities of the embryo are not compatible with life, except in extremely rare cases. A complete molar pregnancy will not develop a recognizable fetus.

Partial moles are much less likely to become malignant than complete moles.

Treatment

Some molar pregnancies will miscarry without intervention, but if doctors detect molar pregnancy by ultrasound, they usually recommend a D&C or medication in order to reduce the risk of further complications. Surgery can usually remove most complete and partial moles. However, in about 20% of cases, some of the abnormal tissue remains, which can cause persistent gestational trophoblastic disease.

Persistent gestational trophoblastic disease is when molar pregnancies develop into invasive moles or choriocarcinoma. In these cases, the tumors continue to grow from abnormal placental tissue. Chemotherapy may be used to shrink the tissue growth. If the abnormal tissue grows through the uterine wall, severe, sometimes life-threatening, bleeding can occur.

Rarely, molar pregnancies might occur in twin conceptions with a hydatidiform mole alongside an otherwise viable pregnancy. In these cases, continuing the pregnancy can pose serious maternal health risks. In fact, there is a significant risk of developing persistent gestational trophoblastic disease, and many choose to terminate the pregnancy.

If left untreated, molar pregnancy can have serious maternal adverse effects, including the rupture of the uterus.

However, as routine prenatal ultrasound and other diagnostic techniques have become commonplace, most molar pregnancies are caught and treated early enough in pregnancy to avoid dangerous complications that occurred more often in the past when the condition was more likely to progress undetected into the second trimester.

Medical Monitoring

Because of the risk of developing an invasive mole or choriocarcinoma, doctors recommend that women who have had molar pregnancies be monitored for several months. Treatment usually includes weekly or monthly hCG blood tests. If the hCG levels fail to decrease after the pregnancy loss, or begin to rise again, this can be a symptom of persistent GTD. Monitoring usually lasts from six months to a year.

If the woman has three consecutive negative hCG blood tests, they are most likely out of the danger zone. Some doctors are less aggressive about monitoring women who have had partial molar pregnancies because the odds of complications are lower.

Coping

The emotional recovery from a molar pregnancy can be just as challenging as the physical one. Coping with this devastating pregnancy outcome can involve the same stages of grief as other miscarriages, but like ectopic pregnancy (another potentially dangerous condition), grieving from molar pregnancy can be both a relief that the condition was detected as well as grief for the loss of the expected baby.

You might hear comments along the lines of “at least they caught it in time” or “at least it wasn’t a real baby,” but it is perfectly normal to be sad and to grieve. Be sure to be patient with yourself as you take the time you need to recover. Consider seeking out support groups and other resources to help you get through the process as needed.

Getting Pregnant Again

Exact recommendations vary, but doctors usually advise waiting at least six months to try to get pregnant again after a molar pregnancy. This advice should always be followed and has a clear medical basis.

Why? Rising hCG levels can be the first indication of invasive moles or choriocarcinoma. Both conditions are highly treatable when detected. A new pregnancy would also cause hCG levels to rise, and if this happened, doctors would not be able to distinguish whether the rising hCG was from the new pregnancy or from a potentially malignant condition.

Because the treatment for invasive moles and choriocarcinoma can involve chemotherapy, pregnancy should be avoided until doctors can be sure that the need for chemotherapy will not arise.

A Word From Verywell

While experiencing a molar pregnancy can be heartbreaking, there is a good chance you will be able to conceive again—and have a healthy pregnancy next time. That said, about 1% to 2% of women who had a molar pregnancy will have another one, so your doctor may want to follow up with early ultrasounds and hCG blood tests in your next pregnancy to rule out a repeat molar pregnancy.

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