Molar pregnancies happen when fetal tissue becomes an abnormal growth in the uterus instead of a fetus. Although it is not an embryo, the transformation triggers the symptoms of pregnancy.

Such pregnancies are relatively rare, occurring in one out of 1,500 women. But once discovered, a molar pregnancy needs to be treated immediately by removing all of the molar tissue. Otherwise, serious health problems can occur.

Molar pregnancies can be traced to genetics. Somehow, the genetic information contained in a sperm or egg is damaged. Normal human cells contain 23 pairs of chromosomes. In a normal pregnancy, one chromosome in each pair comes from the father and one comes from the mother. However, in a molar pregnancy, all of the fertilized egg chromosomes come from the father. For reasons that are not clear, the chromosomes from the mother’s egg are lost or inactive, while the father’s chromosomes duplicate. The egg may not have a nucleus or may have an inactive nucleus.

In a partial molar pregnancy, a mother’s chromosomes remain, but the father provides two sets of chromosomes. Thus, the embryo has 60 chromosomes instead of 46. This happens when two sperm fertilize a single egg or when the father’s chromosomes are duplicated.

Two Forms of Molar Pregnancy

There are two types of molar pregnancy:

1. Complete molar pregnancy. This happens when sperm fertilizes an egg that lacks complete genetic information. The tissue then grows as abnormal tissue, resembling a cluster of grapes as it fills the uterus.
2. Partial molar pregnancy. This form of molar pregnancy occurs when two sperm fertilize an egg. The placenta becomes the molar growth, and any resulting fetal tissue develops severe defects.

There are a number of risk factors for potential molar pregnancies. These include age (those age 35 and over increasingly are at risk); a history of molar pregnancies (particularly if two or more have occurred); a prior history of miscarriages; and a diet low in carotene, a form of Vitamin A.  

The majority of molar pregnancies are not cancerous and are confined to the uterus. A more aggressive tumor in molar pregnancy cases is the chorioadenoma destruens. In this instance, the grape-like clusters grow into or through a muscle layer or the uterine wall. In about 15 percent of cases, the cluster will spread to tissues outside of the uterus.  

All molar pregnancies, including choriocarcinoma, are more likely in women of Asian or African descent.

Molar Pregnancy Symptoms

Molar pregnancies resemble a normal pregnancy in its early stages. Missed periods and morning sickness may occur. But there are other symptoms that differentiate the molar pregnancy. These include vaginal bleeding, a larger-than-normal uterus, severe nausea and vomiting, and symptoms resembling hyperthyroidism, including a fast or irregular heartbeat, extreme tiredness, nervousness, and excessive sweating. Women may also experience an uncomfortable feeling in the pelvic area or vaginal discharge that has a grape-like shape.  


Making it difficult to determine is that many of these symptoms accompany a normal pregnancy, miscarriage or multiple pregnancy.

Diagnosing a Molar Pregnancy

Doctors will determine whether a pregnancy is a molar pregnancy by a pelvic exam, pelvic ultrasound and a blood test to measure pregnancy hormones. It also may be discovered during a routine ultrasound early in pregnancy.  

A molar pregnancy may be diagnosed as early as the eighth or ninth week of pregnancy. At that point, an ultrasound image test may show no embryo or fetus; no amniotic fluid; a thick cystic placenta that may nearly fill the uterus, and potentially ovarian cysts. A partial molar pregnancy will show a growth-restricted fetus, low levels of amniotic fluid and a thick placenta.

When a molar pregnancy is discovered, doctors usually will test for hyperthyroidism, preeclampsia and anemia.

Molar pregnancies are dangerous because they can cause heavy uterine bleeding. They may also cause gestational trophoblastic disease, which is the growth of normal tissue inside the uterus that may continue growing even after the molar pregnancy’s removal. A complete molar pregnancy will see about 150 to 200 cases develop into trophoblastic disease that will grow after molar removal. Out of 1000 cases of partial molar pregnancy, about 50 will develop into trophoblastic disease. Almost all women are cured with treatment, but in a few cases, it can develop into cancer, with the abnormal tissue spreading to other parts of the body.


Molar Pregnancy Treatment

The treatment for a molar pregnancy is immediate removal of all of the tissue growth from the uterus. This can be accomplished in several ways, including using vacuum aspiration, which sucks the tissue out of the uterus or a surgical procedure known as a dilation and curettage (D&C), in which the molar tissue is surgically removed in an outpatient procedure using a vacuum device.  

Some women who no longer wish to have children may have the entire uterus removed via surgery.

After surgery, doctors will take regular blood tests to make sure trophoblastic disease is not present. Patients will be required to use birth control pills or another contraceptive method for the next six to 12 months to avoid pregnancy in the wake of a molar pregnancy. Chemotherapy may be required if cancer is detected.

The trauma of losing a pregnancy and the waiting time required before trying to achieve another pregnancy is, of course, traumatic. Individual counseling or support groups are recommended in the wake of a molar pregnancy. The good news is that the odds of another molar pregnancy occurring are extremely small, estimated to be approximately 1 to 2 percent.