Molar Pregnancy


Molar pregnancies are an uncommon and very frightening complication of pregnancy. The formal medical term for a molar pregnancy is "hydatidiform mole."

Molar pregnancies are an uncommon and very frightening complication of pregnancy. The formal medical term for a molar pregnancy is "hydatidiform mole." Simply put, a molar pregnancy is an abnormality of the placenta (afterbirth), caused by a problem when the egg and sperm join together at fertilization. The following is a brief review of this complicated subject.

Types of Molar Pregnancy

There are two types of molar pregnancy, complete and partial. Complete molar pregnancies have only placental parts (there is no baby), and form when the sperm fertilizes an empty egg. Because the egg is empty, no baby is formed. The placenta grows and produces the pregnancy hormone, called HCG, so the patient thinks she is pregnant. Unfortunately, an ultrasound (sometimes called a sonogram) will show that there is no baby, only placenta. A partial mole occurs when 2 sperm fertilize an egg. Instead of forming twins, something goes wrong, leading to a pregnancy with an abnormal fetus and an abnormal placenta. The baby has too many chromosomes and almost always dies in the uterus. Thus, molar pregnancies are "accidents of nature" that are not anyone’s fault. They are not caused by behavior, but they are more common in older women and in certain geographic locations. Also, although most molar pregnancies occur after a miscarriage, some occur after an ectopic (tubal) pregnancy or even a normal delivery.

Risk Factors

The incidence of molar pregnancy varies depending on where one lives. For example, in the US about 1 out of every 1000 pregnancies is a molar pregnancy. In Southeast Asia the incidence is 8 times higher. Interestingly, women from Mexico, Southeast Asia, and the Philippines have higher rates than white US women, who themselves have higher rates than black US women. Age over 40 is a risk factor for molar pregnancy, as is having a prior molar pregnancy. In fact, the chance of having another molar pregnancy is about 1 out of 100. The reasons for the geographic and age differences are currently unknown.

Symptoms and Diagnosis

Women with a molar pregnancy usually feel pregnant and complain of vaginal spotting or bleeding. Many women with molar pregnancies develop nausea and vomiting. Some even develop rare complications like thyroid disease or very early preeclampsia (toxemia). Preeclampsia occurring earlier than 20 weeks is very worrisome for a molar pregnancy. The doctor or midwife more than likely will check them for a possible miscarriage, and may order or perform an ultrasound (sonogram). The pelvic exam may reveal a larger, or smaller, than expected uterus. It may also reveal enlarged ovaries, caused by non-cancerous ovarian cysts stemming from abnormally high amounts of the pregnancy hormone HCG. The ultrasound will often show a "cluster of grapes" appearance or a "snowstorm" appearance, signifying an abnormal placenta. If a baby is present it’s a possible partial mole, while if the baby is absent it’s probably a complete mole. Treatment consists of a D&C (dilation and curettage) of the uterus, where a small vacuum device is inserted into the uterus, under anesthesia, to remove the molar pregnancy. This must be done very carefully or excessive bleeding and blood clots to the lungs can occur. The placental tissue is sent to the pathologist, who looks under the microscope to make the final diagnosis. An HCG level, and sometimes a thyroid level, are also obtained. In unusual cases, where the patient has completed her childbearing, a hysterectomy may be preferable. Although most cases of molar pregnancy occur after a miscarriage, some occur after ectopic pregnancies or a normal pregnancy. Therefore, women with abnormal bleeding or a persistent cough (especially if it produces blood) should see their doctor for an HCG level to make sure they do not have a molar pregnancy.


After evacuating a molar pregnancy it is critically important that the patient see her doctor frequently, as molar pregnancies can recur. Follow-up usually consists of a baseline chest x-ray, review of the pathology specimen, physical examination of the vagina and uterus every 2 weeks until the uterus returns to normal then every 3 months for a year, contraception like the pill or shot with no attempt to become pregnant for 1 year, and, most importantly, weekly HCG blood levels until zero then every month for a year. As one can see, this involves a lot of trips to the lab and the doctor’s office! This is important because molar pregnancies can "come back" even after a thorough D&C. When they come back the patient may need chemotherapy to prevent the microscopic placental cells from spreading to other organs like cancer. Fortunately, this only occurs in about 20% of complete molar pregnancies; it is even more uncommon with partial molar pregnancies. Many women are frustrated when their doctor recommends waiting one year to become pregnant. This is actually important, because a rise in HCG levels may indicate a normal pregnancy when the patient is trying to get pregnant, or a recurrent molar pregnancy, which requires chemotherapy. To avoid this confusion we ask for a 1 year period without becoming pregnant.

Treatment for recurrent molar pregnancy

When the HCG levels drop then increase again it means that the molar pregnancy has grown from microscopic cells in the wall of the uterus to larger cells. These cells can act like a cancer, and metastasize (spread) to other organs, like the lungs, brain, bones, and vagina. Treatment for recurrent molar pregnancy, called gestational trophoblastic neoplasia, or GTN, in medical terms, usually consists of a chemotherapy medication called methotrexate. Fortunately, methotrexate is a pretty "easy" chemotherapy on the system, and can be given as an intramuscular shot. Sometimes only 1 shot is necessary. In other cases, multiple shots, or even the addition of other medications, is necessary. Also, when GTN is suspected, the patient usually gets a CT scan of the brain, lungs, and abdomen, and a battery of blood tests. Again, weekly HCG tests are obtained until they fall to zero, then careful follow-up is undertaken for a year. Patients can expect an almost 100% cure rate using chemotherapy.

Future Pregnancy

Fortunately, the risk of having another molar pregnancy is about 1% (1 in 100). Most doctors will perform an ultrasound to make sure the pregnancy is normal when a patient has had a prior molar pregnancy. It is also a good idea to send the placenta to the pathologist after the delivery just to make sure there are not abnormal areas.

D. Ashley Hill, M.D.
Associate Director
Department of Obstetrics and Gynecology
Florida Hospital Family Practice Residency
Orlando, Florida Articles by Dr. Hill

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