A molar pregnancy, also called hydatidiform mole, is an abnormal growth of placental tissue in a woman's uterus. It is an unsuccessful pregnancy that occurs after a man's sperm has fertilized a woman's egg and when tissue that would normally develop into the placenta to nourish the developing fetus forms an abnormal growth, or mass, containing hundreds of grape-sized cysts (fluid-filled sacs).
Usually the first clue to a molar pregnancy is when a woman has vaginal bleeding in the early months of pregnancy. She may also have a larger-than-normal sized uterus, pain from ovarian cysts and severe morning sickness.
Many women who are diagnosed with a molar pregnancy have never heard of it before, and were unaware that this pregnancy complication exists, said Dr. John Schorge, chief of gynecology and oncology at Massachusetts General Hospital in Boston. There doesn't seem to be a reason why it occurs aside from a woman's age, he said. Becoming pregnant before age 20 or after age 40 increases a woman's risk of having a molar pregnancy, Schorge said.
Complete mole vs. partial mole
There are two types of molar pregnancies: a complete mole and a partial mole. Both types are caused by an abnormally fertilized egg. A complete mole pregnancy is a more common occurrence than a partial mole pregnancy, Schorge said.
In a complete molar pregnancy, the embryo does not develop but placental tissue grows rapidly into an abnormal mass. This usually occurs because a woman's egg lacks maternal chromosomes (the mother's chromosomes are either lost or inactivated), and the egg is fertilized by one or two sperm cells. This results in a fertilized egg that only contains chromosomes from the father, or 46 chromosomes. (An ultrasound will show that there is no fetus only placental tissue.)
In a partial molar pregnancy, a woman releases a normal egg, but two sperm fertilize the egg instead of one. This leads to an abnormal embryo that contains too many chromosomes: one set of chromosomes from the mother and two sets of chromosomes from the father, or 69 chromosomes instead of the normal 46 (23 from the mother and 23 from the father).
In a partial molar pregnancy, there are too many chromosomes and the embryo develops abnormally and cannot survive.
Signs & symptoms
Vaginal bleeding is the most common symptom of both types of molar pregnancies, Schorge told Live Science. He said that a complete mole pregnancy might have heavier bleeding than a partial mole pregnancy, in which bleeding is usually relatively minor.
Vaginal bleeding may be dark brown to light red in color, and it typically occurs in the second or third month of pregnancy. The blood may contain little cysts that look like tiny grapes.
Every pregnant women should know that when she is having any bleeding during pregnancy, she needs to contact her doctor immediately, Schorge said.
In addition to vaginal bleeding, other signs of a molar pregnancy may include:
Abnormal growth of the uterus. For a complete mole, the size of the uterus is larger than normal during early pregnancy. For a partial mole, the size of the uterus is smaller than usual.
Enlarged ovaries. High levels of hCG (human chorionic gonadotropin), a hormone produced during the early phase of pregnancy, may cause the ovaries to be larger than normal during pregnancy.
Severe nausea and vomiting.
High blood pressure during the first trimester or early second trimester of pregnancy.
In the United States, about 1 in every 1,000 pregnancies may be a molar pregnancy, making them a rare occurrence.
The following factors may be increase a woman's risk of having a molar pregnancy:
Age. Women at the extreme ends of their childbearing potential — under age 20 or over age 40 — may be at higher risk of a molar pregnancy, Schorge said. The thinking is that girls who get pregnant in their teens, when their eggs are more likely to be irregular and not ideal, and women who are older than 40 and trying to become pregnant with their own eggs, are more likely to have irregular eggs, he explained.
Prior molar pregnancy. If a woman has had one molar pregnancy, her risk, on average, is 1 to 2 percent higher of having another one, Schorge said. But he also noted that a woman's chances are also good that her next pregnancy will be normal and she will carry the fetus to term. If a woman had two prior molar pregnancies, her risk is 15 to 20 percent higher of having another one, Schorge said.
Asian ethnicity. Women from Southeast Asia, including Vietnam and Korea, might be at a slightly increased risk for a molar pregnancy, and there have been some dietary theories proposed about why this may be the case, Schorge told Live Science.
For the most part, doctors don't truly know what causes a molar pregnancy, Schorge said. Some dietary theories, such as a deficiency ofcarotene, a plant pigment found in some red and orange fruits and vegetables that is converted by the body into vitamin A, have been proposed as one possible reason, he explained.
Diagnosis and tests
A molar pregnancy can usually be diagnosed by abdominal ultrasound, which can show the presence of cysts in the uterus. A complete mole pregnancy may be easier to detect by ultrasound than a partial mole pregnancy.
A woman will also be given a blood test to measure her levels of hCG. In women with a complete mole pregnancy, levels of hCG may be higher than expected at that stage of the pregnancy. Rapidly growing placenta tissue triggers the release of hCG.
In women with a partial mole pregnancy, levels of hCG may be normal or slightly increased for that point in the pregnancy, making it harder for doctors to diagnose, according to the American Cancer Society.
Treatment and medication
Some women who have a molar pregnancy may see a cancer doctor (oncologist) who specializes in gynecology for treatment.
A molar pregnancy is typically treated by removing the embryo and placenta from a woman's uterus by a procedure known as dilation and curettage (D&C). To confirm the diagnosis of a molar pregnancy, a pathologist will examine under a microscope a sample of molar tissue that was removed during the D&C.
After the D&C, a woman's levels of hCG in her blood will be regularly checked and monitored for six months to 1 year to make sure her hCG falls to normal levels and that no molar tissue remains in her uterus.
If hCG levels don't fall to normal levels or start to rise again, her doctor may recommend additional treatment.
A woman who is older and not planning to become pregnant again may elect to have a hysterectomy, a surgery to remove the uterus, instead of undergoing a D&C.
Risks to mother
There is a short-term risk to a mother that a molar pregnancy could develop into gestational trophoblastic disease (GTD), an umbrella term for a condition in which molar tissue remains in the uterus after it has been surgically removed and continues to grow, Schorge told Live Science.
This can be treated with the chemotherapy drug methotrexate. GTD is one of the few cancers that can almost always be cured by chemotherapy no matter how advanced the disease is, according to the American Cancer Society.
In extremely rare cases, a very small proportion of molar pregnancies can develop into an invasive mole or choriocarcinoma, a cancer that can spread to nearby tissue or other parts of the body. This can be treated with chemotherapy, involving several different cancer drugs.
Women who have had a molar pregnancy should wait about one year before trying to become pregnant again, Schorge recommended. By waiting one year to conceive, this will help doctors to know that an increase in hCG levels on a blood test is because of a new pregnancy and not because abnormal molar tissue has come back, he said.
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Cari Nierenberg has been writing about health and wellness topics for online news outlets and print publications for more than two decades. Her work has been published by Live Science, The Washington Post, WebMD, Scientific American, among others. She has a Bachelor of Science degree in nutrition from Cornell University and a Master of Science degree in Nutrition and Communication from Boston University.