In a molar pregnancy, the early placenta develops into an abnormal mass (called a hydatidiform mole) that looks a little like a bunch of white grapes. The embryo either does not form at all or is malformed and cannot survive. About 1 in 1,500 pregnancies is molar.
There are two types of molar pregnancy: complete mole (there is no embryo and no normal placental tissue); and partial mole (there is an abnormal embryo, and there may be some normal placental tissue.) Both types of molar pregnancy are caused by an abnormal fertilized egg.
In a complete mole, all of the fertilized egg’s chromosomes (tiny thread-like structures in cells that carry genes) come from the father. Normally, half come from the father and half from the mother. In a complete mole, shortly after fertilization, the chromosomes from the mother’s egg are lost or inactivated, and those from the father are duplicated.
In most cases of partial mole, the mother’s 23 chromosomes remain. However, there are two sets of chromosomes from the father (so the embryo has 69 chromosomes instead of the normal 46). This can happen when two sperm fertilize an egg.
Molar pregnancy poses a threat to the pregnant woman because it can occasionally result in rare pregnancy-related types of cancers called invasive mole and choriocarcinoma.
The most common symptom of a molar pregnancy is abnormal vaginal bleeding in the first three months of pregnancy. Other symptoms may include severe nausea and vomiting, rapid uterine growth, high blood pressure, cysts (fluid-filled sacs) on the ovaries, and hyperthyroidism.
Health care providers use an ultrasound to diagnose a molar pregnancy. The provider also measures the levels of the hormone hCG in the mother’s blood, which often are higher than normal with a molar pregnancy.
A molar pregnancy is a frightening experience. Not only does the woman lose a pregnancy, she learns that she has a slight risk of developing cancer. To protect her, all molar tissue must be removed from the uterus. This usually is done with a D&C. Occasionally, when the mole is extensive and the woman has decided against future pregnancies, she may have a hysterectomy.
After mole removal, her provider again measures the level of hCG. If it has dropped to zero, the woman generally needs no additional treatment. However, the provider will continue to monitor hCG levels for 6 months to 1 year to be sure there is no remaining molar tissue. It’s important that a woman not become pregnant during this time, because a pregnancy would make it difficult to monitor hCG levels.
After the uterus is emptied, about 20 percent of complete moles and less than 5 percent of partial moles continue on. The remaining abnormal tissue may continue to grow in the wall of the uterus. This is called persistent gestational trophoblastic disease (GTD) or invasive mole. The diagnosis is made when hCG blood levels stop decreasing or begin to increase. Treatment with one or more cancer drugs cures persistent GTD nearly 100 percent of the time. Rarely, a cancerous form of GTD, called choriocarcinoma, develops and spreads to other organs. Use of multiple cancer drugs is usually very successful at treating this cancer.
All of this may sound scary, but the good news is that a woman who has had molar pregnancy usually can go on to have healthy pregnancies. The risk that a mole will develop in a future pregnancy is only about 1 to 2 percent.