A molar pregnancy — also known as hydatidiform mole — is a noncancerous (benign) tumor that develops in the uterus. A molar pregnancy starts when an egg is fertilized, but instead of a normal, viable pregnancy resulting, the placenta develops into an abnormal mass of cysts.
In a complete molar pregnancy, there’s no embryo or normal placental tissue. In a partial molar pregnancy, there’s an abnormal embryo and possibly some normal placental tissue. The embryo begins to develop but is malformed and can’t survive.
A molar pregnancy can have serious complications — including a rare form of cancer — and requires early treatment.
A molar pregnancy is caused by an abnormally fertilized egg. Human cells normally contain 23 pairs of chromosomes. One chromosome in each pair comes from the father, the other from the mother. In a complete molar pregnancy, all of the fertilized egg’s chromosomes come from the father. Shortly after fertilization, the chromosomes from the mother’s egg are lost or inactivated and the father’s chromosomes are duplicated. The egg may have had an inactive nucleus or no nucleus.
In a partial or incomplete molar pregnancy, the mother’s chromosomes remain but the father provides two sets of chromosomes. As a result, the embryo has 69 chromosomes instead of 46. This can happen when the father’s chromosomes are duplicated or if two sperm fertilize a single egg.
A molar pregnancy may seem like a normal pregnancy at first, but most molar pregnancies cause specific signs and symptoms, including:
- Dark brown to bright red vaginal bleeding during the first trimester
- Severe nausea and vomiting
- Sometimes vaginal passage of grape-like cysts
- Rarely pelvic pressure or pain
If you experience any signs or symptoms of a molar pregnancy, consult your doctor or pregnancy care provider. He or she may detect other signs of a molar pregnancy, such as:
- Rapid uterine growth — the uterus is too large for the stage of pregnancy
- High blood pressure
- Preeclampsia — a condition that causes high blood pressure and protein in the urine after 20 weeks of pregnancy
- Ovarian cysts
- Overactive thyroid (hyperthyroidism)