Hydatidiform mole


What is a hydatidiform mole?

A hydatidiform mole is a pregnancy that starts with the wrong amount of genetic information (chromosomes). It is also called a molar pregnancy. Our genetic information holds the instructions for the body to grow and develop normally. The genetic information is packaged into structures called chromosomes. In humans, most cells contain 23 pairs of chromosomes, or 46 total chromosomes. The two types of molar pregnancies are called complete and partial.

In pregnancy, an egg is fertilized by the sperm. Usually, the egg and sperm each provide half of the chromosomes. In a complete molar pregnancy, the egg is missing its chromosomes. The missing information is replaced by either making a second copy of the chromosomes from the sperm or being fertilized by two sperm. The resulting pregnancy doesn't have any genetic information from the mother, only the father. Without genetic information from the mother, a baby cannot develop. The fertilized egg grows into a mass of tissue that implants in the uterus. On ultrasound this mass looks like a cluster of grapes. There are no signs of a fetus. The growth of the mass is considered a type of tumor that is usually not cancerous. It is called gestational trophoblastic disease. However, without treatment and complete removal, it can become cancerous. This happens about 15-20% of the time.

Partial molar pregnancies usually happen when an egg is fertilized by two sperm. It can also happen when an egg is fertilized by one sperm that duplicates its genetic information. This results in a pregnancy that has too many sets of chromosomes. It can't develop normally due to the extra information. On ultrasound the placenta looks like a cluster of grapes and there is a fetus developing. The fetus has many growth problems and birth defects. The fetus typically dies within a few weeks of conception. Partial molar pregnancies have a small chance of becoming cancer, but this is uncommon.

Signs of a molar pregnancy include:

  • Vaginal bleeding in early pregnancy
  • Very high levels of a pregnancy hormone called human chorionic gonadotropin (hCG)
  • Severe nausea and vomiting
  • Abnormal growth of the uterus

Ask your doctor if you have questions about molar pregnancies. The type of doctor who manages pregnancy is called an obstetrician. To find an obstetrician near you, use the Find an OB-Gyn tool on the American College of Obstetricians and Gynecologists website.

Slim R, Mehio A (January 2007). "The genetics of hydatidiform moles: new lights on an ancient disease". Clin Genet. 71 (1): 25-34. doi:10.1111/j.1399-0004.2006.00697.x. PMID 17204043. Review.

Di Cintio E, Parazzini F, Rosa C, Chatenoud L, Benzi G (1997). "The epidemiology of gestational trophoblastic disease". Gen Diagn Pathol 143 (2-3): 103-8. PMID 9443567.

Cavaliere, A., Ermito, S., Dinatale, A., & Pedata, R. (2009). Management of molar pregnancy. Journal of Prenatal Medicine, 3(1), 15–17.


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