A hydatidiform mole, or molar pregnancy, occurs when a nonviable fertilized egg implants in the uterus. Occurrence is about 1 in 1000 pregnancies in the United States. The true etiologies for these are unknown. There are 2 distinct classifications of moles: complete and partial.

• Partial moles typically have a triploid genetic complement (69,XXY) and exist with fetal tissue. They also could be 96,XXXY. These usually arise from an egg being fertilized by 2 (or more) sperm. They have a lower malignant potential (< 5%). Although fetal tissue may be present, these are never normal pregnancies.
• Complete moles are created by placental proliferation without identifiable fetal tissue. These usually have a genetic complement of 46,XX derived solely from the male partner. This is likely due to the egg having lost its genetic complement at some time point and the sperm doubling its DNA. They have a much higher malignant potential (15%-25%).
• Rarely, hydatidiform moles may be related to a pregnancy with spermatogonia so that all tissues have the complete male genotype.

Patients with molar pregnancies will classically present with painless vaginal bleeding, uterine size measuring greater than gestational dating, and increased nausea / emesis or hyperemesis. They may also have elevated blood pressure or preeclampsia. Some will have signs of hyperthyroidism. Theca lutein cysts can occur on the ovaries due to elevated beta-human chorionic gonadotropin (beta-hCG, or BCHG) levels.

Risk of molar pregnancy is higher in patients younger than 20 and older than 40, as well as in patients with diets low in protein, folic acid, and carotene.

Twenty percent of patients with a complete molar pregnancy go on to develop persistent gestational trophoblastic disease.