A molar pregnancy is a type of abnormal pregnancy that occurs when the tissue that normally develops into a fetus instead forms a mass of cells. This can happen when an egg is fertilized by a sperm but does not develop properly, leading to the growth of a non-viable tissue called a mole. There are two main types: complete and partial moles, with complete moles having no normal fetal tissue. Symptoms of a molar pregnancy can include vaginal bleeding, rapid growth of the uterus, and high levels of the hormone hCG. Diagnosis is typically made through ultrasound and blood tests. Treatment usually involves the removal of the abnormal tissue to prevent complications.