From MedlinePlus Genetics
Recurrent hydatidiform mole is a condition that affects women and is characterized by the occurrence of at least two abnormal pregnancies that result in the formation of hydatidiform moles. A hydatidiform mole is a mass that forms early in pregnancy and is made up of cells from an abnormally developed embryo and placenta. Normally, the embryo would develop into a fetus and the placenta would grow to provide nutrients to the growing fetus. When a hydatidiform mole occurs once, it is known as sporadic hydatidiform mole; if it happens again, the condition is known as recurrent hydatidiform mole.

The first symptom of a hydatidiform mole is often vaginal bleeding in the first trimester of pregnancy. During an ultrasound examination, the abnormal placenta appears as numerous small sacs, often described as resembling a bunch of grapes.

Hydatidiform moles are not naturally discharged from the body and must be surgically removed, typically by the end of the first trimester. After removal, there is up to a 20 percent risk that any tissue left behind will continue to grow and become a cancerous (malignant) tumor called a persistent mole. If the tumor invades the surrounding tissue of the uterus, it is called an invasive mole. In rare cases, this malignant tumor can transform into a different form of cancer called gestational choriocarcinoma that can spread (metastasize) to other tissues such as the liver, lungs, or brain.  https://medlineplus.gov/genetics/condition/recurrent-hydatidiform-mole