General Discussion

https://medlineplus.gov/ency/article/000909.htm
Hydatidiform mole (HM) is a rare mass or growth that forms inside the womb (uterus) at the beginning of a pregnancy. It is a type of gestational trophoblastic disease (GTD).

Causes
HM, or molar pregnancy, results from abnormal fertilization of the oocyte (egg). It results in an abnormal fetus. The placenta grows normally with little or no growth of the fetal tissue. The placental tissue forms a mass in the uterus. On ultrasound, this mass often has a grape-like appearance, as it contains many small cysts.

The chance of mole formation is higher in older women. A history of mole in earlier years is also a risk factor.

Molar pregnancy can be of two types:
Partial molar pregnancy: There is an abnormal placenta and some fetal development.
Complete molar pregnancy: There is an abnormal placenta and no fetus.