A partial hydatidiform mole is one of two types of molar pregnancy. In a partial hydatidiform mole, the pregnancy has chromosomal abnormalities that result from two sperm having fertilized a single egg. In the resulting molar pregnancy, a baby may develop in the uterus, but with an abnormal, overgrown placenta. These pregnancies are not viable; the chromosomal abnormalities are not compatible with life.
A partial hydatidiform mole is less likely than a complete molar pregnancy to result in persistent gestational trophoblastic disease after treatment, but 2% to 4% of women with partial molar pregnancies may develop complications like an invasive mole or choriocarcinoma.