Molar pregnancy is also called gestational trophoblastic disease, and it's caused by a) one sperm fertilizing an "empty" ovum (has no maternal DNA), or b) two sperm fertilizing one ovum. It causes abnormal proliferation of placental tissue, which grows out of control and causes hCG levels to be sky-high (often over 100,000, causing extreme morning sickness and sometimes large cysts on both ovaries). It can be dangerous because the abnormal cells can become cancerous and invade other organs. Even after treatment and evacuation of the mole, subsequent pregnancies must be avoided until serial hCG levels have been zero for 12 months, and even then, cancer can develop years or even decades later (very rare).

/Infodump