What causes molar pregnancies?

Abnormal pregnancy. Molar pregnancies are when the placenta overgrows and takes over. There may or may not be a fetus or embryo present. They are caused by abnormal genetics. Normally the sperm contributes half the genetic material. If the sperm contributes most or all of it, a molar pregnancy occurs.
Wrong speciality. Don't know. I am a dentist and the molars that i treat don't get pregnant. Your question went to the wrong place.