Final answer:
A complete molar pregnancy occurs when an egg with no genetic material is fertilized by a sperm, causing abnormal growth of what should become placental tissue with no fetus present.
Step-by-step explanation:
A complete molar pregnancy is caused when an egg with no genetic information is fertilized by one or two sperm, and all the genetic material is paternal. This condition is a type of gestational trophoblastic disease, which results in an abnormal growth of trophoblasts; the cells that normally develop into the placenta. There is no viable fetus in a complete molar pregnancy, only an abnormal growth of placental-like tissue. In some cases, the abnormal tissue can become cancerous and spread to other parts of the body. A molar pregnancy can't continue as a normal viable pregnancy.