Genetics + more. A complete mole is formed when a sperm fertilizes an empty egg & duplicates, or when two sperm fertilize an empty egg. It has 46 chromosomes(all paternal). A partial mole develops when two sperm(or one sperm that duplicates) fertilize an egg & has 69 chromosomes. Fetal parts are usually only found in partial moles. Complete moles inc risk for choriocarcinoma(a type of tumor) more than partial moles.
Molar pregnancy. A partial mole develops at either at conception (2 sperm fertilize 1 egg) or immediately after conception (1 sperm enters an egg and duplicates). A complete mole forms similarly but with an empty egg. A partial mole has about a 5% and a complete mole 15% chance of becoming a gestational trophoblastic neoplasia.