What is a mole and why it forms A mole is a yolk cyst, an organism that forms early in pregnancy but does not survive because the fetus fails to develop. In a molar pregnancy, both the placenta and the embryo grow differently than they would in a normal pregnancy, forming a sac-like structure similar to a grape. This is a rare pregnancy complication, occurring in approximately 1-2% of all pregnancies. A molar pregnancy may occur for many reasons, including chromosomal abnormalities in the embryo, failure of the egg to fertilize, or