The Guardian reports on this study that shows IVF embryos have alot more genetic defects than they previously thought, even when the eggs are donated by younger women.  In the study, 42% of the embryos screened had damaged chromosomes, and the damage ranged from 28% to 83%.  The genetic defects can prevent the embryo from being carried to term.  Researchers believe that the defective eggs are common in the general population but the body knows to reject them early on if they happen to become fertilized.  I guess science still hasn't caught up to nature.