What contributes to a good egg and healthy embryo?

A few ideas: A good embryo comes from a good, egg, and to a lesser extent, good sperm. Nutrition, stress, alcohol use, smoking have all been shown to contribute to egg quality. The type of stimulation used in ivf has a big effect, and there are medications available that will improve the egg yield in women experiencing problems with stimulation.

Age: Generally, age is the biggest predictor of healthy eggs and embryos, however other factors may also contribute such as pcos, endometriosis, sperm fragmentation, genetics...