Can you have a positive/negative influence on the health of the egg cells before pregnancy? so that you have less chance of down syndrome, congenital abnormalities etc?

You have al the eggs you will ever have from birth on. Every day of your life/exposure may have an unknowable influence on egg quality. We know that DS and other chromosomal defects begin increasing in frequency with advanced maternal age. We know of certain drugs/agents that increase risk. What is done in the few months before conception should match the healthy lifestyle known for best results.