Slim down if you're overweight. Research shows that women who are obese before pregnancy are much more likely to have babies with birth defects. Start a healthy eating and exercise plan now. Another healthy move: Avoid high-mercury fish (such as swordfish, king mackerel, and tilefish) to rid your system of the potential toxin; a buildup of mercury in your body can harm an unborn baby.

When: Ideally, 1 year before conception -- or as soon as you start thinking about getting pregnant -- since losing weight healthfully takes time