Avoid junk food, lack of folic acid, alcolol, tobacco, drugs, chemicals, MSG, aspartame (Nutrasweet, Equal, Spoonful), and physical shock.
Eating poorly. That is, you should eat well balanced meals that include adequate protein and omit excessive amounts of fat. This is not the time to be eating a lot of junk food!
Inadequate vitamin intake. That is, take your prenatal vitamins. The prenatal vitamins should contain at least 400-800 micrograms of folic acid.
Alcohol. Avoid alcohol, especially in early pregnancy.
Tobacco (or other weed). It should be well known, by now that tobacco is not good for the mother and certainly not good for the fetus. The placentas of mothers that are heavy smokers are usually obviously different from those of mothers who do not smoke. And, of course, second-hand smoke will not be good for the newborn infant. If ever there is a time to stop smoking or at least drastically cut back, it is during pregnancy. That goes for mother AND father. Second hand smoke affects mother as well as baby.
Drugs (illegal, and many legal drugs as well). Don't addict your baby to cocaine, heroin, or other "street drugs." Check with your midwife or physician (or pharmacist) before taking any over the counter or prescription medication. Medications that are relatively "harmless" under normal circumstances can cause problems for pregnant mothers and their babies to be.
Chemicals. Avoid exposure to large quantities of organic chemicals. Use common sense. Don't spray insecticides, herbicides, organic solvents, etc. in such a way that you may absorb large quantities of these organic chemicals.
MSG and also aspartame (otherwise known as Nutrasweet, Equal, or Spoonful). There are conflicting reports about the safety or toxicity of these chemicals, but I would rather err on the side of caution and avoid them during conception and pregnancy. After reviewing the literature you can make up your own mind, but you might want to avoid them altogether.
A starting point in the discussion about aspartame can be found here.
Information particularly pertinent to pregnancy can be found here.
Information about medications that contain aspartame can be found here.
Or, simply type "aspartame" as a keyword in any of the common search engines. You will receive many hits explaining the pros and cons of aspartame. Along the way you will also run across discussions about MSG, because some of their actions are similar.
Physical shock to mother (and baby within). While you can't always avoid accidents, such as falls, automobile accidents, and things of that nature, you can avoid putting yourself in situations where such trauma is likely. For example, bungee jumping or race card driving activities should be postponed until after pregnancy.