In general improve your diet. Eat more green leafy vegetables, fruits (limit by calorie intake), and baked or grilled meats. Drink lots of water, avoid canned drinks and caffeine. Milk is great, but can account for a lot of calories in your diet. Vitamin D supplementation in pregnancy is especially important for darker skinned women, and up to 1000 mg per day, along with Omega-3 should be important supplements.
Calcium supplements can be very helpful in the second trimester to stop night time leg cramps. Don't take calcium at the same time as your prenatal vitamin, the iron and calcium mixed together will interfere with absorbing either one. Folic acid is critical for all women who are considering pregnancy or are pregnant, and is found in high levels in green leafy vegetables.
Don't be paranoid about eating, but use good common sense, good kitchen cleanliness and limit your portion sizes for weight control.
What about fish and seafood?
You have probably heard that there are foods that should be avoided during pregnancy. Most of the restrictions are directed at avoiding mercury which has gotten concentrated in fish. The fish to be avoided are the predatory fish from the ocean, and include tuna, shark, mackerel, and salmon.
In fact, most fish should be limited to 4 ounces per week. The dietary benefits of fish can be compensated for by including omega 3 fatty acids in your diet. Most prenatal vitamins now include omega 3s, and will be listed as either derived from fish oil or from plant sources such as algae.
Studies have shown that babies born to mothers with Omega-3 supplementation have better vision and higher IQ scores at 18 months. Omega-3 can be added to your regular vitamin with a specific pregnancy supplement called Expecta. The studies which showed benefit were performed on Omega-3 derived from fish oil. These restrictions probably do not apply to farm raised fish, which are usually fed artificial fish food.
What about lunch / deli meat?
The second concern which you may have read about lists luncheon meats and deli meats as foods to avoid. Listeria is a bacterium which is very common in our environment, but pregnant women have a peculiar susceptibility to becoming sick with listeria.
Luncheon meats and hot dogs are safe to eat if they have been heated to steaming. You should wash your hands after handling these foods. Soft cheeses and products made from unpasteurized milk products should be avoided. If in doubt, don't eat it. If you handle it, wash your hands afterwards.
What about cooked meat?
A third concern comes from under-cooked meats, especially venison (deer meat). Cook pork, chicken, ground beef, and venison thoroughly, and do not return cooked meat back to the surfaces which were used for preparation without washing your hands and the plates and utensils used before cooking.
Under cooked venison is a major source of toxoplasmosis, which can cause birth defects if contracted during pregnancy. Under cooked meats are also potential sources of E. Coli bacteria and trichinosis.
What about caffeine?
There is some evidence that large amounts of caffeine may cause subtle problems during pregnancy. Caffeine does not need to be eliminated, but should be limited to 100 mg per day or less. Two colas or one 6 ounce cup of coffee or tea contain about 100 mg of caffeine.
What about artificial sweeteners?
If you are diabetic, artificial sweeteners are acceptable. If you are not diabetic, it is probably better to avoid excessive use of Nutrasweet, Splenda and especially saccharin.