Pre-pregnancy: 

If you're planning a pregnancy within the next year, now's the time to start changing your diet. It's easier to give up caffeine, alcohol, and cigarettes before you conceive. Eat the way you want your child to eat. Do you want him or her living on fast food and fatty desserts? Do you want your kids to think dinner comes out of a box? Wouldn't you rather picture your child munching on fresh fruit, and eating whole grains? Get yourself in good shape, physically and nutritionally. A 1995 report in the journal Medical Science Sports and Exercise revealed that women who exercised in early pregnancy experienced fewer discomforts later in their term. 
Choose a healthcare practitioner and go for a checkup. Have blood work done to see if you are anemic. Get your B12 level checked too. It's best to start taking prenatal supplements now to correct and prevent any nutritional deficiencies. 

If you've been on oral contraceptives, remember that the pill depletes your stores of folic acid, a nutrient shown to prevent neural tube defects (like spina bifida). All women of child bearing age should have 400 mcg folic acid a day. As early as 1993, The Journal of the America Medical Association (JAMA)reported that having enough folate cuts your risk of having a baby with a neural tube defect by 60 percent. Think foliage! Green leafy vegetables, as well as oranges, whole grains, wheat germ, and legumes, are great sources of folic acid. Soon, folic acid will be even more accessible; by January 1, 1998, the FDA will require American food manufacturers to add folic acid to most enriched breads, flours, corn meals, pastas, rice, and other grain products. 

The First Trimester (Months One to Three) 

Weight gain 

During the first three months or pregnancy, you should gain about 2-4 pounds. Some women may even begin to "show", especially if they are fairly thin to begin with. Although your mother or grandmother may have been strongly encouraged by her doctor to keep her weight low while she was expecting, we now know that adequate weight gain leads to healthier babies. A recent report in the journal Obstetrics and Gynecology recommends gaining between 25 and 35 pounds during the course of the term. Even if you are severely overweight , you should still gain at least 15 pounds If you have trouble gaining weight, eat more energy dense foods like nuts and nut butters, dried fruits, and bean spreads. Small frequent meals will help you meet your energy needs. Though some fats may be warranted to help you gain weight, your baby does not necessarily need hot fudge sundaes or french fries. 
Don't consider pregnancy a license to overdo the weight gain. Very large weight gains can lead to problems too. For instance, you may find it difficult to lose the weight after the baby is born. You only need 300 extra calories per day when pregnant. "Three hundred calories is a fairly small increase compared to the increases seen for other nutrients, so it is important to use those calories wisely," says Dr. Reed Mangels, PhD, RD, FADA, nutrition advisor to the Vegetarian Resource Group. "If you are eating a lot of sweet or fatty foods, replace them with fruits, vegetables, grains, and legumes." Make every calorie count.