The best time to start thinking about good nutrition is before a woman becomes pregnant. Then she can be certain that her baby will have all the essential nutrients from the moment of conception. Babies born at low weights (less than 5.5 pounds) have a greater likelihood for development of health problems.

Mothers-to-be can help prevent this from occurring by eating the well-rounded diet that women their age would ideally eat, as well as achieving an appropriate weight for their height. Women who are 15 percent or more underweight present a special risk for a difficult pregnancy and childbirth.

Before becoming pregnant, talk to your health care provider about your need for a folic acid (a B vitamin) supplement. Doctors and scientists agree that use of folic acid supplements can reduce the occurrence of a birth defect called a neural tube defect.

One form of this defect is spina bifida, an incomplete closure of the spine. Folate levels also may be affected by consuming alcohol and smoking cigarettes, both of which are toxic to the developing fetus and should not be used when you are pregnant. Taking oral contraceptives is also associated with low levels of folate.

It is essential to talk with your health care provider about folic acid because too much of any supplement can harm your health. The best dietary sources of folic acid include fortified breakfast cereals and enriched grain products.

Folate (the natural form of this vitamin found in foods) is found in leafy green vegetables, oranges and grapefruit, black-eyed peas, kidney beans, and other cooked dried beans. Even if you eat a well-balanced diet, prenatal vitamins are recommended.

Research has proved that women who have a normal weight at the time of conception have the healthiest pregnancies and babies if they gain 25 to 35 pounds. Women who are underweight may need to gain additional weight. Even women who are overweight should plan on gaining about 15 to 20 pounds.

During the first trimester, additional calories may not be needed. It is important, however, to make sure that your diet provides the best nutrition possible for you and your unborn baby. In the second and third trimesters, you will need about 300 extra calories per day beyond your normal diet.

Concentrate on foods such as lean meat, low- or no-fat dairy products, and dark green vegetables, all of which provide generous amounts of vitamins, minerals, and protein. If you enjoy milk and dairy products, it will be easy to meet your need for calcium, a crucial mineral during pregnancy.

If you do not, ask your doctor or registered dietitian about calcium-fortified foods. In some cases, a supplement may be recommended. Even with the increased need for calories, it is nearly impossible to get sufficient amounts of iron, which is needed in double the amounts recommended for nonpregnant women.

The added iron is needed for the expanded blood volume that accommodates the changes in your body during pregnancy. Iron is also needed for the formation of tissue for both your baby and the placenta. At birth, a newborn needs enough stored iron to last for the first 6 months of life.

The best advice is to eat a healthful diet and take only the supplements recommended by your caregiver. Once a baby is born, the mother who chooses to breastfeed still needs extra calories—typically about 500 calories per day. Continue to concentrate on eating nutrient-rich foods.