At a minimum, prenatal vitamins should contain two things that mothers-to-be and growing babies especially need: folic acid and iron. Folic acid decreases the chance that a baby will be born with spina bifida, a series of birth defects that affect parts of the nervous system, including the brain and spinal cord. The CDC states that up to 50 percent of spina bifida cases can be prevented simply by taking enough folic acid. It suggests that all women who could become pregnant consume at least 0.4 milligrams, or 400 micrograms, of folic acid per day, the recommended daily allowance for both adult men and women. All prenatal vitamins should provide this, but be sure to double-check the label, and take this supplement every day if pregnancy is a possibility for you.

Iron is included in prenatal vitamins because pregnant women are especially at risk for iron deficiency, or anemia. Anemia simply means lacking in blood. Within a few months, expectant mothers transition from carrying enough blood for one person to having enough for two. This is so that the growing baby has the oxygen he or she needs to breathe, and any blood lost during the birthing process will be less traumatic for the mom.

By the end of her second trimester, a pregnant woman may have twice her usual blood volume. All this extra blood may be the source of the so-called pregnancy "glow." It is definitely the source of a mother-to-be running low on iron as she rushes to make all that extra blood. That's why the National Institutes of Health recommends that pregnant women over the age of 19 consume at least 27 milligrams of iron per day, while the recommended dose for nonpregnant women is only eight mg and lactating women are advised to consume nine mg of iron every day. Taking this amount of iron before and during pregnancy helps ensure there will be enough for both the mother and the baby to have an iron-rich blood supply.

It's important to remember that vitamins and supplements cannot take the place of a healthy diet. For example, pregnant women should eat multiple servings of fresh green vegetables and foods rich in omega-3 fatty acids. Higher doses of certain vitamins might be important for you, depending on your own personal health and health history. Talking with your doctor can help you determine any other specific vitamins and supplements that you might need based on your situation.

The early weeks of pregnancy are important to the development of your baby. If you're trying to get pregnant or think there's a chance you may become pregnant, work with your doctor to ensure that your vitamins are filling the right needs in your diet.