A healthy diet is the best way to get the vitamins and minerals you need. But during pregnancy you might fall short on key nutrients. If you're pregnant or hoping to conceive, prenatal vitamins can help fill any gaps.

During pregnancy, you need more folic acid and iron than usual. Here's why:

• Folic acid helps prevent neural tube defects. These defects are serious abnormalities of the fetal brain and spinal cord. Ideally, you'll begin taking extra folic acid at least 3 months before you become pregnant.
• Iron supports the development of the placenta and fetus. Iron helps your body make blood to supply oxygen to the fetus. Iron also helps prevent anemia, a condition in which blood has a low number of healthy red blood cells.

Prenatal vitamins are available over-the-counter in nearly any pharmacy. Your health care provider might recommend a specific brand or leave the choice up to you.

Beyond checking for folic acid and iron, look for a prenatal vitamin that contains calcium and vitamin D. They help promote the development of the baby's teeth and bones. It also might be beneficial to look for a prenatal vitamin that contains vitamin C, vitamin A, vitamin E, B vitamins, zinc and iodine.

In addition, your health care provider might suggest higher doses of certain nutrients depending on the circumstances. For example, if you've given birth to a baby who has a neural tube defect, your health care provider might recommend a separate supplement containing a higher dose of folic acid — such as 4 milligrams (4,000 micrograms) — before and during any subsequent pregnancies.

But in general, avoid taking extra prenatal vitamins or multivitamins with dosing in excess of what you need on a daily basis. High doses of some vitamins may be harmful to your baby. For example, extra vitamin A during pregnancy can potentially cause harm to your baby.

Omega-3 fatty acids, a type of fat found naturally in many kinds of fish, help promote a baby's brain development. If you don't eat fish or other foods high in omega-3 fatty acids, your health care provider might recommend omega-3 fatty acid supplements in addition to prenatal vitamins.

Ideally, you'll start taking prenatal vitamins before conception. In fact, it's generally a good idea for women of reproductive age to regularly take a prenatal vitamin. The baby's neural tube, which becomes the brain and spinal cord, develops during the first month of pregnancy — perhaps before you even know that you're pregnant.

Sometimes the iron in prenatal vitamins contributes to constipation. To prevent constipation:

• Drink plenty of fluids
• Include more fiber in your diet
• Include physical activity in your daily routine, as long as you have your health care provider's OK
• Ask your health care provider about using a stool softener

If these tips don't seem to help, ask your health care provider about other options.