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A prenatal vitamin is vital for a healthy pregnancy and a healthy baby, but knowing when to start taking them, which brand to buy and how much of each vitamin and mineral to look for can be daunting.

Here, get the 411 on everything you need to know about prenatal vitamins.

Why prenatal vitamins?
It can be hard to eat healthy when you’re dealing with morning sickness and pregnancy cravings, but prenatal vitamins aren’t meant to make up for unhealthy food choices.

“This should complement a healthy diet,” Dr. Mary Rosser, an assistant professor in the department of obstetrics and gynecology and women's health at Montefiore Medical Center in New York City said. “It’s really not a substitute for anything.”

Plus, because we tend to lack a lot of vitamins and minerals due to depleted soils, getting those in a prenatal vitamin is even more important, according to Michele McRae, Senior Director of Research, Formulation and Quality at Rainbow Light, a supplement company .

When should you start taking prenatal vitamins?
If you’re hoping to get pregnant, you should ideally start taking prenatal vitamins at least three months before you plan to get conceive. Prenatal vitamins have folic acid, which prevents neural tube defects like spina bifida, which develops within 28 days after conception.

Plus, folic acid may reduce the risk of autism, according to a report in the Journal of the American Medical Association.

“The defects can occur very early in pregnancy, many times before women know that they’re pregnant,” Rosser said.

For this reason and because 50 percent of pregnancies are unplanned, experts recommend all women of reproductive age take a multivitamin with 400 micrograms of folic acid, whether they’re trying to get pregnant or not.

What to look for in a prenatal vitamin.
Prenatal vitamins are available by both over-the-counter and by prescription. Although formulations can vary from brand to brand, these are the most important vitamins and minerals to look for:

Folic acid
Anywhere between 400 mcg and 800 mcg of folic acid is ideal. If you previously had a baby with a neural tube defect or if you have other risk factors, your doctor may prescribe 4 mg (4,000 mcg) of folic acid.

Iron
Iron is essential for a baby’s growth and development and it prevents you from having anemia. Getting more iron is especially important because your blood volume almost doubles to deliver oxygen and nutrients to the baby.

30 mg is best but your doctor may prescribe more if you’re iron deficient, which most moms are, McRae said.

Calcium
Your developing baby needs calcium for growth and bone development and strength and she’ll draw that nutrient from you. Because of this, it’s important for moms to get enough calcium— aim for 250 mg a day.

Magnesium
Look for a prenatal vitamin with at least 400 mg of magnesium, which is crucial for baby’s bone development. It will help you relieve muscle aches, improve energy, and aid relaxation.

Vitamin D
Vitamin D not only helps mom absorb calcium and supports healthy blood pressure, but research shows its linked to better birth outcomes, too.

Plus, a recent study in the Journal of Clinical Endocrinology and Metabolism found that moms with higher vitamin D levels were more likely to have children with stronger muscles.

Aim for 400 IU to 800 IU of vitamin D. If you’re at risk for bone issues or are a vegetarian, your doctor may prescribe up to 2,000 IU a day.

DHA and EPA
Studies show that DHA and EPA are important for baby’s brain and eye function and may even prevent pre-term birth and depression during pregnancy and postpartum.

Most over-the-counter prenatal vitamins don’t include DHA and EPA, but you can take a supplement or ask your doctor for a prescription. Take at least 250 mg per day.

Vitamin B
Vitamin B6 is important for a baby’s brain development and nervous system, and can maintain your energy and keep your mood balanced. Look for 2 mg of B6, but a prenatal vitamin with a vitamin B complex is even better.

Vitamin C
Vitamin C but is essential for immunity, especially because we don’t produce it. 50 mg a day is best.

Choline
Studies show that choline may help with cognitive abilities and IQ later in life. At least 30 mg a day is ideal.