You know your nutritional needs change when you’re expecting, but it’s easy to feel overwhelmed by the wide variety of supplement options. To help you make the best choice, we asked experts to identify the most important nutrients for pregnant women, why they’re crucial, and how much you need of each.

Folic Acid

This B vitamin is crucial before and during the early weeks of pregnancy because it helps prevent serious brain and spinal cord abnormalities known as neural tube defects. It may also protect against heart defects, cleft palate, and other health problems, according to the March of Dimes.

Research shows: Women who took folic acid before and during pregnancy were about 40 percent less likely to have a baby later diagnosed with autism, according to a study conducted at the Norwegian Institute of Public Health.

Good to know: If you’re planning to add to your family, experts recommend increasing your folic acid intake ASAP. To get the maximum benefit, women should start taking folic acid at least one month before getting pregnant. Eating fortified bread and cereal can also help you meet the daily requirement.

How much you need: 400 mcg a day

Iron

During pregnancy, your body requires twice as much iron so it can make extra blood to deliver oxygen to your baby.

Research shows: If you develop iron-deficiency anemia—and about half of pregnant women do—you’re at a higher risk of giving birth prematurely, according to the National Heart, Lung, and Blood Institute.

Good to know: Ferrous sulfate, a form of iron used in many multivitamins, can be constipating. If this occurs, switch to an iron-free multivitamin and take a separate iron supplement containing heme iron, ferrous gluconate, or ferrous glycinate. Top dietary sources of iron include meat, fish, chicken, and cooked beans.

How much you need: 27 mg a day

Calcium

This mineral builds your baby’s bones, teeth, heart, nerves, and muscles. It may also reduce the risk of preeclampsia and preterm birth.

Research shows: If you don’t get enough calcium during pregnancy, your body draws it from your bones and gives it to your baby. This drain on your body’s calcium reserves could put you at risk for osteoporosis later in life.

Good to know: Most prenatal multivitamins don’t contain enough calcium, so you may need a separate supplement. (See our suggestion in “Our Prenatal Picks” below.) Or you can include more calcium-rich foods, such as milk, yogurt, cheese, fortified orange juice, broccoli, and kale, in your diet.

How much you need: 1,000 mg a day

Vitamin D

Vitamin D works with calcium to build your baby’s bones and teeth. It’s also vital for healthy skin and eyesight.

Research shows: A study conducted at the University of Calgary suggests that pregnant women with insufficient levels of vitamin D may be at greater risk of gestational diabetes, preeclampsia, and having a lower-birth-weight baby.

Good to know: Fetal needs for vitamin D increase during the second half of pregnancy, when your baby’s bone growth peaks. But as long as you get your daily dose throughout your pregnancy, your body will have enough for your baby.

How much you need: 1,000 IU a day

DHA

This nutrient is a type of omega-3 fatty acid found in fish that boosts your baby’s brain and eye development.

Research shows: Infants born to mothers who took DHA supplements during pregnancy had fewer colds and shorter illnesses at 1, 3, and 6 months of age, according to a study from Emory University.

Good to know: Most prenatal multivitamins don’t contain DHA, so you may need to take a separate supplement. You should also aim to get DHA from your diet: Experts recommend that pregnant women eat at least 8 ounces of omega-3-rich fish a week. Choose safe, low-mercury options like salmon, cod, sardines, and tilapia.

How much you need: 300 mg a day

Iodine

Iodine is necessary for the production of thyroid hormones, which are crucial for the proper development of your baby’s brain and nervous system.

Research shows: Even mild iodine deficiency in pregnant women may be linked to lower IQ in children, according to researchers at Bristol University and the University of Surrey in England.

Good to know: If you have hyperthyroidism, avoid iodine supplements and get your iodine from food. Good sources include fish, milk, cheese, yogurt, and fortified bread and cereal.

How much you need: 220 mcg a day

Our Prenatal Picks

These multis are natural, safe for you and your baby, and provide 100 percent of your daily needs for most of the nutrients on our list.

Early Promise Prenatal Gentle Multiple ($30.36, iherb.com)

MegaFood Baby & Me ($29.99, megafood.com)

Now Prenatal Gels + DHA ($28.92 amazon.com)

Rainbow Light Prenatal One ($11.54, rainbowlight.com)

If the supplement you choose doesn’t contain DHA, consider adding Nordic Naturals Prenatal DHA ($29, nordicnaturals.com).

And since most multis don’t contain enough calcium, you may need a separate calcium supplement. Our pick: Nature’s Way Calcium Citrate ($10.98, iherb.com).