You already know that taking a prenatal vitamin for a few months before conception and throughout each pregnancy is important. You’ve probably heard that folic acid helps prevent neural tube defects, which develop in the earliest weeks of embryonic development, before a pregnancy is even recognized. What you might not know is why prenatal vitamins continue to be important throughout the childbearing cycle, and how to pick the vitamin that’s best for you.

In reality, there is precious little evidence about the effects of prenatal multivitamins in the body. What we do know are a few basics: multivitamins improve health outcomes in malnourished populations; supplemental iron reduces the risk of iron deficiency anemia; and adequate folate reduces the risk of defects in neural tube development.  There is virtually no evidence regarding the best formulation of the specific micronutrients that make up a typical prenatal multivitamin. Most health care practitioners, physicians included, receive little formal training in nutrition and even less in the area of dietary supplements like prenatal vitamins. You may be told that it doesn’t matter which vitamin you buy, as long as the requirement for folic acid is fulfilled. A prescription vitamin is not necessarily more beneficial than one that’s available over the counter.

But there’s got to be more to the story, right? Here are some key points to consider when selecting a vitamin:

1. Whole food versus synthetic

Conventional prenatal vitamins contain synthetic ingredients. This is not a bad thing if you don’t care where and what the ingredients came from. Synthetic sources allow the vitamin to be smaller, pressed into a single tablet. A whole food vitamin, on the other hand, is derived from real food sources (broccoli, sprouts, carrot, etc.), and may be organic and/or non-GMO by popular demand. Whole food vitamins are usually powdered and encapsulated, and require three or four capsules to be swallowed each day. This can be a problem for women who gag. Nevertheless, many women feel that whole food vitamins are easier to tolerate than synthetic options. There is also at least one chewable whole food organic prenatal vitamin on the market at this time. In addition to the whole food and synthetic categories, there are cultured and fermented options that are neither completely food-based nor completely synthetic. Some prenatal multivitamins contain probiotics, but most do not. If this all seems confusing, and you want to know the details of a particular product, you should contact the manufacturer. Don’t rely on marketing materials alone.

2. Forms of folate

Folic acid is the synthetic form of folate (Vitamin B9) that most vitamins contain. Look for a prenatal vitamin with 800-1000 micrograms of folate. If you’ve already had a pregnancy affected by a neural tube defect or you take anti-seizure medication, you will be advised to take extra folate in a separate supplement. Pick a vitamin that contains the biologically active form of folate rather than synthetic folic acid. Many people have a genetic mutation which inhibits the conversion of folic acid into its active form. Unless you’ve been tested and know your status, assume that this could be a problem. Vitamins with active folate may be labeled with the brand names Metafolin or Quatrefolic, or the term L-5-MTHF. If the label simply reads folate, call the manufacturer and find out exactly what that means.

3. Iron

The iron component of a prenatal vitamin is the ingredient most often responsible for constipation, dark stools, gastric pain or bloating. Iron is usually supplied in the form of an iron salt: ferrous sulfate, ferrous fumarate, and ferrous gluconate are the most common forms. There is no proof that any particular form is easier to tolerate than another form, but if you experience the gastric symptoms above, try switching to a prenatal with a different iron salt. The iron content in a prenatal vitamin is not sufficient to correct iron deficiency anemia. Your midwife or doctor is likely to suggest a separate iron supplement if you have this type of anemia. Adding 500 mg of Vitamin C can help the gastrointestinal tract to absorb more iron. Floradix is a liquid supplement containing iron from food sources, and is usually well tolerated. It does not substitute for a prenatal multivitamin, however.

4. DHA

Include DHA in your prenatal regimen. DHA is an essential fatty acid that must be consumed from a dietary source like fish or algae. DHA is important for the development of a baby’s brain and eyes. Most pregnant women have inadequate dietary intake of omega-3 fatty acids, including DHA. Adequate fish intake in pregnancy is associated with better cognitive function, motor skills, communication and social development in offspring. Eating fish in pregnancy is a highly charged topic, however. Contamination with methylmercury means that some women who regularly consume seafood have mercury levels above the safe threshold. The FDA recommends 8 to 12 ounces of fish per week for pregnant women. This should be fish with low methylmercury levels and high DHA levels, like wild caught salmon and pollock. DHA can also be found in omega-3 enriched eggs. There is no established daily dosage of DHA for pregnant women, but a reasonable target may be 200 to 400 mg daily. Check to see if your prenatal vitamin includes DHA (most do not). A separate DHA supplement can be taken or a purified fish oil supplement of 1000 mg daily can be continued throughout pregnancy. Some women may wish to obtain omega-3 fatty acids from plant sources like flaxseed oil. These sources are rich in the essential fatty acid alpha-linolenic acid (ALA), which has limited conversion to DHA. For this reason, it is more difficult to estimate the usefulness of these sources.

5. Vitamin D

Last but certainly not least, women should insure Vitamin D adequacy before, during, and after pregnancy. Vitamin D deficiency is extremely common, and unlikely to be corrected through diet alone. A 2016 review in the Cochrane Database suggests that Vitamin D supplementation in pregnancy reduces the risks of preterm birth, low birthweight, and preeclampsia. Look for a prenatal vitamin with Vitamin D3, or cholecalciferol. The amount of Vitamin D contained in prenatal vitamins varies widely by product, and may not be sufficient to correct a true deficiency. A simple blood test can determine serum Vitamin D levels, and repeat testing can confirm the adequacy of supplementation. Talk to your health care provider about the pros and cons of any supplements you are considering, and get their advice about ideal forms and dosages. If your provider is not well-versed in this area, consult a nutritionist, midwife, or functional medicine doctor with a special interest in pregnancy nutrition.