Between morning sickness, fleeting cravings and random aversions, getting good nutrition during pregnancy can be rough. Even if you’re eating a balanced diet, it’s still possible to miss some essential nutrients.

Enter prenatal vitamins. They’re an easy way to fill in any gaps and support both your body and your baby’s growth and development. Here’s what pregnant women need to know about them.

Why is it important to take prenatal vitamins?

Pregnant women have increased nutritional demands, both because of the needs of the baby developing inside and the changes that their body is going through to accommodate the pregnancy. While most women get sufficient nutrition through a well-balanced diet, it’s helpful to take a multivitamin to make sure that all the nutritional requirements of the pregnancy are met.

What are the benefits of taking a prenatal vitamin?

For certain vitamins and supplements, there are specific benefits. For example, folic acid supplementation helps to prevent certain birth defects (like spina bifida) in babies. Taking iron helps to prevent anemia (low blood count), which may lead to fatigue and fainting. Taking DHA (a type of polyunsaturated fatty acid) may improve fetal neural development and fetal growth when taken in the third trimester. Extra calcium is good for bones, and vitamin B6 may be useful for suppressing nausea in pregnancy.

Which key nutrients should women look for in a prenatal?

Although prenatal vitamins typically contain the full range of vitamins and minerals, the ones that are most important are folic acid, B vitamins, calcium and iron. Calcium and iron are particularly important because the requirements for these minerals are higher in pregnancy than they are for non-pregnant women.

However, these are the ingredients most often left out of gummy vitamins, which many women like because of taste and ease of use. So if they choose the gummy, they may need to take extra iron or calcium if these have been left out.

When to start and stop taking prenatal vitamins

You should start using a multivitamin that contains folic acid whenever you think you might conceive. Because the kind of birth defects that arise from a lack of folic acid start to form before many women even realize they are pregnant, it’s important to supplement this nutrient even before conception.

Other nutrient demands increase only after pregnancy starts, so it doesn’t have to be a “prenatal vitamin” until after the pregnancy test is positive. If you’re unable to tolerate the vitamin because you feel sick in the first trimester, it’s OK to start once you’re feeling better.

You should continue to take a prenatal vitamin for the duration of breastfeeding, however long that may be. This is because lactation (making breast milk) also increases nutritional demands, especially of calcium and iron.

Do prenatal vitamins have any side effects?

The ingredient that’s most likely to cause gastrointestinal side effects is iron, which may cause indigestion and constipation. There are different iron preparations available, so women who experience these effects with one vitamin may want to try a different vitamin that has a slower-release form of iron.

Other vitamins don’t necessarily have side effects, but may be harmful if too much is ingested. Too much vitamin A can have harmful effects on the fetus, but is fine when taken as beta-carotene, a plant-based precursor that’s safe. Too much iodine and fat-soluble vitamins (D, E and K) may also be harmful.

Finally, prenatal vitamins that pack in all these nutrients may be big—some women find them hard to swallow and they may cause gagging. Using a chewable alternative may be an option.

This advice applies to most low-risk women and pregnancies. There are certain cases (women who have had gastric bypass surgery, women with eating disorders or women carrying twins and triplets), where the nutritional needs are different. These women should contact their prenatal care providers about the best regimen of nutritional supplement to use.

Jonathan Schaffir is an ob/gyn at The Ohio State University Wexner Medical Center and a professor at The Ohio State University College of Medicine.