Folic acid is the main thing, and usually it will include other vitamins that are important to supplement to your
diet. Some brands make certain women feel nauseous, so you might have to try a few. Talk to a pharmacist and find out which one they recommend. Mine recommended the cheaper version of one that he found women generally seemed able to take, and I lucked out in that they didn't make me sick. Take them with food, it will help keep the nausea away too.