There is no study available that has investigated determinants of prenatal multivitamin adherence among pregnant women, based on gastrointestinal (GI) adverse events. The objective of this study was to identify determinants predicting adherence to prenatal multivitamins in pregnant women who were randomized to take 2 different supplements. The authors recruited and interviewed 70 women on the importance of various factors that may have affected adherence to previous and assigned multivitamins. The different factors included GI symptoms and swallowing difficulty. The authors used a 5-point scale to measure degree of importance. The highest scoring factors for not taking or discontinuing any previous multivitamins were fear of or experience of nausea, vomiting, and gagging. For women who never took the assigned prenatal multivitamins, the highest scoring factors contributing to that decision were fear of nausea, fear of vomiting, and health care provider advice. For women who started taking the assigned supplements, the most important factors affecting adherence were dosing regimen, health care provider advice, and mode of product distribution. Adherence to assigned prenatal multivitamins significantly correlated only with the importance of constipation in deciding to discontinue any previous multivitamins. It is concluded that predictors of adherence to recommended prenatal multivitamins during pregnancy are rooted in women's prior experiences with multivitamin use.