The aim of this study is to determine the level of adherence to dietary guidelines among men and women during preconception, and pregnant women, and factors associated with adherence. Searches were conducted in CINAHL, AMED, EMBASE, and Maternity and Infant Care from inception to March 2018. Observational studies assessing the primary outcome (adherence to dietary guidelines and/or nutritional recommendations) and/or secondary outcome (factors associated with adherence) were eligible. Study quality was assessed using the National Institutes of Health Quality Assessment Tool for Observational Cohort and Cross-sectional studies. Men or women (aged ≥18 years) who identified as trying/intending to conceive or were pregnant. Eighteen studies were included. The quality of studies was fair (44%) to good (56%). Most studies indicated preconceptual and pregnant women do not meet recommendations for vegetable, cereal grain, or folate intake. Pregnant women did not meet iron or calcium intake requirements in 91% and 55% of included studies, respectively, and also exceeded fat intake recommendations in 55% of included studies. Higher level education was associated with improved guideline adherence in pregnant women, whereas older age and non-smoking status were associated with greater guideline adherence in preconceptual and pregnant women. The findings of this review suggest that preconceptual and pregnant women may not be meeting the minimum requirements stipulated in dietary guidelines and/or nutritional recommendations. This could have potential adverse consequences for pregnancy and birth outcomes and the health of the offspring. Major knowledge gaps identified in this review, which warrant further investigation, are the dietary intakes of men during preconception, and the predictors of guideline adherence. © 2019 The Authors. Maternal & Child Nutrition published by John Wiley & Sons, Ltd.