Introduction The aim of this study is to evaluate the knowledge of infant oral health in mothers who have breastfeeding for a period longer than 6 months. Methods A descriptive, observational and cross-sectional study was carried out in 1126 mothers who had breastfed for more than 6 months, using a validated questionnaire. Results 57% of the mothers surveyed had a high or very high level of knowledge about oral health during pregnancy and breastfeeding; a statistically significant association (p < 0.05) was found between this and the variables of mother's age, duration of breastfeeding, level of education and previous information received about oral health. Conclusions The age of the mothers is positively related to the level of oral health knowledge, the elder the mothers the higher the knowledge. Mothers with a longer duration of breastfeeding beyond 24 months have a higher degree of oral health knowledge compared to the rest of the respondents, while their level of educations was also positively related to their degree of knowledge. There is a direct relationship between mothers having received previous information on oral hygiene and their surveyed degree of knowledge.