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Oxytocin has an important function in breastfeeding via its role in the milk ejection reflex and in attachment and bonding processes. Genetic factors account for a significant part of the individual differences in breastfeeding behavior. OXT and OXTR have been proposed as gene candidates for breastfeeding. Previous studies have focused on certain single‐nucleotide polymorphisms (SNPs) within these genes, finding null or inconsistent results. The present study analyses the associations between a wide coverage of polymorphisms in OXT and OXTR and breastfeeding duration from 2 large and independent unselected samples comprising a total of 580 and 2112 female twin mothers from the Murcia Twin Registry (Spain) and QIMR Berghofer Medical Research Institute (Australia), respectively. A total of 19 SNPs in OXT and 137 in OXTR SNPs were covered in both samples. Effects of the OXT and OXTR polymorphisms on breastfeeding duration were calculated by means of linear regression controlling for age at survey time, educational level, interaction between age and educational level and principal components of genetic ancestry. The analyses were conducted independently in the 2 samples and also meta‐analyzed. Although some SNPs were associated at an alpha level of .05 with breastfeeding, they did not survive multiple testing correction. We conclude that SNPs within or nearby OXT and OXTR are unlikely to have large effects on breastfeeding behavior. [ABSTRACT FROM AUTHOR]