Simple SummaryBreastfeeding is encouraged worldwide due to its nutritional and bonding benefits, but more attention needs to be given to the potential psychological challenges it poses to new mothers. This study investigated whether breastfeeding pain relates to mothers’ and infants’ bonding behavior. Seventy-one mothers with varying levels of breastfeeding pain were videotaped with their infants during face-to-face interactions. We found that mothers with severe breastfeeding pain express less affect and less infant-directed gaze during interactive moments of engagement and play than mothers with no or moderate pain. Infants of mothers experiencing pain also express less affect and more mother-directed gaze than those of mothers not experiencing pain. These findings suggest that maternal pain can interfere with the behaviors of both mothers and infants, potentially impacting development and bonding. Since the mother–infant dyad is a codependent allostatic unit, the allostatic challenges of one partner can influence both partners. Therefore, nutritional advances should be considered along with additional allostatic consequences of breastfeeding to ensure the well-being of both mothers and infants.AbstractBreast milk is considered the ideal infant nutrition, and medical organizations encourage breastfeeding worldwide. Moreover, breastfeeding is often perceived as a natural and spontaneous socio-biological process and one of the fundamental roles of new mothers. While breastfeeding is beneficial, little scientific consideration has been given to its potential psychological challenges. Here, we investigate the phenomenon of breastfeeding pain in mothers and its association with maternal and infant behavioral regulation. During the postpartum weeks, the mother–infant dyad can be considered one allostatic unit directed at infant regulation and development. We hypothesize that pain comprises an allostatic challenge for mothers and will thus impair the capacity for dyadic regulation. To test this, we recruited 71 mothers with varying levels of breastfeeding pain and videotaped them with their infants (2–35 weeks old) during spontaneous face-to-face interactions. We quantified the individual differences in dyadic regulation by behaviorally coding the second-by-second affective expressions for each mother and infant throughout their interactions. We tested the extent to which breastfeeding pain alters affect regulation during mother–infant interactions. We discovered that mothers with severe breastfeeding pain express less affective expressions and less infant-directed gaze during interactive moments of engagement and play than mothers with no or moderate pain. Moreover, infants of mothers experiencing pain during breastfeeding express less affective expressions and more mother-directed gaze while interacting with their mothers than infants of mothers who are not in pain. This demonstrates that the allostatic challenge of maternal pain interferes with the behavioral regulation of both mothers and infants. Since the mother–infant dyad is a codependent allostatic unit, the allostatic challenges of one partner can impact the dyad and thus potentially impact child development, bonding, and mother and infant well-being. The challenges of breastfeeding should be considered in addition to the nutritional advances.