This study aimed to analyze the position of the lumbopelvic region and lumbar muscle activity in the most common breastfeeding positions. We recorded the curvatures of the lumbar spine and pelvis by means of an electrogoniometer, and the muscle activation levels of the erector spinae with electromyography, in 34 women in erect standing and breastfeeding their children in several positions. Both side lying and clutch-hold positions showed a greater degree of lumbar spine flexion compared to standing. In all sitting postures it was observed that the pelvis was placed in retroversion when compared to standing and side lying. In muscle activity, it was observed that the activation intensity of the right erector in the right side-supported side lying position was significantly lower compared to the rest of breastfeeding postures and standing. Side lying may be a better position to avoid muscle fatigue.