How do I know if my baby's knees are not too far apart in a wrap / sling ?

Before 3-4 months: the baby's knees should be at the same width as the pelvis, the ankles in line with the knees. When carrying with arms, you can observe that he gathers on the chest of the carrier, this is a good indication of the most physiological position for him.
Around 3 to 4 months: when the baby's motor skills progress and he can grasp his ankles, he naturally opens his pelvis more and his knees "embrace" the carrier more than before. This is when you can remove the booster seat with the baby carrier.
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