Researchers in Spain obtained hip ultrasound studies while babies were in three different types of baby carriers. [https://www.ncbi.nlm.nih.gov/pubmed/30928246] The results were encouraging for babies with normal hips when carried inward-facing even if the position was less than the optimum M-position. With the thighs spread around the mother’s torso, the hips were deeper in the socket than when the infant was lying on the exam table for the standard ultrasound study.
This study shows that slight variations from the M-position are acceptable when the hips are normal. The infants studied were approximately two months old and had stable hips. The International Hip Dysplasia Institute still suggests avoiding outward facing carriers until six months of age and avoiding carriers with narrow support between the thighs. Approximately one in ten newborn infants has mild hip instability so this study may not apply to younger infants, or to infants with hip dysplasia that’s known or unknown. However, it is encouraging that proper babywearing may help center the hips into the socket.
The carriers studied were the Original Babyjörn®, BabyBjörn® One and Manduca®, but many baby carriers can be found that have been acknowledged as “Hip-Healthy” by the International Hip Dysplasia Institute.