Many cultures have practiced swaddling their babies. Native Americans used a cradleboard or “papoose” carrier. More modern use of this positioning relies on a blanket or diapers. The child is placed in a straight position with arms and legs held to the sides. The effect is to create neutral warmth, which is calming to the nervous system. Babies sleep better and cry less in swaddles. There is a decreased risk of sudden infant child syndrome for babies who are swaddled.
But the question has been raised about the potential harm this positioning may have on the hips. A straight position held too long can lead to hip subluxation (partial dislocation) or even full dislocation. Children’s hips need the opportunity to move in all directions, especially hip abduction (away from the body) in order to fully form a deep, stable hip socket.
In order to study the effect of swaddling, researchers from China swaddled baby rats. Studying this problem in humans would not be considered ethical so animal studies are the next best way to investigate potential harm to humans. They used four groups of rats.
One group (control group) was not swaddled at all. The other three groups were swaddled for varying lengths of time using surgical tape to hold the hind legs together against the body. Group two (early swaddling) was held in the swaddling position immediately after birth and for the first five days of life. Group three was swaddled for the second five days. Group four was swaddled early (first five days) and for a prolonged or extended period of time (total of 10 days).
The rats were allowed to return to normal movements for three months and then the hips were X-rayed and examined with dissection. They found that the rats who were swaddled the earliest and longest had more cases of small, deformed femoral heads (round bone at the top of the thigh bone that fits into the hip socket). This group was also most likely to have hip subluxations and/or dislocations. And the cartilage growth that forms the hip socket was deformed or distorted.
At the other end of the continuum, the control group had perfectly formed hips with no problems of any kind. In the two remaining groups, the early swaddling group had the worst results with more hip dysplasia (shallow hip sockets), more subluxations, and more dislocations compared with the late swaddling group. Female rats were more likely to have hip problems in general but especially in the early swaddling group.
The value of this study is to help prevent developmental dysplasia from forming in human babies. The authors suggest that the practice of swaddling should be avoided in humans because of the potential negative effect on hip development. It is important to find other ways to achieve the same benefits of swaddling without binding the hips/legs.
The authors were unable to explain why some hips subluxed or dislocated while others did not. They did observe that one side did not dislocate more than the other. In human infants, left hip dysplasia with subluxation or dislocation is more common that right-sided problems. The reason for this may be the position of the child while in the mother’s uterus. Ultrasound images show a tendency for babies’ left hips to be pressed up against the mother’s sacrum, which could contribute to the greater number of cases of left hip dysplasia in infants.