Contrary to popular belief, this W-sitting posture is normal for many children, and should be allowed even though children who can sit like this often walk with their feet turned in – called pigeon toed walking. The medical term that allows W-sitting is called Internal Femoral Torsion because the thigh bone has an increased twist in some people.
Treatments for this common variation were debunked more than twenty years ago, but myths still persist about the best way to stop this way of sitting and walking. This way of sitting and walking is often noticed around the age of three years while the child is very flexible. During further growth, the bone twist goes away naturally in 99% of children without intervention. When this goes away, parents and doctors often credit whatever they did to “help” it go away. Studies have shown that W-sitting does not cause any harm to the developing hips, and does not contribute to hip dysplasia. The Seattle Children’s Hospital has a short brochure for those who want to learn more about this common variation of normal. https://www.seattlechildrens.org/pdf/pe245.pdf
Each of us have differences in body shapes and abilities, which explains why some people are gymnasts or sprinters and others are basketball players or swimmers. These are variations of normal. Internal Femoral Torsion is one of those variations like being left-handed or having reddish hair except that Internal Femoral Torsion decreases with age. Research studies show that adolescents who could W-sit during childhood have athletic abilities that are just as good in those who did not W-sit during childhood. [https://www.ncbi.nlm.nih.gov/pubmed/608277] Some doctors have noticed that these children may engage in different sports than those who did not W-sit with sports such as swimming and gymnastics being preferred.
There has been concern that increased Femoral Torsion can cause hip dysplasia, but this is not the case. While increased femoral torsion is more common in older children with a dislocated hip, this is because the twist does not improve spontaneously when the hip is out of the socket. Early treatment to get the hip back into the socket can allow the hip to develop naturally and this is another reason why early treatment is important for hip dysplasia. Pediatric orthopedic surgeons who treat older children for a dislocated hip are aware that these older children may need correction of internal femoral torsion when surgery is performed. The surgeons can evaluate this during surgery and adjust as needed. Stopping W-sitting has no influence on femoral torsion whether the hip is in or out of the socket.