It looks like Martha Hawes picked this up from existing literature:

"Although debate continues as to whether the deformity is due to.....positioning in the cot (Mau 1968), prone lying does appear to have an inhibitory effect; it must be insisted upon, particularly for the hypotonic infant (McMaster 1983)"

-- R.A. Dickson, "Conservative Treatment for Idiopathic Scoliosis", The Journal of Joint and Bone Surgery Vol 67-B, No. 2 March 1985.

Btw I know Prof Dickson, he's still a scoliosis surgeon and is Head of the Departmnt of Orthopaedic and Trauma Surgery at the university in Leeds here in the UK