Tying or not tying the legs of newborns does not cause rotundity, because there is no necessary causal link between rotundity and leg tying. The main reason for the appearance of rotund legs in newborns is due to vitamin D deficiency rickets, because growth and development is very rapid, plus the number of sun exposure is very low, so it is very easy to develop vitamin D deficiency. Vitamin D deficiency will cause a significant decrease in calcium absorption, and if the calcium level is significantly low, it will cause calcium softening, resulting in rotundity, X-leg, O-leg, etc. This is caused by calcium deficiency in newborns, and there is no necessary relationship between tying or not tying the legs. Therefore, it is wrong to say that untied legs of newborns will lead to rotundity.