Parents who come to the clinic with x-rays often have the question, “Some doctors say that our baby’s ‘acetabular index’ is over 25°, which is on the large side, is that a problem? Does it need to be treated?” We know that the change of the acetabular index reflects the ossification of the acetabular structure and is a manifestation of the maturity of the acetabulum, which is closely related to age. There is a dynamic process of change within the human tissue structure, and as we age, each tissue structure tends to mature changes, and for the osteoarticular system, it is mainly manifested as the ossification that can be seen in x-rays. The ossification of the acetabulum is reflected in the gradual decrease of the acetabular index on pelvic radiographs. The criteria we currently use to determine this are mostly based on data from a study published in the 1970s by the German scholar T. Nnnis. According to T. Nnnis’ study, the normal value of the acetabular index should be a range (Figure 4), and there is no definite line of demarcation between “normal” and “pathological”. When the value is at the so-called “limit of the range”, it is difficult to determine the presence of pathological changes. A certain number of pathological hip measurements can also fall within this “limit range”. Therefore, it is important to consider not only the age of the child, but also the distribution of the normal acetabular index at that age to determine whether the “acetabular index” is too large. Determining the presence of DDH is not like failing a school exam with a score of 60. The acetabular index has its own “passing line”, but this is a relative and dynamic passing line, not an absolute value.