ðɪs ɪz bɪkˈʌz ðɪ ɚɹˈɪdʒɪnəl tʃˈɑːɹts pɹədˈuːst ɪn nˈaɪntiːnhˈʌndɹɪd sˈɛvənti sˈɛvən wɜː bˈeɪst ˌɔn sˈæmpəlz ʌv mˈɪdəlklˈæs wˈaɪt ɐmˈɛɹɪkən bˈeɪbiz ˌɔn hˈaɪpɹˈoʊtiːn bˈɑːɾəlfˈɛd dˈaɪəts ɪn oʊhˈaɪoʊ.
This is because the original charts produced in 1977 were based on samples of middle-class white American babies on high-protein bottle-fed diets in Ohio.