The suffix 'ko' was originally used to indicate a male, as in the case of kuninomiyatsuko (provincial governor) ONO no Imoko; however, as the Fujiwara clan had monopolized all the names for females, because it only a portion of females of high position, such as the Empress, were able to have the names with 'ko' up until the modern era, 'ko' has become a popular suffix for female names at present.
「コ」は国造（ミヤツコ）小野妹子など、元は男性を表したが、藤原氏が女性名として独占し、近世までは皇后など一部の身分の高い女性しか名乗れなかった事から、現代では女性名として定着した。