They say they gave the child a boy's name because they were anticipating celebrating kitanokata (legitimate wife; here, Yoshihime) giving birth to an heir' (from 'Date Jige Kiroku').
「御男子の故を以て名付け玉へるは、北御方（正夫人の意、ここでは愛姫を指す）の御腹に嗣君誕生し玉はん事をあらかじめ祝し玉ふと云云」（「伊達治家記録」より）。