FUJIWARA no Senshi, the real mother of Emperor Ichijo, became a nun and entitled Higashi Sanjoin, which set a precedent for kokumo and Junbo to become nun and then entitled nyoin (a close female relative of the Emperor or a woman of comparable standing).
一条天皇生母の藤原詮子が出家して東三条院となって以来、国母・准母は出家後に女院号を受けるのが通例となった。