Speaking to reporters Friday, Koizumi said he "may not stick to" plans for an early submission of the bill.
"Circumstances have changed," he said, apparently referring to this week's announcement of Princess Kiko's pregnancy, which raised the possibility of a new male heir to the Chrysanthemum Throne.
The news has taken the steam out of Koizumi's drive to let empresses reign to avert a succession crisis in the royal family. No male heir to the throne has been born since 1965.
Koizumi, who had earlier called for quick approval of the law change, has been rapidly backing away from that position since the pregnancy announcement.
"I don't think the issue should trigger political conflict," he said earlier Friday, after several government ministers backed delaying action on the bill until the gender of Kiko's child is determined. The birth is reportedly due in September or October.
"Depending on the outcome (of the pregnancy), I would expect an argument that there is no need to change the law," Justice Minister Seiken Sugiura said Friday.
Koizumi has reportedly told ruling party leaders he'd prefer to wait until the change can gain broader support in Parliament.