Chichi no Jikan

A male biology student living near a supportive female neighbor is visited by aliens who require him to care for their human-like infant for a year to prevent global destruction. To facilitate the task, the aliens provide a device that transforms his body into a woman's with a button press, complete with physical attributes suitable for nursing. His neighbor assists him as he adjusts to parenting, including purchasing a breast pump, visiting a maternity facility for a massage to relieve milk buildup, and bathing the child together. The protagonist approaches the experience with scientific curiosity despite the emotional and physical challenges involved.