Chichi no Jikan

Chichi no Jikan follows Kyūichi Shinogi, a male biology student who lives in an apartment near a helpful female neighbor/friend, Ayaka Takarai. One day, he is visited by aliens, who demand that he raise their baby for a year... or else the world is history. The baby is apparently human, and is raised the way humans are: through breastfeeding. Kyūichi's sex is not a problem for the aliens: they give him a simple device that switches his body to a woman's with the mere touch of a button. Conveniently, Kyūichi's female form is very well-endowed.

Although she's understandably confused and uncomfortable by the whole situation, Ayaka helps Kyūichi out as he navigates the unfamiliar experience of raising a child, and breastfeeding it in particular. That means going to a store to buy a breast pump, visiting a maternity home to get a massage to release Kyūichi's stored-up milk, and giving the baby a bath together. Kyūichi, as a biology student, is fascinated by the whole thing from an academic perspective.