Chichi No Jikan

Chichi no Jikan follows Shinogi Kyichi, a male biology student who lives in an apartment near a helpful female neighbor/friend, Takarai Ayaka. 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 breast-feeding. Kyichi'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, Kyichi's female form is very well-endowed. Although she's understandably confused and uncomfortable by the whole situation, Ayaka helps Kyichi out as he navigates the unfamiliar experience of raising a child, and breast-feeding it in particular. That means going to a store to buy a breast pump, visiting a maternity hospital to get a massage to release Kyichi's stored-up milk, and giving the baby a bath together. Kyichi, as a biology student, is fascinated by the whole thing from an academic perspective.