There is a single scene only with breast expansion, though since she start off with pretty small breasts, not sure if it counts, and it is only for the duration of that scene, but well it kinda exists I geuss. If it matters to you as well the 'birth scene' at the other end of the pregnancy is in the forms of eggs. My all-time favorite H-game 'Naedoko Demon's Ground' which is sadly banned from this site due to Rule 7, does fit that bill, with a female protagonist, though she doesn't fight directly herself, but rather using monsters birthed from pregnancy that have occurred among other things from defeat rape, mainly by tentacles of various sizes and shapes.