It will definitely not be an end game/epilogue only thing. It will likely be available starting at a certain point through a girl's storyline, and will "pause" her story until the pregnancy is done. Some girls will be happy to have children (e.g. Beth, Kirana) while others will take some big convincing (e.g. Tirria, Sacha).
My plan was to use elements of both of those kinds of pregnancy systems. The girls would go through the stages of pregnancy (bellies getting bigger, breasts growing, etc.) and likely have a unique sex scene while pregnant (in addition to their normal scenes with the changes). As for the kids, I would like to have some kind of system where the first 1-2 kids a girl has the player can interact with (or choose to opt out), but I'm not concrete on the details yet.