The twin symbol
Sometimes after entering a dungeon and returning to fort city, when a heroine is chosen to classmate with, this symbol appears. This indicates that there is a higher rate for twins to be conceived (as far as it's known triplets cannot be created and only appear in the demo). To greater the chances further, it is best to attempt trimating with the Heroines to preform better chances for conceiving twins. That doesn't mean that it won't work through ordinary classmating, however, there are just greater chances for it to work.