You can't. For a complete list of girls who can-
Mimi, Elsie, Chloe, Davy (post game), Isadora, Eris (post game), Hides-In-Barrels, Keke, Yulia, Historia (post game), Dobbi, Tali, Wendy, Mary, Alraune (Or whatever you named the plant girl), Fiona, Moona, Alice, Bessie (post game), Cheaki, Breeki, Ray (post game), Yamamoto from Hayakawa (after 3 encounters), Swiftstroke (post game), Sofi, Liada The Light, and Hestia (post game).
Notably, none of the girls in the capital city get pregnant, though that's mainly due to the fact they're all still adventuring, and have made plans to do so far in the future. (I think ZAP for example says something along the lines of waiting until she's 30)
A general rule is, if a girl is actively fighting a lot, she's not gonna get pregnant. Not always the case, but mainly, as the dev intentionally tries not to make scenarios where a pregnant person might get into a battle. Grand Magus and a lot of other characters for example have notable post-game battles in the arena, so that's probably why they don't have pregnancy, as it'd be sort of a bad look to beat up a bunch of pregnant baby mamas.