That's crazy. In the CK2+ mod female rulers often went into regency for the last couple of months of pregnancy, it/when regencies are implemented in CK3 that might be an idea here too.
Like, there isn't a lot of precedent for female military leaders during the period already......probably even fewer or no heavily pregnant rulers.
Thinking back to when my partner was heavily pregnant and wildly hormonal, it does make me laugh imagining her lead a medieval army though. The mood swings might have interesting military traits/effects.