There's a new variable "fertile" for Tina and Jenny in the game. At a certain state it is changed to "True" so you can impregnate them. Seems to be a failsave to make sure you can't lock yourself out from progressing the game.
As far as I see in the code you can impregnate Tina after the long talk in the pizza shop with Tony, Maria, MC and Tina.
For Jenny it seems to be switching to "True" after you try to give her the necklace and unlock the girlfriend experience.
(EDIT) Seems after loading and converting an 20.1 save to 20.5, Jennys fertile state does not get set to "true". Seems like a bug to me.
The responsible check in the impregnation minigame is:
Code:
if player.pregnancy_chance > 0 and machine.get('fertile', True) and not machine.pregnancy.is_pregnant:
I think this is quite interesting. It opens up possibilities for girls to deliberately go off or on birth control after a change in relationship with the MC.
Would be fascinating to implement a more thorough impregnation system with various contraceptives, different fertility during the girls ovulation cycles (MC keeping track or getting hints when the girls are most fertile to either avoid getting them pregnant or increase his chances to impregnate them)