Sex scenes with non-brainwashed colleagues never result in pregnancy.
Sex scenes with brainwashed colleagues result in pregnancy with expected probability, and I don't think there is a way to influence that, short of them hitting pregnancy limit. Their rooms can't be upgraded, right?
You can't upgrade the brainwashed colleagues' rooms, no. You also can't give them milk for some reason either, so you can't add new traits to them either. You CAN give them drugs you, or at least, you can give them the trait upgrade thing, which upgrades traits at random, so you can somewhat improve their birthing performance, and I suspect you can also reset their birth counters with that other drug.
Shame that you can't milk them or fuck them with monsters or whatever though...
Still, they have some really interesting unique traits that they can pass on to their kids. Like how one of the sheep twins has a unique trait that makes her immune to diseases. So breed her once. Then improve the characteristics of the kid to your liking. Then bam, infinite brothel workers...
If you even care about brothels, given the whole 'tentacle eggs sell for 5000 gold' thing... >.>"