Ok, I've been looking into my checkpoints and saves looking for the point where Jake can get Sofia pregnant. Without using checkpoints, I think it's incredibly hard. First you need to get her off birth control early on and then later on make her a bimbo and call up Jake. Problem is, as far as I can tell, there's only 1 time you can make Sofia a bimbo and can call Jake for a threesome. I'm also unsure if you need to make her your girlfriend first.
The passage that leads to that event is called "Afternoon063BaseHub1" and you need to pick "Go see Rio and Sofia." That brings you to a passage called "RioSofia063BaseAfternoonStart1". Once there, you need to pick "Cast 'Bimbofy' one of the girls" and Sofia in the next choice. You can then call Jake. I get that event on Sunday, August 11 but since I use the checkpoints, that date could be wrong.
That's the only time you can do it as far I can tell from a quick look at the code that I did a few months ago. Also, I don't think there is any pregnancy talk from Sofia if Jake gets her pregnant.