I lean toward random, but there is such a thing as 'too' random for something like pregnancy, that goes on a cycle. That said, it would depend on how many sex scenes we get with the character and how repeatable they are. If you are only going to get a few with each woman, then you need it to be more likely than if you can go back and do them as often as you want with repeatable events. It also depends on how much of the pregnancy we get to see in the game, do we get to see them at each stage, see our kid be born, etc., or it is simply a cause and effect?
With some games, they have certain events at certain points in the story, where if you finish inside, then she will get pregnant. That is really the most realistic, but in those games, it generally only affects the ending, not the rest of the game, in part because it generally happens near the end of her story.
Another thing to keep in mind is that women who spend enough time around each other, have a tendency to have their cycles sync up, so the house full of women here ought to be ovulating around the same time, for example, since all four of them basically live together (not literally).
I don't really care how you do it to be honest, though I might suggest having at least a sex scene with each of them during each trimester, for the fans of that. It got pretty nasty over in Summertime Saga when it came out that they would not be getting a 3rd trimester sex scene with Diane any time soon (you can still have her in the first 2, but it does not change the scene, as her belly is hidden).
