Well, I would love for it to be more random and unpredictable but like that it can also work and can turn out to be (at least for the player) unpredictable to a point.
In StoryInit I put:
<<set $gameDate to new Date(2017, 1, 1, 8, 0)>>
<<set $laterDate to new Date(2017, 10, 17, 3, 30)>>
<<set $earlierDate to new Date(2017, 1, 1, 16, 0)>>
The earlierDate is set to a time near the current game date for testings, later on it's going to be somewhere more far away.
Now in the different rooms I put:
<<if $earlierDate > $gameDate>>You will go pregnant somewhere here when going to [[your bedroom|Your Bed]]
<<else>>You are Pregnant, congratulations future mommy.<</if>>
<<if $laterDate < $gameDate>>You feel a sharp pain... better go to the [[hospital|Hospital]]. Your child is about to be born<</if>>
both go in the same passages always together like that the player can wander around the house and the event would trigger and somewhere along you go pregnant and then at another point you deliver...
Of course, in the "Your Bed" passage I put this so the stats would change correctly:
<<if $earlierDate > $gameDate>>
<<set $Status to "Pregnant">>
<<else>>You are pregnant.<</if>>
at some point you go from bed to somewhere else and have a pregnant status.. tested it, it works like that...
It's not exactly what I wished for but it works... :)