Looks like there's actually a bug in the formula for daily pregnancy chances. The chance to conceive your first child is
probably 3x lower than the devs intended. Interestingly though, because of the bug it seems the second child is 100% guaranteed to be conceived on the first night you spend together. The third child's conception chances are 3x greater than the first's. Fourth is half as likely as the first. It's a bit messy lol. I know the cause, and I'll make a bug report sometime soon.
To clear everything else up though, the only things that matter for pregnancy chances are as follows:
- Must be waiting together in the same settlement at the moment of the daily tick (when your income is tabulated). Does not matter if your spouse is in your party or the settlement. Villages don't count as settlements in this case, only castles and towns. You can actually leave the settlement as much as you want, as long as you are inside at the daily tick when it does the check.
- The female character must be between the ages of 18 and 45. Chances drop as the female gets older. Male age does not matter.
- The chances of pregnancy are supposed to drop with each successive child, but this isn't the case at the moment because of the bug.
- It should go without saying that the birth and death cycle needs to be enabled and the female character can't already be pregnant.
None of the other stuff matters. Relations do not affect it and it appears fertility was removed as a mechanic.
Edit: In the meantime, you can use the command "campaign.conceive_child" in the console to instantly get your spouse pregnant if you want.