Well, you had the right setting. If you use the pregnancy percentage settings for each age group and set them all to 0, NPC sims will never randomly get pregnant and the "Bypass played households" setting will be ignored.
However, this will not effect sims in LIVE mode.
Oh, with the pregnancy percentages set to "0", sims in LIVE mode won't randomly get pregnant either... but if you manually get one pregnant, they'll have the normal gestation period. I don't think there's a way to "pause" pregnancy.
I think the best option is to set the pregnancy percentages all to 0, and set what @Pamtastic72 mentioned. Set the pregnancy duration to whatever you want. You won't be able to pause it... but at least it'll be the length you want.
