I play a lifespan of 200 days:
Babies: 5 days; Toddlers: 10 days; Children: 20 days; Teens: 30 days; Young Adults: 50 days; Adults: 65 days; Elders: 20 days.
In my game, based on the 200 day lifespan, pregnancies are 10 days long. This is how I changed the settings:
Click on City Hall or any computer and go to NRaas>Retuner>Settings>General>By Tuneable XML>Sims3.Gameplay.ActorSystems>Pregnancy. Then, change the original settings to the new ones, marked by the red dots.