The Sim Manipulator
is a wonderful hack, and I just discovered yet another feature (those that know bear with me).
With the sim manipulator you can self-click on a sim and get a variety of information under the menu item "Stats" but, in addition, I found out that if you self-click on a pregnant sim there is a submenu "My Pregnancy" where you can find out 1)The number of babies coming, and 2) How many hours the pregnancy has lasted so far.
If you use the sim manipulator statue (an artist's dummy) you can change either - the duration of the pregnancy, or the number of babies. If twins are on the way and you find twin toddlers more than a handful
you can change the upcoming birth to a singleton. If you have a singleton coming, and you love
toddlers, you can change to twins.
Either way, the pregnancy runs to around 72 hours - there may be some random variation. In the case I observed, birth happened at 75 hours.