There are about seven settings associated with pregnancy length that are reachable by way of NRaas Retuner. To see them, visit this page and do a Ctrl+F kind of Find for the word pregnancy.
http://www.nraas.net/community/Members-Retuner-Settings
The advantages to doing this the Retuner way are that one can change the settings to whatever one wants by an in-game menu interface without having to look for or figure out how to edit a standalone tuning mod that does the exact same thing, and that they can be different for different ongoing games because the settings are game save/world specific. And yes, there should be NO pregnant sims in town at all before trying to edit these settings or chances are they will never go into labor.
It becomes important when considering this to not allow the settings to conflict with each other mathematically or else unexpected things might happen. And if making the pregnancy term really long, we have to remember that sims cannot age up while pregnant nor should they be able to become pregnant if their birthday is approaching. So if, as a wild example, the YA and Adult age stages are each 100 sim days and the length of pregnancy is 120 sim days, then no sims will ever become pregnant because it would always be too close to an upcoming birthday (assuming aging is on). If the length of pregnancy is let's say 90 sim days, then the windows of opportunity there for both age stages are very narrow.
There's all kinds of ways to influence the gender (of the first born if multiples) and chances of number of babies per birth with mods and with in-game actions, but officially neither one are really set until the sim gives birth. We can expand on those if needed. There is no way to influence or set the gender of babies after the first one in a multiple birth (the second twin, the second and third triplet).