Pregnancy and Birth (Human)
Your human Sim must first be good friends with another Sim of the opposite gender. Use social interactions listed under the "Romantic.." sub-menu until the other Sim starts to find you irresistable. Usually, simply "Kiss" repeatedly will do the trick. Once irresistable, you may choose the "Try for Baby" social interaction, also listed under "Romantic.."
The Sims will automatically walk to a double-bed (keep in mind this will not work if you only have single-beds available) and do their business. Certain problems, such as other Sims in the room, low stat bars, or no room in the household can prevent pregnancy. Listen for the happy-bells sound effect, which signifies pregnancy in both Sims and pets. Keep repeating the "Try for Baby" interaction until you hear the bells.
After about 12 hours or so, your Sim will automatically pull you out of the game for a quick change of clothes. That belly gets big and your choices are limited. Thankfully, it's only for a couple of days, so just be patient!
The Sim will be pregnant for about 2 1/2 days before going into labor. Use this time wisely! Take them out to eat at the restaurant, get a massage, and whatever you do, keep those stat bars high!
When the Sim goes into labor, you do not have to go to the hospital, you may choose to stay home for a good old-fashioned home birth, but going to the hospital will give your Sim the benefit of returning home with the other parent in tow to help with the new baby(s) if they don't live in your Sim's house with them.
Now, just keep the baby(s) happy to keep to Social Services worker from taking them away due to your attrocious parenting skills! ;)
Added by: XDevilz RogueX Nov 6th 2012, ID#8789