Instructions
Click on the Sim with whom you wish to have a baby. A radial menu appears displaying possible options.
Select "Try for Baby" when it appears as an option. This generally only appears when both Sims are in good moods, are romantically involved, and have recently performed options under the "Romance" tree. If accepted, the two Sims will make the baby, who will be born after three in-game days.
Repeat Step 1 and Step 2 after the baby is born. You can keep having new babies as long as your Sim is not presently pregnant and you have not exceeded eight Sims in one house.