How to Have a Baby on Sims 2

Steps

Find two adult Sims of the opposite sex. This is a requirement unless you have mods installed in your game allowing same sex couples to have children together. The two Sims you have chosen also must have their relationship level high in the "love" category.

Have both Sims "relax" in a double bed. You can't do any actions until they start talking to each other in bed. Instead of a bed, you can also use hot tubs, cars, elevators, photo booths, clothing changing rooms and supposedly other things. However, you must have the correct expansion packs for different places.

Click on one of the Sims and choose "Cuddle". Then you have to kiss, then make out. They do not have to kiss or make out, but chances can sometimes increase if you do this.

Click on them again and find "Try for Baby". If the option is not there, you may need to get their lifetime relationship number higher.

Listen for a "Rock-a-bye Baby" tune right after the woohoo. This will tell you that your Sim is pregnant.

Alien Baby

Purchase the most expensive telescope.

Have your male sim stargaze using it (at night, obviously).

Eventually your sim will get abducted by aliens. This may take a while, so you must be patient.

Your Sim is now impregnated with a cute little alien baby!

Alternately, you may want to use a cheat if you are not patient enough for this. Save and go back into the neighborhood, then open up the cheat box (using ctrl-shift-c) and type in boolprop testingcheatsenabled true and then return to your house. Then, select the sim you want abducted and shift-click on the telescope, then select 'be abducted'.