Instructions
Save up at least 5,000 simoleons -- preferably 10,000 -- so that you will have enough money to buy the things that you will need for the baby. You will need enough food in the fridge to feed the baby, a crib and its own room for when it gets older.
Order a nanny service over the phone if you want someone other than your Sims to take care of the child. However, the game features maternity leave, so you can let one Sim stay home during the child's infancy even if both Sims are employed.
Place two Sims, male and female, on the same bed together. Preferably, they should be in love.
Allow them to interact with each other on the bed; you should get a range of options such as "cuddle."
Choose the "Try for Baby" option, and you will have a baby in no time.