Instructions
Begin conversing with a Sim of the opposite gender by clicking on the other Sim character and choosing the "Talk" option. The other Sim character does not have to be someone from your own household; you can talk to neighbors and strangers as well.
Wait until the friend level between you and the other Sim reaches 50. The Sim will be labeled as a "friend" and you will be able to select other communication methods such as "Tickle," "Dance" and "Flirt.
Continue interacting with the other Sim to raise your friend level to 80. You will be able to use the options "Hug" and finally "Kiss." A heart will appear in the friend meter to signify you are romantically involved with the other Sim after you have completed the kiss.
Walk to the bedroom, click your partner and select the "Call Over" option. Left-click on the bed and select the "Relax" option once the Sim has walked into the bedroom. Left-click on the other Sim character and select the "Relax" option as well.
Wait until the message "Should we have a baby?" appears. Continue to romance your partner Sim if the message does not appear. Click the "Yes" button and the female Sim will become pregnant. The female Sim will give birth to a child after four or five days.