Trying on the Bed
To try for a baby on the bed, you have to have a male and female sim relax on a bed together. The option to 'try for baby' or 'woohoo' will show up. Choose 'try for baby' if you wish your Sim to have a baby.
If you listen closely after 'trying for baby' you will hear a lullaby afterwards. You can have both relax on the bed and see if the 'try for baby' option shows up. If it doesn't, then your Sim is pregnant. Or you can just wait and see if the telltale signs of pregnancy show up.