How do you get teenage pregnancies on the sims 2?

You cannot make a teenage sim pregnant. The youngest age a sim can become pregnant is Young Adult. If you don't want to wait a while for the sim to age up, press the buttons Ctrl then Shift then C. A bar where you can type things in will appear at the top of the screen. Type in exactly this:

testingcheatsenabled true

This will enable the cheat world of The Sims 3. Then Shift click on the sim, and click the option "Trigger Age Transition" and the sim will age up (only once). So if this is the teen you would like to be pregnant, she will become a Young Adult with this cheat, and then you can make her pregnant.

(Also here's another tip: if you type inmotherlode into the box at the top of the screen, you will receive $50,000 of which you could use to buy things for the baby)

I hope I helped ;)