If you want to have a teen mom, you must follow these steps:
1. Go to http://www.modthesims.info/d/388170 and download: AutoRomanticSocials.rar. With this file, your teen will have the actions of "Woohoo", "Try for baby", "Marriage". But the actions of "Woohoo" and "Try for baby" only appears when you talk with the other sim at the option of "Romantic". Never appears if yours teens are on the bed, on the shower...
2. Go to http://www.modthesims.info/d/350022 and download: Adult_Maternity_For_Teens_All.zip. With this file, your teen will have clothes that will show the growth of her baby. If you don't use this, your teen won't have body.
3. Move all of these packages to: PC/C:/Program Files/Electronic Arts/The Sims 3/Mods/Packages.
If you haven't got the folders "Mods" and "Packages", you have to go to "The Sims 3" and create a folder called "Mods". Into a "Mods" you have to create another folder called "Packages". Into "The Sims 3" you have to put the file "Resource.cfg", that you can find in: http://www.mediafire.com/?6ow5aa0w23lp886.
4. Now, you can enter on the game and check if your teen can have a baby.
Only use these steps if you have "Generations". If you use this steps without "Generations" it's probable that yours sims can't interact, or that your game doesn't runs.
Some people says that if you have "Generations", you can have a teen mom without AutoRomanticSocials. They say that you have to follow these steps, but I don't recommend this, because my teen mom can't give birth, but maybe it's only my problem and it works for you:
1. On the game, press Ctrl+Shift+C and write: testingcheatsenabled true.
2. Click Shift+The future teen mom and select "Edit in Cas"
3. Make your teens adults.
4. Go to "Try for Baby" with the adults, and just when they are going to do this, click on the sims while you press shift and select "Edit in Cas", and make your adults teens.
5. Now, if the music sounds, your teen are pregnancy.
Sorry if I don't write well in english but I'm spanish. Any question, write me.