If you don't wish to use mods there is always another option, though they wont be a teenager when they have the child. This process involves aging up the teenage sim/s to adult (Both if the couple are both teenagers) and making them try for baby, than using CAS to change the male back to a teenager, (If he already wasn't an adult), letting the female give birth to the child and than turning her back to a teenager.
Open the cheat command box using Ctrl+Shift+C and enter 'TestingCheatsEnabled true' (Case sensitive), this activates testing cheats. Hold shift on the sim you want to get pregnant, than select either edit in CAS or Trigger Age Transistion. The difference is that if you use TAT the sim will have a birthday like normal, so using CAS is the better option if you want to do this quickly. In CAS, change the age from teenager to young adult and save. Don't bother changing anything else as this is only temporary. If the male you want to use is also a teen, do the same thing. Now have them try for baby until you hear the little noise. This will mean your sim is pregnant. Use the cheat to turn the male back to a teenager using CAS. Now you will have to wait untill the female gives birth. Once she does you can do the same to her. Now you have a teenage mother!!!
And just to be respectful, you can also make sims have children with another of the same sex using the same prosses. Simply make on of them the opposite sex in create-a-sim and change them back after they've given birth.