First she needs to find a Sim that she likes and when they know each other well enough make your Sim relax on the bed (must be the bed for two) then click on the guy and ask him to join you then click on him again when he gets on the bed and click on try for a baby.
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It takes three Sim days for the baby to turn into a child.
There is no cheat in Sims 3 to get your teen pregnant but if you enter the cheat in testingcheatsenabled true you can right click on your sim and then you click trigger age transition and then get your sims pregnant once your sim has delivered the baby right click on your sim again and click edit my sim and then you can make it a teenager again and your sim will still have the baby and that is how you can have a baby when you are a teenager
4 sim days
It usually takes three Sim days.
your sim will show sign of naseau and back pains and in a few days a "baby bump" will appear on your sim.