Thank you for your query at AskTheDoctor.com
For starters, sex at the age of 12 should be avoided. The woman's body is not ready for physical intimacy at such a tender age. Even if it is consensual it should be avoided if possible.
Regarding pregnancy, if she already attained menarche ( had her period ), then she has the potential to get pregnant. Other ways to avoid pregnancy would be to use the withdrawal method ( withdrawing penis right before ejaculation ), or calendar methods, where sex is avoided from the 8th to 22nd day of cycle, considering the cycles are regular. However, a condom is the safest and the best way to avoid pregnancy or STDs.
Hope this was helpful.
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