As i've said before the first rule of thumb is to not freak out until you've missed a period. If you do so before hand, it will cause your period to be late. That's how strong our mind is. It's definately not effective at all to use plastic wrap, so definately buy condoms. Buy them now so that you'll be adequately prepared for next time. If you plan on being sexually active, you should plan to be on birth control too. You can get this from your local health department or planned parenthood without your parents having any knowledge of it.
You are definately at risk because precum can get you pregnant. I'm not really sure how effective plastic wrap would have been at keeping that out. But I don't think you should worry until you miss a period.
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