appropriate time for first pap

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This isn't a question about me specifically, but just in general.

I went to the doctor a couple months ago and we got on the topic of pap smears. She told me these days the rule of thumb as to when a woman should get her first pap is 2 years after she first becomes sexually active, regardless of age.

Is this true???

I've always been under the impression it was within the same year a woman initially becomes sexually active, or by the time she is 21 (whichever comes first).
Then again, I've also heard you should go when you turn 18, and I've even heard when you first start your period (which seems sort of ridiculous).

But anyway... 2 years after you first have sex? Isn't that a little risky to wait so long, especially with the risk of contracting HPV and developing cervical cancer?

When, exactly, should one go for her first pap?

Sorry for all the questions, I'm just really curious about what is fact here!