Why would you even do that?
Condoms are so easy to get.
Have you heard of STDs/STIs??
Do you really want to risk having a baby at your age?
I dont mean to sound like some parent or school teacher moaning on about safe sex, but you ask if there is "any threat?"
Any threat to you, to her, ...or to your possible unborn child that happens as a result of unprotected sex?
Please use protection.
As for when girls are most fertile...the answer is during ovulation, which usually happens around 14 days before her period (and a couple of days either side).
However, that definately doesnt mean she cant get pregnant if you have unprotected sex with her at other times.
Sorry for sounding like a bitch
, ive just seen the possible results of unprotected sex way too many times and i wouldnt wish it upon anyone...