Actually, the 7 days BEFORE a girl's period are the safest. If
she's on a regular 28-day cycle, her ovulation will be on day 14,
where day 1 is the start of her period. The egg will be alive for
24 hours, but your sperm can live inside her for a max of 5
days.
So, if you take 14 days, plus when you penetrated her:
DDDDDDDDDDDDDO
________P
You're still in the "safe" zone, so you shouldn't worry.
And the best place to learn about your gf's fertility cycle:
www.google.com
