I got my period on 4th december. i had unprotected sex today i.e 13th. will i get pregnant?

Unlikely: Ovulation , releasing an egg generally occurs around 14 days after you start your cycle, but sperm can stay viable for 2-3 days. A woman's fertile window can be from 11 days to 17 days from start of menses. You had sex on day 9? Unless you generally have a short cycle, less than, 25 days, your probably safe. But never trust your cycle!