My period usually lasts for seven days. i had sexual intercourse on the fifth day. can i become pregnant by doing so?

Yes: Pregnancy can result from intercourse on any day of a woman's cycle. You should use contraception regularly, unless you wish to get pregnant.

Cycle length?: Need to tknow how long between periods which is called 'length of menstrual cycle'. You ovulate 14d before the period starts. So if your cycle is 28 days, then you probably are not pregnant as the egg lasts ~72hrs and sperms ~48hrs.

Probably not: Its not how long your period lasts but how long your cycles are that determines when you ovulate. The average cycle (from the first day of menses to the next first day of menses) is 28 days. Ovulation occurs on the fourteenth day of the cycle. So intercourse on the 7th day is not around ovulation. If your cycle is 21 days, however, then ovulation occurs on day 7 and pregnancy is possible.