is possible only,if unprotected sexual intercourse is done with ejaculation around ovulation time in regular period. Sometimes precum or prostatic fluid,secreted prior to ejaculation,causes pregnancy in low percentage.
Condom has failure rate of 14-15%. It is better to wait 1 wk and if period not comes,do a urine/blood test
to confirm/exclude pregnancy.
It is better to take Emergency pill like Unwanted 72/I-pill within 72 hrs to clear doubt from mind and it is safe with effectiveness more than 99%.
Practice safe sex
by using safe period/OCP/Condom in correct method etc to avoid unwanted pregnancy.