Thanks for the query.
In case of regular periods there is less chance for pregnancy
Because usually in regular 28 to 30 days cycle ovum will release on around 14th to 16th day.
If there is any irregularity in your cycles previously, you better go for emergency pill as 72 hours not yet over after the intercourse.
If you are not planning for pregnancy better to use some regular method of contraception to avoid these problems in future.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429