If women whose cycles are usually between 26 and 32 days in length:
Days 1-7 is infertile
Days 8-19 are considered fertile; considered unsafe for unprotected intercourse
Day 20 through the end of the cycle are considered infertile.
Failure rate is around 25% by calendar method,
Failure rate by withdrawal method is around 22%.
Next Steps
Advisable to use post coital pill if it's within 72hours. These tablets are available over the counter without prescription.
Health Tips
Barrier method will be very useful as it can prevent pregnancy and infections too.