Thanks for the query.
The chance of pregnancy is around 0.5 % only, which is very less, when one has unprotected sex on birth control if you have been taking them regularly every month.If you have just started it for the first time, you should use a backup like condom for first 7 days on the pill.
Any missed pill can lead to failure if not supplement with immediate pills accordingly.
Hope this helped.
These Q&A’s are for educational purposes and should not be relied upon as a substitute for medical advice you may receive from your physician. If you have a medical emergency, please call 911. These answers do not constitute or initiate a patient/doctor relationship.