Hello, and I hope I can help you today.
Birth control pills definitely have decreased effectiveness when you do not take them properly. So, if you have been having unprotected sex and miss more than 2 pills in one month (without backing them up) you do have a chance of having conceived.
There is also no way to know if you ovulated during the time when you missed pills, and in addition, sperm can live for up to 5 days within the reproductive tract. Furthermore, some women have spotting when they ovulate.
So, even if you were bleeding, you could have conceived from this act of
unprotected intercourse. Unfortunately there is no way to quesntify that risk.
Over-the-counter
pregnancy tests are very accurate and will become positive within 10-14 days of conception. So, if you do not get a real period within 2 weeks of the date you had unprotected intercourse, I suggest you perform a urine pregnancy test.
I hope this information was helpful and answers your question.
Best wishes,
Dr. Brown