There is ALWAYS a chance that you can become pregnant if you have sex unprotected, or even protected. Contraceptions fail…..at one point or another!! The withdrawel method is no more reliable than any other. You are taking a risk when ever you have sex, that you will conceive a baby!
I’m not sure about your dates you posted. But regardless, if you become late for your period you should go have a blood test done to get an accurate results.