Hi my girlfriend had sex with someone on the 10th

Patient: Hi my girlfriend had sex with someone on the 10th of march and had her period on the normal day (22nd) she has a 28 day cycle. She had her perod for 5 days and normally 3 . this month she didnt have it she is now 4 days late and i know the other guy pulled out but so do i. The question is. Is there a possibility that it is his? Even though she had her period 12 days after intercourse?

Doctor: