Chances of pregnancy

Patient: My girlfriend had her period around December 12-14 and we had unprotected sex on the 18th. She drank althea pills everyday for 21 days prior to her period on said dates. She never took any kind of contraceptive pills after that. Then she experienced spotting and cramps around December 24/25 and up until now her period has not yet arrived. What is the possibility of her being pregnant? December 18 should\’ve fallen on the \”safe\”” dates to have sex based on the calendar method