What Causes Delay In Periods After Having Unprotected Sex?

hi, i penetrated [unprotected] my fiance but withdrew before ejaculation. she took a emergency contraceptive "unwanted 72" about 15 hrs after the incident as a precaution. she was expecting her periods to start today. but that hasnt happenned. what do u suggest should be our course of action. {male,30yrs}