Guidance has been given by the FSRH (1):
More detailed guidance (1):
If pills are missed in the first week (Pills 1-7)  | If pills are missed in the second week (Pills 8-14)  | If pills are missed in the third week (Pills 15-21)  | 
EC should be considered if unprotected sex occurred in the pill-free interval or in the first week of pill-taking.  | No indication for EC if the pills in the preceding 7 days have been taken consistently and correctly (assuming the pills thereafter are taken correctly and additional contraceptive precautions are used).  | OMIT THE PILL-FREE INTERVAL by finishing the pills in the current pack (or discarding any placebo tablets) and starting a new pack the next day.  | 
Notes:
Reference:
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