Thanks for the query.
No, if you take the pills as per the schedule they can prevent the pregnancy
irrespective of the period of menstrual cycle.
So no need to worry.
But emergency pill is having some amount of failure rate in general. So if you miss the period, once go for pregnancy test.
Better to avoid repeated use of emergency pills.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429