From the NHS website:
"If you start the pill up to and including the fifth day of your period, you will still be protected from pregnancy straight away, unless you have a short menstrual cycle (your period is every 23 days or less). If you have a short menstrual cycle, you will need additional contraception, such as condoms, until you have taken the pill for seven days.
If you start the pill on any other day of your cycle, you will not be protected from pregnancy straight away and will need additional contraception until you have taken the pill for seven days. "
Source:
http://www.nhs.uk/Con...s/how-to-take-it.aspx
Chris