hi I would like to know whether it is less effective if i start my pills on the 5th day of my period. I am on 21 monophasic birth control pills pack for the last 6 months and have been taking the microgynon 30 pills for 21 days each month, starting from the 5th day of my period. Also, I would be grateful if you please let me know, whether I can take 18 pills instead of 21 for next month to advance my period. thanks