The other post explain how it work but not why the placebo are there. You could remove them and do nothing for a week but then you need to remember to star taking the pills again a week later and that is not that hard to miss and if you did for few days then you start to risk getting pregnant.
So the placebo pill is there so you can follow a routine and take a pill a day each day without any break. That is a lot simple to follow and the explanation for them. So they are a memory aid.
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