I learned an interesting and startling fact when I went to the GYN to have an IUD placed. When she inquired about past birth control methods I mentioned that originally I was on the weekly patch, up until my insurance no longer covered it and they put me on pills. Not regular pills though, the progesterone only pill (aka mini pill, aka POP, aka low dose hormone pills). She seemed a little confused by the swap and asked why they would put me on the mini pill over regular ones. To that I had no answer; it was simply what they had chosen, without my consent, and were pretty unwilling to work with me about.
She then asked since I was only getting the IUD due to breastfeeding, why I didn't want to go back to the mini pill -- since I was already familiar with its usage and placing an IUD is considerably invasive. This answer did not surprise her nearly as much as it had surprised every other doctor I'd talked to about it: because after four years of zealot-like religious use I got pregnant on the mini pill. Her reply was instead, "Oh, yeah... that is not uncommon." Wait. What? It isn't uncommon for women who use their mini pills by-the-book everyday to get pregnant regardless?
She explained to me that despite the instructions your health care provider gives you for the mini pill being practically identical to those given to you for the regular pill, how you should actually take them is quite different. Both brochures will instruct you to take your pill at the same time every day. What the mini pill instructions do not warn you about is that there is essentially a two hour long window after you have taken your pill that you may still conceive! So, if one day you take your pill then get frisky right afterwards, you run a considerable risk of getting pregnant despite your strict pill use.
No where within the pill's included patient literature is this mentioned. My primary care physician, who prescribed them, never explained this to me either. According to my stand-in GYN this is normal, most physicians don't seem to know about this very key difference and therefor don't warn anyone about it. If you do some research, the information is there, but why would you? You think you've been given all the relevant information. According to my GYN, while the mini pill claims an effectiveness similar to the regular pill, it actually has a similar fail rate to condoms.
That's a pretty huge difference. She says it isn't brought up much because most people have very vanilla at-night-only sex and take their pill in the morning. So the average person's risk is pretty minimal. However, if you lead a more adventurous sex life, or you know, don't plan your sex around the sun's position in the sky your risk is higher.
So, my friends, from me to you: remember to close that window before getting it on after your progesterone only pills!