I got pg with dd1 when I was 17, while using protection, after 4 months of not using it.
After she was born, I went on the pill, was on for 7 years, until I met DH and we got married. During those 7 years, I was one of those people who was just awful at taking it. Would frequently forget, never took it at the same time of day...really I probably should have gotten pg at some point during that time. But thankfully did not.
After I met DH and we got married, we started ttc right away, having no reason to think that we would have a problem. Well, we did, it took 6 years and an IVF to get pg with dd2. At the time of our IVF, we were told that it was basically our only option, that DH had antisperm antibodies and that my eggs were a little older and the "shells" were a little "thicker" than they should be (there was a lot of technical mumbo jumbo that I can't recall off the top of my head, but that was the jist of it.)
DD2 was born November 2008. I did not go back on the pill, and AF showed up at 8 wks post partum despite exclusively nursing. She wasn't regular at first, due to the nursing, but once we finally gave it up at 5 months, I was having regular cycles again. While we knew we wanted more, by that point, we knew that we just couldn't swing it right away. We were in a financial mess, I was stressed out and dealing with ppd, we were just not in a good spot. And besides, we assumed we needed an IVF anyway. So we weren't trying, but we weren't preventing.
I went on PPD meds, which did help, but had awful side effects. One of which was that it increased my prolactin levels, which inhibited ovulation. Great, birth control as a side effect! Well, because of the other side effects, I weaned myself off them in December. Between the antidepressants and the holidays and so on, DH and I weren't "together" much anyway.
Well, I wasn't sure when to expect AF, and when she didn't show up by mid January (last one had probably been around the first of December, last AD was around mid December) I decided to take a test. Super dark line, as soon as the pee touched the dye! It was like 1:30 in the morning, DH was in bed, I started shaking and ran upstairs and woke him up. All I could spit out was "Look! Look! Look!" First, he thought it was a thermometer and I was freaking out because the baby was sick. Once he realized what it was, he was like, uh, take another. So I did and it too came up darkly positive. We sat up for the next hour or two, in shock, jusk kinda repeating, I can't believe it. Oh My Gawd.
I called my dr office for a blood test to confirm, had the test the next day and got my numbers the day after that. Very positively pg. Got a u/s to date with my OB, I was already about 5 weeks or so along, with due date of 9/22! I am still, even at 30wks now, just AMAZED that we did it on our own. The best guess is that I ovulated right after I weaned off the ADs and the only time DH and I were together like that month worked!