Thank you! :)
You're probably right about the CM. I realise that it would have been more reliable if I'd been able to observe that a bit better, but my problem was that I always started having CM from a couple of days after my period finished and I could never quite tell the difference between the different sorts of CM. In hindsight, (and it does seem quite obvious now), I probably should have realised that I'm quite fertile and should have been more careful during the first half of the cycle, but I guess one is always wiser in retrospect...
On the other hand, I don't think my husband would have been very impressed if I'd told him that we should be using condoms for the whole of the first part of the cycle
Congratulations on your little miracle, JuliMummy!
Out of more curiosity, what was your earliest day of temperature rise? If you have 6-12 cycles experience of temping, and CD 18 or even CD 17 was your first day of temperature rise, then this was indeed an NFP failure by a certain set of rules. Dr. Doering's temperature-only rule is first day of temperature rise minus 7 is the last "safe" day. You need at least 6 cycles of experience for this (which it sounds like you had), though the effectiveness can be increased by having 12 cycles.
By the sympto-thermal method (more effective than temperature-only), you would only use this rule if you were dry on a particular day and all the days leading up to it. By this more conservative set of rules, it is not a method failure. However, according to TAONFP 4th Edition, if you have a history of long cycles with all-the-time mucus and consistently have at least 5 (better to have 6) days of more-fertile mucus before your temperature rise, then you can use the Doering rule in the presence of less-fertile mucus, but do not ignore more-fertile type mucus. I am curious as to whether or not you met the requirements for this STM rule since it is supposed to be very highly effective.
It is important to note that there are many ways to use NFP, and not all methods are equally effective, and no method is 100% effective besides strict abstinence. You can't really say that NFP isn't as effective just because some sets of rules that you might choose are not as effective as some other methods of family planning because there are some sets of rules that are just as effective as the pill. Nor can you say that somebody didn't have an NFP method failure just because they were not following the set of rules that you were taught. There are many, many ways to practice NFP, and I really do encourage all of you to choose sets of rules that meet the effectiveness that you need for your own situation. Not everybody needs a method that is over 99% effective.
Thank you, JMJ. My earliest ovulation was day 17, so that means temperature rise on day 18. Usually ovulation was on day 19, so we used protection from day 12 onwards. So yes, I agree, using the temperature only method you could call it a method failure. But in hindsight I probably should have paid more attention to CM.