Once you get the hang of it, NFP is actually very easy to use with a normal 28-day cycle. However, my desire to practice NFP was due to the fact that I had tiny babies(three months and six months, respectively) and felt that my body wasn't ready to sustain another pregnancy yet. Most women I talk to have some kind of hormonal variable(breastfeeding being the most common) which affects the length and predictability of their cycles, making it more difficult to track ovulation. Jack had difficulty breastfeeding, so for six months I used a breast pump and fed him by bottle. I ovulated when he was three months old and it took another five months after that for me to have normal cycles. While I was still pumping, my cycles varied in length; one was 62 days long, with ovulation occurring on day 51. Fun times. Here's why:
Prolactin is a hormone produced by breastfeeding. It keeps estrogen levels low. The rise and fall of estrogen levels is what dictates menstrual cycles. Some women I know nurse on-demand, co-sleep, and otherwise do everything you're "supposed" to do to keep prolactin levels high and naturally prolong nursing-induced infertility, and lots of them get pregnant. Don't assume you're infertile just because you're breastfeeding. With Jack, I was pumping and not producing enough prolactin to ward off my cycle. Matteas was a champion nurser and I still got my cycle back when he was 6 months old. You never can tell.
There are lots of different philosophies on the most effective way to avoid pregnancy. I would like to state that the following information is what I found to be most effective for me. What works for you may be something different; if you have other tricks up your sleeve, by all means, share!
In a nutshell, here's my NFP routine: abstain from sex beginning on Day 1 of my cycle until three days after I ovulate. During a normal cycle, this works out to 17 consecutive days of abstinence and 11 days of being "in the clear(able to have sex without getting pregnant)." The keen observer may note that the ratio is weighted in favor of NOT having sex, to which I can only respond: quality, not quantity. Moving on...
On Having Sex Prior to Ovulation
The short answer is: don't do it. Certain methods(like Billings) of NFP claim that, when certain indicators are present, you can have sex prior to ovulation without getting pregnant. The reasoning behind this is that sperm can only survive for a few hours unless conditions are sperm-friendly. Sperm need an alkaline environment to survive, and the pH of a woman's vagina is typically very acidic. Vaginal pH shifts to alkaline prior to and during ovulation to allow sperm to survive for as long as possible. Theoretically, vaginal pH is too acidic for sperm to survive for the first 2-3 days immediately following menstruation. The theory is that even if you have sex on these days(which would correspond to approximately days 7-9 of your cycle) the sperm will not survive long enough to reach the uterus. In my experience, this is not the case. I know several women(myself included) who have become pregnant while using NFP: in every case, pregnancy resulted from having sex prior to ovulation.
On Having Sex During Menstruation
Again, the short answer is: don't do it.
Blood is very alkaline, making it possible to get pregnant if you have sex during your period. Sperm can survive just fine in blood, and once they reach a fallopian tube vaginal pH is no longer a factor. Further, prostaglandins in semen can trigger early ovulation. I know this because of Matteas; he is what we call our Six Day baby. Here comes too much information: we had sex on Day 5 of my cycle, while light spotting was still present. Six days later I had an appointment with my OB/GYN for a regular checkup. Upon examining me, she told me I was ovulating. That moment. On Day 11 of my cycle. I suspected as much, but I always appreciate professional confirmation. This tells me two things: sex during menstruation can indeed trigger early ovulation, and; sperm can survive just fine for at least 6 days.
So the bottom line is, for the greatest possible accuracy maintain complete abstinence until after ovulation.
Which brings me to my next topic: how do you know when you've ovulated?