All in all, this typical woman has about 5 fertile days out of 28, and only 2-3 with a high chance of pregnancy. These days are days 12-16. But your actual mileage may vary. The reason women can get surprised by a pregnancy is because ovulation and menstruation aren't always so nicely linked in reality as in theory. An egg could be released at anytime.
Few women have perfectly regular cycles.
In addition, sometimes women mistake spotting for their periods, throwing off their estimations. Nevertheless, once the deed is done, this counting method is the best way to estimate one's risk of pregnancy. The 14 days to ovulation is fairly consistent among all women, even though the number of days from ovulation to menstruation varies considerably. Of course, it works better on women with regular cycles. In any case, if your period does not arrive on schedule this month, wait a few days before panicking.