The most fertile time of the month is just before or at ovulation. Ovulation usually occurs two weeks before a period starts, so it is necessary to count backwards from the anticipated start of the next period in order to find the most fertile time. Take the number of days in the usual cycle (from the beginning of one period to the beginning of the next) and subtract 14. For example, a woman with a 32 day period would likely ovulate around day 18 (32-14=18), while a woman with a 28 day cycle would ovulate around day 14 (28-14=14).
Because the sperm live longer than the egg does, it is best to have intercourse before ovulation rather than afterwards, so a woman who ovulates on day 14 would have a good chance of conceiving if she has intercourse on either day 12, 13 or 14. For women with monthly menstrual cycles but with slightly irregular intervals (e.g. 27-33 day menstrual cycles), one way to decide on the potential fertile period is by taking the shortest cycle and subtracting 16, then taking the longest cycle and subtracting 12. This would give the fertile time and allow for even more fluctuation in cycle length than usual. For example, the fertile period for a woman whose cycles vary from 27 to 33 days should be sometime between day 11 (27-16=11) and 21 (33-12=21).
Alternatively, women may want to use an ovulation predictor kit, which can be downloaded as an app or purchased over the counter at drug stores. One important fact to note is that, if a woman is using a basal body temperature chart, that the temperature will rise after ovulation, and therefore after the most fertile period. The couple should therefore not wait until the temperature has risen to start to have intercourse, as they will have missed the most fertile time.