The fertility window is the time in which egg fertilization by sperm can occur, and this includes only 6 days in a menstrual cycle; the day of ovulation and 5 days prior to that.
The easiest way to determine the fertility window is the calendar method. First, you have to measure the time between periods, the day first bleeding occurs. For example, the day of your first period is day 1, than wait until the next period starts, and this is the last day of your menstrual cycle; the length of the menstrual cycle is between 24 to 35 days; if you measure less than 24 or more than 35 days (more than two consecutive cycles), please consult your Doctor. Second, to determine the day of ovulation, subtract 14 days from the length of your cycle (i.e., if your menstrual cycle is 30 days, subtracting 14 will result in ovulation at day 16 from first bleeding); Third, subtract at additional 5 days from the result (i.e., if 16 days is the day of ovulation, 16 minus 5 is day 11). Your fertility window includes the five days prior to ovulation, in our example, it is day 11 to day 16 after first bleeding.
However, if the length of your menstrual cycle changes between cycles, or if your menstrual cycle is not regular, a more accurate method is required. In addition, please be aware that about 3% of women suffer from luteal phase dysfunction, and in this case, medical intervention is required. Accurate prediction of day of ovulation is required is all of the aforementioned cases. This include measurement of basal body temperature between periods, determination of the time at which your cervical fluid becomes more stretchy (occurs around ovulation), and the use of a an ovulation test which determines hormones levels indicative of ovulation; the challenge with ovulation test is that it needs to be done during ovulation (24 hours in your cycle), and this it is usually too late to be used to get pregnant at the measurement time. Thus ovulation tests are used to determine the time within the menstrual cycle in which you ovulate. Most over the counter ovulation tests guide you in the process of determining basal body temperature, cervical fluid and ovulation test. After you have determined the day of ovulation with ovulation tests, your fertility window is the same as described above, that is, the day of ovulation and 5 days prior to that.