The first step in using any fertility calculator based on ovulation dates is to chart the dates of your last few menstrual periods, according to WebMD. Note the date that you began menstruating on a calendar, and mark it as day one.
If your menstrual cycle, or the length of time between the last day of one period to the first day of the next, is regular, you may only need the date that you started your last period to begin. However, charting several months is more accurate, WebMD explains. There are several methods of calculating ovulation dates using this information. To use the one provided on WebMD, enter the dates of your periods in the spaces provided and click Calculate. The calculator provides you with an estimate of your most fertile days for three months.
Another way to track ovulation is to measure the levels of leutinizing hormone, or LH, in your urine using a digital urine test. LH levels increase about 24 to 36 hours before ovulation, which allows you to accurately predict your most fertile days, explains Clearblue, the makers of the Clearblue digital ovulation test. To use the Clearblue calculator, fill in the date of the first day of your last period and your average monthly cycle length in the spaces provided on the webpage. The calculator then tells you when to begin testing using the Clearblue test strips. To ensure that your urine contains enough hormones to give an accurate result, do not empty your bladder for four hours before you test. Plan to have sex on the day the test is positive and the next to maximize your chances of getting pregnant.