|First, determine the length of your normal cycle. It could be anywhere from 20 to 45 days long. Start counting on day one of your period (the first day of bleeding or spotting) and stop counting on the first day of your next period. This is the length of your cycle.
If the length of your cycle is different each month by more than just a few days, then simply take the average number of days over the last three months.
Once you determine the length of your cycle or an average number of days, you can use the chart below to help determine the day you should begin testing. On the top row of the chart, find the number that corresponds to the length of your cycle. Directly below that number is a smaller number. This is the day of your cycle when you should begin testing.
Here's an example: Ms. Smith has a regular cycle of 28 days and started her period on the 4th day. Beginning with the 4th as Day one, she counts forward 11 days and begins testing on the 14th.