Clearblue advanced fertility refill tests are suitable for women whose natural cycle normally lasts between 21 and 42 days the majority of women will need 10 fertility test sticks for each cycle however women with irregular or long cycles may have to use 20 test sticks each cycle in order to detect their most fertile time both the fertility and pregnancy tests in this pack are for use with the clear blue advanced fertility monitor and are 99 accurate in detecting the lh surge for fertility