Which cycle tracker iphone app is most accurate at identifying the fertile window?

"Fertile window" ? : Ovulation occurs at 14 days prior to the onset of a menstrual event, but a simple basal body thermometer (BBT) is fairly accurate to determine the rise in temperature at the onset of ovulation. Fertility over a 4-5 day "window" is possible, since sperm lives for several days, and the egg is available for 2-3 days. The BBT is calibrated to read in tenths of a degree.