Abstract: A fertility determining system employing hormone level measuring apparatus on selected days in a menstrual cycle to determine the levels of analytes, i.e., estrone-3-glucuronide (“E3G”) and luteinizing hormone (“LH”), in early morning urine. The fertility/infertility signalling algorithm relies upon data characterizing the user earlier in that menstrual cycle, and also upon her analyte pattern in past recent cycles. At a point in a menstrual cycle which is user-dependent, the early morning E3G urine level is compared against the lower E3G level earlier in the cycle. The beginning of the fertile period is signalled when a sufficient E3G increase occurs. The end of the fertile period (and, correspondingly, the reestablishment of a period requiring no contraception) is signalled a period after the LH surge. In the absence of an LH surge in any subject, the end of the fertile phase is signalled a period of time following the peak of the E3G urinary metabolite of estradiol.