Importance of Daily Sex Hormone Measurements Within the Menstrual Cycle for Fertility Estimates in Cyclical Shifts Studies

Recent discussions have highlighted the importance of fertility measurements for the study of peri-ovulatory shifts in women's mating psychology and mating-related behaviors. Participants in such studies typically attend at least two test sessions, one of which is, at least in theory, scheduled to occur during the high-fertility, peri-ovulatory phase of the menstrual cycle. A crucial part of this debate is whether luteinizing hormone (LH) tests alone are sufficient to accurately assign test sessions to the peri-ovulatory phase. This article adds to this ongoing debate by presenting analyses of a detailed database of daily estradiol levels and LH tests for 102 menstrual cycles. Based on more than 4,000 hormonal measurements, it is clear that individual differences in length of the cycle, length of the luteal phase and, perhaps most importantly, the discrepancy between the timing of the LH surge and the drop in estradiol that follows it are pronounced. Less than 40% of analyzed cycles followed the textbook pattern commonly assumed to occur in fertility-based research, in which the LH surge is assumed to occur not more than 48 hr before the estradiol drop. These results suggest that LH tests alone are not sufficient to assign test sessions to the peri-ovulatory phase and that analyses of sex hormones are essential to identify whether the participant was tested during the peri-ovulatory phase.