Timing is crucial: Some critical thoughts on using LH tests to determine women's current fertility

Naturally cycling women reportedly go through a variety of psychological and behavioural changes over menstrual cycle. Evolutionary informed scholars have interpreted such changes as maximising reproductive success. However, concerns have been raised regarding this ovulatory shift hypothesis, since recent studies have yielded inconsistent findings. We suggest that the inconsistent findings regarding the ovulatory shift hypothesis may result from a too simplistic definition of the fertile window. Presently, most studies use LH tests to determine the fertile window. The problem with this "gold standard" is that it builds on the misconception that fertility peaks with ovulation and that ovulation regularly occurs 24 to 48 hours after the LH surge. While commercially available urinary LH test strips are a cheap and easy way to reliably detect LH surges, the LH surge itself marks the impending end of the fertile window. So if women are invited to the laboratory after the LH surge (as is often done for practical reasons) there is a high probability of misclassifying women as fertile when in fact the fertile window has already closed. We discuss possible advancements that may help to increase the accuracy and reliability of determining a woman's individual fertile window, during which any adaptive changes that increase the chance of reproduction should be best observable.