Prediction and detection of the fertile period: the markers

The occurrence and duration of the fertile period in women are strictly related to the time of ovulation. Since the only positive confirmation of ovulation is the identification of an ovum in the female reproductive tract or the subsequent detection of a pregnancy, the prediction and detection of its occurrence have to be based on markers or indicators that lie at varying physiological distances from ovulation itself. These may be variations in hormones and other substances that can be detected in different body fluids or the evidence of their effects on specific target organs. Recent advances in the knowledge of reproductive physiology have allowed the identification of distinct substances and biological phenomena that accompany the occurrence of the fertile period. This article is intended to update and classify the available fertility markers based on their particular nature and modality of expression and, additionally, consider the temporal relationship between the appearance of their specific signals and the time of ovulation. Consequently, those indicators directly related to changes at the ovarian level were defined as direct markers, including ovarian morphology, the reproductive hormones, and the intraovarian regulatory proteins, whereas those reflecting variations observed in different target organs were considered indirect markers, and were further qualified as biochemical, biophysical, and clinical. Subsequently, fertility markers were classified as prospective, immediate, or retrospective, depending on whether they allow the prediction, detection, or confirmation of the ovulatory event, respectively.