Quantitative Hormone Analysis Reveals Sources of Variability in the Menstrual Cycle

Although the fluctuations of hormones over the menstrual cycle are well recognized, this study investigated the variability in these patterns for individual women. This study examined a set of daily blood hormone results from an underlying data set of previous research. Hormones included follicle-stimulating hormone (FSH), luteinizing hormone, progesterone, and estradiol taken over one menstrual cycle in 20 women of reproductive age (age 20–36 years), with ultrasound-confirmed ovulation. Although every woman’s profile of hormone changes was consistent with the expected physiological sequence of events, there was notable variability in the timing and peak levels. Variability in the length of the follicular phase was greater than in the length of the luteal phase, with the greater part of the variability in the cycle length being explained by variability in the follicular phase. Lower levels of FSH at the beginning of the cycle were associated with a longer follicular phase. Variability in the timing of events around ovulation was relatively consistent across all women. Variability in the length of the luteal phase was associated with the duration of elevated levels of progesterone. Differences were seen in the extent to which FSH increased at the end of the cycle. This study suggests the variability of the first part of the follicular phase may be due to initial FSH changes. Understanding the variability of menstrual cycle hormones is relevant for all women. Identifying subclinical ovulatory disturbances and variability is important to understand female health, across different populations including menstruating women and female athletes.