Variation of haemoglobin concentration during the menstrual cycle: a simple view of the evidence

The variation of haemoglobin (Hb) concentration during the menstrual cycle has been studied often, but the available data are inconsistent. We analyse data reported in 41 studies published between 1948 and 2021. A number of others are informative, but lack some details and are considered only in passing. A statistically significant change in Hb concentration was reported in 20 of the 41 studies (49% ), but the effect size (Cohen’s d) ranged from 0.18 (very small) to 3.2 (huge). Despite this, in 10 of these 20 studies the absolute menstrual cycle-related change reported was smaller than the average diurnal variation in Hb concentration (= 4.9%). Two particular factors distinguish these 20 studies from the 21 studies in which no statistically significant effect was observed. First, the assessment of menstrual cycle phase was based on calendrical considerations rather than being based on or confirmed by measurements of reproductive hormones. Second, while few of these studies involved large numbers of subjects, those in which a statistically significant result was reported tended to involve more subjects. While the available data do not demonstrate a systematic cycle-related variation in Hb concentration, it is possible that there is a small effect and those changes may be clinically significant for women who have a low Hb concentration. To demonstrate any cycle-related variation the covariates should be more tightly controlled and consideration should be given to the number of subjects required and to the statistical analysis