Is there a best time of the month for a woman to run a race? It depends.
While some female runners don’t notice differences in their races between phases of the menstrual cycle, others do. In general, the best endurance performances occur when estrogen is the dominant hormone (end of week 1 and all of week 2) and worst when progesterone is the dominant hormone (week 3 and week 4) and during the few days prior to and during the period.
Racing may also be better during the middle part of the luteal phase (a week after ovulation), when estrogen is at its secondary peak. However, at that time, progesterone, which negatively affects body temperature, hydration, and breathing, is also peaking, so racing, like training, may only be better when the increase in estrogen is high relative to the increase in progesterone (i.e., the ratio of estrogen to progesterone is high).
Two of the most powerful running-related effects of estrogen are that it promotes a greater use of fat for energy (which spares muscles’ limited store of glycogen) and stimulates a greater storage of muscle glycogen, both of which are important for long races, like half-marathons, marathons, and ultramarathons. To exploit this unique female metabolic characteristic, long races, if possible, should be planned in the late-follicular phase of the menstrual cycle.
Therefore, if the date of a planned marathon (especially in the heat) falls during the luteal phase, when progesterone is high, it’s important to focus on keeping cool and staying hydrated. Given the pros of estrogen and the cons of progesterone, for racing to be improved during the mid-luteal phase, the effects of the rising estrogen would need to outweigh the effects of the rising progesterone. For races that have a large anaerobic component (5K and shorter), there is no evidence or expectation that the phase of the menstrual cycle affects racing performance.