Why is my period late after a half marathon?

Let’s discuss what we classify as an abnormal menstrual cycle, and also why someone's period would be late after a half or full marathon.

Menstrual Cycle Irregularity 

Firstly, some menstrual cycle irregularity is normal, or rather, doesn't necessarily represent issues with our reproductive system. Consequently, we usually only recommend investigation if someone has had menstrual cycle abnormalities three cycles in a row. That is, if someone's period was late three cycles in a row or absent for three months, medical review would be required.

When a menstrual cycle is late, it's due to the follicular phase being prolonged rather than the luteal phase. A quick reminder: the follicular phase is the time from when the period starts until ovulation, and the luteal phase is from ovulation until the next period starts. The luteal phase is almost always two weeks or less, as this phase follows ovulation and progesterone can only be produced for ten to twelve days if the egg hasn't been fertilised. It's the drop in progesterone that results in the next period starting.

The follicular phase, however, is when the small follicles are developing due to stimulation from our pituitary hormones, i.e. LH and FSH. If these are decreased or abnormal, such as in functional hypothalamic amenorrhoea or polycystic ovarian syndrome, follicle development may be prolonged, resulting in a longer menstrual cycle.

Common causes of a late period

In a case where there is one late period post a race, it's likely it wasn't the marathon per se that caused the late period, but rather the training leading up to the race. If a period is late, it can't be attributed to something done in the week or two prior, as this corresponds to the luteal phase, which lasts approximately two weeks and cannot be prolonged. Consequently, the delay would generally stem from the events leading up to the half or full marathon: increased training volume, potentially some minor low energy availability, and perhaps some psychological stress associated with the race. This may have resulted in a degree of functional amenorrhoea, i.e., decreased LH from the pituitary, which may normalise by the next cycle, particularly if the athlete has decreased their training volume and restored their energy availability.

Medical review

For an athlete in this case, provided their ongoing menstrual cycle returns to normal (which is generally 21 to 35 days, but for most people it's around 26 to 30 days, give or take a few days each cycle), I would not recommend medical review currently.

Other signs the athlete may have had low energy availability include increasing fatigue, minor illnesses, poor sleep, and general lethargy. Focusing on good recovery and adequate nutrition can not only improve performance, but also help keep menstrual cycles regular during a high-volume training block.