Why is the apparent alignment between the menstrual cycle and the lunar cycle generally considered coincidental rather than causal?

Human cycles are highly variable, ranging from 21 to 35 days

Although the term menstruation derives from the Greek word *mene*, related to the Moon, the current scientific understanding discounts a direct causal link between the lunar cycle and the human female reproductive cycle. This association arises because the average menstrual cycle length is roughly 29.5 days, closely mirroring the synodic lunar month of 29.53 days. However, the key differentiator is variability; human menstrual cycles are not uniform, commonly ranging between 21 and 35 days. If the Moon were the dictating factor, all cycles would adhere precisely to the 29.5-day standard. The documented high variability demonstrates that the alignment in timing is largely coincidental, not a mechanism driven by lunar gravity or light affecting critical hormones like progesterone or estrogen.