The belief that the moon influences fertility draws on the apparent synchronization between the lunar cycle (approximately 29.5 days) and the average menstrual cycle (approximately 28 days). Across cultures, this perceived alignment led to the idea that moon phases could affect ovulation, conception timing, and even the gender or personality of a child. Full moon conceptions are often associated with boys, new moon conceptions with girls, and certain phases are thought to increase fertility or ease childbirth.

This belief system used the moon as a natural and visible calendar for reproductive planning. Before the development of scientific gynecology or modern tracking methods, lunar observations offered a structured way to interpret and predict biological rhythms. The symbolism of the moon—its cycles of waxing, fullness, and waning—was often connected with the processes of fertility, pregnancy, and motherhood in mythology and medicine.