Absence of Correlation Between Lunar Phases and Birth Rates: A Statistical Analysis of 135,000 Japanese Individuals

Background: The belief that the frequency of births increases during the full moon or new moon (the so-called "lunar effect") persists among the general public and some medical professionals, despite a lack of established medical evidence. This study statistically investigates whether lunar phases influence the timing of childbirth in Japan using a dataset of 135,748 birth records collected via the web service charapla.net between 2010 and 2017.Methods: Lunar age (0–29) at birth was calculated for each individual, and expected frequencies were derived via a calendar-based simulation reflecting astronomical cycle length. The goodness-of-fit was analyzed using the Chi-square test and effect size (Cohen’s w).Results: The distribution was nearly flat, with no increased births near the New Moon or Full Moon. Although a statistically significant deviation was detected (χ²(29) = 55.43, p = 0.0022), the effect size was negligible (w = 0.02).Conclusion: These results demonstrate that there is no substantial association between lunar phases and birth rates in the contemporary Japanese population. The findings challenge popular belief and reinforce evidence-based medical and social decision-making.