- BACKGROUND: The aim of this study was to assess effects of natural and artificial seasonality on reproduction in a large cohort of Muslim and Jewish women, living in the same geographic area. METHODS: Population included all births between January I988 and December 2005 (66I3 daily records of 200 009 births) at the Soroka University Medical Center (southern Israel). A 'classical' time series technique based on generalized linear regression models was used. RESULTS: A systematic increase in the number of births during Ramadan was observed in the Muslim (P < 0.00I). but not in the Jewish, population. This 'Ramadan effect', adjusted for trend and seasonal factors, was statistically significant only in Muslim multiparae (P < 0.00I). where the model explained more than 48% of the variance, compared with 7% in Jewish multiparae. Seasonal birth variations in Muslim primiparae were not associated with Ramadan and were similar to those in Jewish primiparae. There were no differences in length of gestation associated with Ramadan. CONCLUSIONS: The findings suggest an increased conception rate following the Hajj pilgrimage, which support the hypothesis that seasonality of reproductive behavior is influenced by socio-cultural factors more than by geographic and climatic factors, and can help plan preconception interventions in Muslim populations in southern Israel.