RESUMEN
Cleft lip and palate are believed to be caused by multi factorial inheritance in which genes interact with environmental agents. Thus the main aims of this study were to find out seasonal variations in the birth date as well as the sex distribution in the birth of children with cleft lip and palate. In this retrospective study, the birth dates of 358 children born in Khorasan Razavi state from 1992 to 2007 with a diagnosis corresponding to cleft palate, or cleft lip with or without cleft palate, were obtained from Cleft Lip and Palate Research Unit in Mashhad Dental School. The data were analyzed by Chi -square test to look at the sex distribution and seasonal variation. The results showed the peak incidence of cleft palate birth in autumn. Cleft lip and palate showed a different seasonal trend, with the peak occurrence in the spring and autumn for males and in the winter for females. Additionally, this study showed the yearly rate for cleft lip, isolated cleft palate and cleft lip and palate decreased from 1992 to 2007. These seasonal trends may act as a pointer to the environmental factors active in the multi factorial etiology of cleft lip and cleft palate