Seasonal incidence of cleft lips and cleft palates in Sweden, 1965--1974.

Seasonal trends of infants with cleft lips and palates born in Sweden during a ten-year period were investigated. The study is based on reports to the Swedish Register of Congenital Malformations supplemented with hospital data. Three different statistical methods were used: chi2 for heterogeneity between months, Edwards' method, and a squared sinus function technique. The tests were performed on birth date and date of last menstrual period (LMP) with and without correction for fluctuation in monthly birth rate. Statistical significant seasonality was found both for cleft lips with or without cleft palate (CLP) and for isolated cleft palate (CP) but only for LMP data. For CLP. a peak was found in March and for CP, in April.