MONTHLY AND SEASONAL-VARIATIONS IN THE FREQUENCY OF CONGENITAL-ANOMALIES

Castilla E E (Instituo Multidisciplinario de Biologica Celular; IMBICE, La Plata, Argentina), Orioli I M, Lugarinho R, Dutra G P, Lopez-Camelo J S, Campana H E, Spagnolo A and Mastroiacovo P. Monthly and seasonal variations in the frequency of congenital anomalies. International Journal of Epidemiology 1990, 19: 399-404Three large and comparable series of births were used to test the working hypothesis that if there is a real seasonal variation in the frequency of a given congenital malformation; it would have to be shown by adequate analysis; to be more overt in non-tropical areas; and to be six months out of phase in northern and southern hemispheres. The date set were hospital births from tropical (287 165 births) and non-tropical (582 585 births) South America, and from Italy (508 536 births).Sixteen well-defined malformation types were tested: anencephaly, spina bifida, cephalocoele, hydrocephaly, microtia, cleft palate, cleft lip, oesophageal atresia, anal atresia, hypospadias, pes equino-varus, pes talovalgus, postaxial polydactyly, pre-axial polydactyly, diaphragmatic hernia, and Down's syndrome. No seasonal variation was proven (p<0.01)for any malformation type in any of the three series of data by means of Walter and Elwood's1 test, or Hewitt et al's non-parametric test2 applied to seven instances with sample sizes smaller than 50 cases.Variations of borderline significance (p<0. 05) included oesophageal atresia in tropical South America, none in non-tropical South America, and anencephaly in Italy. It is concluded that seasonal variation in the occurrence of congenital malformations is a rare phenomenon when tests are strictly used within their recommended limitations. © 1990 International Epidemiological Association.