Although a seasonal trend in the birth distribution has been reported in cystic fibrosis (CF), this finding is still very controversial. The birth distribution of 113 patients with cystic fibrosis born in Saguenay-Lac-St-Jean (complete ascertainment) was analysed using two different statistical methods. Our results showed no monthly or seasonal birth variation. It is likely that a bias owing to incomplete ascertainment might explain why some previous studies found a seasonal trend in the birth distribution of cystic fibrosis. Saguenay-Lac-St-Jean (SLSJ) is a geographically isolated region located 125 miles north-east of Quebec city that was opened to white settlement around 1840. Its population, 98% French speaking and of Catholic faith, rose from 5000 inhabitants in 1852 to 50 000 in 1911 and 285 000 today. Several autosomal dominant and recessive disorders have a high prevalence (myotonic dystrophy, cystic fibrosis, etc) while others, frequently found in the SLSJ region and Charlevoix, are almost non-existent elsewhere (spastic ataxia Charlevoix-Saguenay type, polyneuropathy with or without agenesis of the corpus callosum, etc).' Over the past 20 years, monthly or seasonal birth variation has been reported in various chromosomal and congenital anomalies, such as Down's syndrome, Klinefelter's syndrome, and anencephaly. A seasonal trend has also been suggested in cystic fibrosis (CF) by several workers.2-5 However, the Human Genetics, Department of Human Sciences, Universitk du Qu6bec A Chicoutimi, 555 Boulevard de l'Universit6, Chicoutimi, Quebec G7H 2B1, Canada. J Daigneault, M De Braekeleer Hopital de Chicoutimi, Chicoutimi, Quebec, Canada. G Aubin, F Simard Correspondence to Dr De Braekeleer. issue remains controversial since two studies failed to show any uneven monthly birth distribution.67 This study was aimed at analysing the monthly birth distribution of patients with cystic fibrosis born in Saguenay-Lac-St-Jean (SLSJ). Material and methods The great majority of the subjects affected with CF living in SLSJ have been followed at the CF clinic in Chicoutimi since spring 1973, the date of its opening. Of a total of 127 CF patients known in the SLSJ region on 31 December 1989, 125 patients had been or were still followed at the CF clinic in Chicoutimi. The diagnosis of cystic fibrosis was based on the patient and family histories, clinical, radiological, and biochemical results. All CF cases were confirmed by at least a positive sweat test. For the present study, only the 111 CF subjects born in SLSJ were considered. Statistical significance was sought using two methods. The total observed number of patients for each month was compared to the expected number and a X2 test performed. The expected rates were calculated using all live births in the area during the range of years of birth of the 111 patients (1954 to 1988). For each month, the expected number of patients was calculated using the formula of Nielsen
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