We studied the pattern of annual cyclical variation in month of birth for people who were born in Denmark and in whom Crohn's disease was diagnosed at age 20 years or under. We identified all cases of Crohn's disease in the Danish hospital discharge registry, which was established in 1977 and which has almost complete coverage of admissions to hospital throughout Denmark. From 1977 to the end of 1992, diseases were classified according to the international classification of diseases, eighth revision (ICD-8) (code for Crohn's disease 563.01). We obtained information on the number of births per month from 1957 to 1992 from the Danish population registry.
To study cyclical variation we used periodic regression in which the underlying regression equation has a sinusoidal form, and we fitted a sine curve to the observed cases per birth month. We adjusted the analysis for variation in the number of births by month. The calculation of the 95% confidence interval for the peak-trough ratio was based on large sample theory in non-linear regression models.
We identified 627 cases of Crohn's disease from 1 January 1977 to 31 December 1992 in people aged less than 21. The figure shows the seasonal pattern by month of birth. From the fitted curve in the figure we observed that the peak in births occurred in August and the trough in March, with a ratio of 1.30 (95% confidence interval 1.04 to 1.55). To assess the goodness of fit of the regression model, we compared the observed number of cases according to month of birth with the expected number using a χ2 test (χ2=5.6, df=9).