Bimodal month of birth distribution in cystic fibrosis.

The month of birth of 923 patients in England and Wales dying with cystic fibrosis as an underlying cause was found to be distributed independently of sex and year of birth during 1954-1976. When adjusted for the monthly variations in births in the national population, birth frequencies were bimodally distributed. This confirmed results obtained previously in the Netherlands and Australia. Peak births occurred in late February and August, regardless of whether a single six-monthly cycle or two annual cycles were assumed. It was not possible to choose the better of these models, each yielding similar parametric estimates.