Characteristics of births in each cycle of the bimodal monthly distribution in cystic fibrosis.

The bimodal monthly birth distribution of patients with cystic fibrosis (CF) in Victoria, Australia, is shown to be similar within and among sibships, so that intrafamilial concentration of births around March (mode 1) or September (mode 2) cannot explain the observed periodic pattern. Mode 2 children are subject to a higher stillbirth rate and have younger maternal ages than mode 1 children. A sample of persons dying with CF in England and Wales displaying a similar bimodal birth pattern is shown to have a reduced infant survival rate for males of mode 2. These findings suggest that two annual cycles are a more appropriate model than a six-monthly cycle. An attempt is made to equate mode 1 in the northern hemisphere with mode 2 in the southern hemisphere by pointing out the high perinatal death rate in the former and the high stillbirth rate in the latter.