Schematic depiction of a survival analysis model for cerebral palsy risk. Schematic depiction of a survival analysis model for cerebral palsy risk with post-menstrual age on the X-axis. Two arbitrary periods of risk are shown. In the first risk period, there is one case of cerebral palsy, 3 live births and 15 fetuses at risk, while in the second risk period there is one case of cerebral palsy, 6 live births and 8 fetuses at risk. Under the conventional formulation of gestational age-specific cerebral palsy, this implies a cerebral palsy rate of 1/3 in the first risk period and 1/6 in the second risk period. Under the fetuses-at-risk formulation, the rates of cerebral palsy are 1/15 and 1/8 in the first and second risk periods, respectively.