Table 3

IRRs indicating yearly changes in the total number of CP cases on the total number of live births, by GA or BW categories and by singletons or multiples

SingletonsIRR95% CIModel Wald χ2 Cross-centre variability in slope?MultiplesIRR95% CIModel Wald χ2 Cross-centre variability in slope?
GA
 <32 weeksLinear yearly decrease0.96*0.95 to 0.98 χ2(1)=21.38*NoLinear yearly decrease0.95*0.93 to 0.98 χ2(1)=14.62*No
 32–36 weeksLinear yearly decrease0.96*0.94 to 0.98 χ2(1)=17.50*NoNo yearly change0.990.96 to 1.03 χ2(1)=0.10No
 37+ weeksNo yearly change0.990.96 to 1.02 χ2(1)=0.29YesNo yearly change0.980.94 to 1.02 χ2(1)=0.77No
BW
 <1500 gLinear yearly decrease0.96*0.95 to 0.98 χ2(1)=20.87*NoLinear yearly decrease0.95*0.93 to 0.97 χ2(1)=19.87*No
 1500–<2500 gLinear yearly decrease0.97*0.96 to 0.98 χ2(1)=18.88*NoNo yearly change0.990.96 to 1.01 χ2(1)=0.73No
 ≥2500 gNo yearly change0.980.95 to 1.01 χ2(1)=1.08YesNo yearly change0.960.92 to 1.00 χ2(1)=4.01No
  • We also reported whether the models indicated cross-centre variability in the rate of change (slope) of CP prevalence. IRRs were estimated using multilevel negative binomial regressions. Parameters of the models are introduced in online supplementary appendix 4 and full parameters reported in online supplementary appendices 5 and 6. Cross-centre variability in slope indicated that different centres displayed varying rates of change in the CP prevalence for that category of birth and GA or BW. Conversely, if no cross-centre variability was observed, the rate of change of CP prevalence was estimated to be consistent across centres. The lack of significant cross-centre variability in the slopes of CP prevalence for singletons and multiples in lower GA and BW categories indicated that the significant decrease of CP cases among these at-risk categories was consistent across centres.

  • *P<0.005.

  • BW, birth weight; CP, cerebral palsy; GA, gestation age; IRR, incidence rate ratios.