Table 2

Risk of first surgical intervention by age 3 years, before, during and after introduction of enhanced developmental dysplasia of the hip detection comparing intervention and non-intervention areas

		Before	During	After
Non-intervention area	Births	212 445	121 991	264 109
	Surgery	277	144	348
	Risk of surgery per 1000 live births	1.31 (1.16 to 1.46)	1.19 (1.00 to 1.39)	1.32 (1.18 to 1.46)
	Relative risk	1	0.91 (0.74 to 1.11)	1.01 (0.86 to 1.18)
Intervention area	Births	61 314	36 129	81 387
	Surgery	69	37	43
	Risk of surgery per 1000 live births	1.13 (0.88 to 1.42)	1.04 (0.73 to 1.40)	0.54 (0.39 to 0.70)
	Relative risk	1	0.91 (0.60 to 1.35)	0.47 (0.32 to 0.68)
Difference	Relative risk interaction	–	1.01 (0.64 to 1.56)	0.46 (0.31 to 0.70)

Data shown are counts for births and surgery, risks per 1000 live births and relative risks and interaction relative risks derived from logistic regression models (assuming that, since the event rate is very low, the ORs are estimates of the relative risks, ie, risk ratios) with the main effects and an area/period interaction. Numbers in parentheses are 95% CIs.