Table 1:

Percentages of boys and girls aged 6–12 years who received a diagnosis of attention-deficit/hyperactivity disorder or treatment for the condition from a cohort of 937 943 children, by birth month

	Boys			Girls

Month of birth	Children, no.	Received diagnosis, %	Received treatment, %	Children, no.	Received diagnosis, %	Received treatment, %
January	39 136	5.7	4.4	37 448	1.6	1.1

February	36 586	5.9	4.6	34 791	1.8	1.3

March	41 512	6.0	4.7	38 658	1.9	1.4

April	40 605	6.1	5.0	38 529	1.9	1.4

May	42 724	6.5	5.2	40 621	1.9	1.4

June	40 720	6.7	5.3	38 808	2.2	1.7

July	41 829	7.3	5.8	40 149	2.3	1.7

August	40 859	7.3	5.9	38 670	2.4	1.8

September	41 111	7.6	6.2	38 739	2.6	1.9

October	39 773	7.9	6.2	37 967	2.6	1.9

November	37 409	7.8	6.2	35 535	2.6	1.9

December	38 977	7.4	6.2	36 787	2.7	1.9

Overall	481 241	6.9	5.5	456 702	2.2	1.6

Trend test (p value)^*		−19.7 (< 0.0001)	−19.3 (< 0.0001)		−16.5 (< 0.0001)	−14.1 (< 0.0001)

RD (95% CI) Dec. v. Jan.^†		1.71 (1.36 to 2.05)	1.80 (1.48 to 2.11)		1.09 (0.88 to 1.30)	0.84 (0.66 to 1.01)

RR (95% CI) Dec. v. Jan.^†		1.30 (1.23 to 1.37)	1.41 (1.33 to 1.50)		1.70 (1.53 to 1.88)	1.77 (1.57 to 2.00)

Note: CI = confidence interval, RD = risk difference, RR = relative risk.
↵* Cochran–Armitage trend test with one-sided p value.
↵† Risk of receiving a diagnosis of or treatment for attention-deficit/hyperactivity disorder for children born in December compared with children born in January.