TABLE 3.

Association Between IPI and ASD in Second-Born Children After Firstborn Full Siblings Who Had a Diagnosis of ASD: Kaiser Permanente Northern California births 2000 to 2009.

IPI, months	ASD (n = 111) n (%)	No ASD (n = 767) n (%)	HR (95% CI)
			Crude	Adjusted
<24	65 (58.6)	392 (51.1)	1.4 (1.0–2.1)	1.5 (1.0–2.2)
24 to 71	40 (36.0)	353 (46.0)	Reference	Reference
≥72	6 (5.4)	22 (2.9)	2.2 (1.0–5.2)	1.7 (0.7–3.9)

Values are expressed as n (%). HR adjusted for maternal age, paternal age, and child gender.