Table 4.

Co-sibling analyses of gestational age at birth (1973–2013) and risk of ASD (1987–2015), Sweden.

	Cases	Prev.a	Adjustedb
	(n)	(%)	PR (95% CI)	P
	Total cohort
Preterm (<37 wks)	3,357	2.1	1.32 (1.29, 1.36)	<0.001
Extremely preterm (22–27 wks)	233	6.3	3.25 (2.92, 3.61)	<0.001
Very to moderate preterm (28–33 wks)	814	2.7	1.53 (1.45, 1.61)	<0.001
Late preterm (34–36 wks)	2,310	1.9	1.20 (1.16, 1.24)	<0.001
Early term (37–38 wks)	9,815	1.6	1.10 (1.08, 1.12)	<0.001
Full-term (39–41 wks)	32,942	1.4	Reference
Post-term (≥42 wks)	4,070	1.5	1.07 (1.04, 1.09)	<0.001
Per additional week			0.96 (0.96, 0.96)	<0.001
Males
Preterm (<37 wks)	2,287	2.7	1.31 (1.26, 1.35)	<0.001
Extremely preterm (22–27 wks)	165	8.5	3.36 (2.96, 3.82)	<0.001
Very to moderate preterm (28–33 wks)	546	3.2	1.47 (1.37, 1.57)	<0.001
Late preterm (34–36 wks)	1,576	2.4	1.18 (1.14, 1.23)	<0.001
Early term (37–38 wks)	6,600	2.1	1.10 (1.07, 1.12)	<0.001
Full-term (39–41 wks)	22,202	1.8	Reference
Post-term (≥42 wks)	2,843	1.9	1.05 (1.02, 1.08)	0.003
Per additional week			0.96 (0.96, 0.97)	<0.001
Females
Preterm (<37 wks)	1,070	1.5	1.51 (1.43, 1.59)	<0.001
Extremely preterm (22–27 wks)	68	3.8	4.03 (3.32, 4.88)	<0.001
Very to moderate preterm (28–33 wks)	268	2.0	1.88 (1.69, 2.08)	<0.001
Late preterm (34–36 wks)	734	1.3	1.32 (1.24, 1.41)	<0.001
Early term (37–38 wks)	3,215	1.1	1.15 (1.11, 1.19)	<0.001
Full-term (39–41 wks)	10,740	0.9	Reference
Post-term (≥42 wks)	1,227	1.0	1.11 (1.06, 1.17)	<0.001
Per additional week			0.94 (0.94, 0.95)	<0.001

^aAdjusted for shared familial (genetic and/or environmental) factors, and additionally for birth year, sex, birth order, maternal factors (age, education, BMI, preeclampsia, other hypertensive disorders, diabetes), paternal factors (age, education), and family history of ASD.