Table 1.

λ_S and sib-sib OR (with 95% CI) for preterm birth.

	All races	White	Black
Population: preterm birth	9759	6232	3354
Population: N	268103	220728	42899
Population risk	0.036 (0.035–0.037)	0.028 (0.027–0.029)	0.078 (0.075–0.081)
Siblings: sibpairs with both siblings affected	1020	514	489
Siblings: sibpairs with first sibling affected	6522	4181	2210
Sibling risk	0.156 (0.147–0.165)	0.123 (0.113–0.133)	0.221 (0.204–0.238)
λS	4.3 (4.0–4.6)	4.4 (4.0–4.7)	2.8 (2.6–3.1)
Sib-sib unadjusted OR (95% CI)	5.6 (5.2–6.0)	5.7 (5.2–6.3)	3.6 (3.2–4.0)
Sib-sib adjusted OR (95% CI)#	4.2 (3.9–4.5)	5.1 (4.6–5.7)	3.3 (2.9–3.7)

#Adjusted for mother < 20 years old, mother < 12 education, Medicaid (index of low SES), no prenatal care, mother BMI < 20 kg/m², cigarette smoking