Table 3.

Odds of Adverse Birth Outcomes in Second-Born Child as Estimated by Matched Sibling-Pair Analysis, According to Interpregnancy Interval Between First and Second Consecutive Live Births (n = 192,041), Western Australia, 1980–2010

Birth Outcome and Interpregnancy Interval, months	No. of Sibling Pairs	No. of Discordant Pairs in Analysis	Crude IOR		Adjusted IORa
			IOR	95% CI	IOR	95% CI
Preterm birth
<6	5,871	799	1.25	1.09, 1.41	1.22	1.06, 1.38
6–11	29,158	2,772	1.10	0.97, 1.21	1.08	0.98, 1.19
12–17	42,756	3,526	1.11	1.01, 1.21	1.10	1.00, 1.20
18–23	34,405	2,944	1.00	Referent	1.00	Referent
24–59	64,926	6,118	1.07	0.98, 1.16	1.09	1.00, 1.18
≥60	14,925	1,681	1.59	1.47, 1.71	1.73	1.58, 1.89
Small for gestational age
<6	5,871	934	0.98	0.83, 1.14	1.01	0.86, 1.16
6–11	29,158	3,773	0.99	0.90, 1.08	1.01	0.92, 1.11
12–17	42,756	5,376	1.03	0.95, 1.12	1.05	0.96, 1.13
18–23	34,405	4,460	1.00	Referent	1.00	Referent
24–59	64,926	9,190	1.04	0.96, 1.11	0.99	0.91, 1.07
≥60	14,925	2,517	1.46	1.36, 1.56	1.21	1.08, 1.34
Low birth weight
<6	5,871	599	1.00	0.81, 1.19	1.00	0.81, 1.29
6–11	29,158	1,971	1.10	0.97, 1.23	1.11	0.98, 1.24
12–17	42,756	2,471	1.06	0.93, 1.18	1.06	0.93, 1.18
18–23	34,405	2,019	1.00	Referent	1.00	Referent
24–59	64,926	4,653	1.02	0.91, 1.13	1.00	0.87, 1.11
≥60	14,925	1,334	1.62	1.47, 1.76	1.52	1.34, 1.70

Abbreviations: CI, confidence interval; IOR, interaction odds ratio.

^a Odds of second-born child being born with outcome under analysis compared with odds of first being born with same outcome; IOR and corresponding 95% CI adjusted for maternal age, socioeconomic status, and calendar year of birth, where an interval of 18–23 months was the reference category.