Table 3. Risks of Adverse Birth Outcomes in Matched-Sibling Analysis by Interpregnancy Interval, Guangdong Province From 2014 to 2020.

Birth outcome and interpregnancy interval, mo	No. of sibling pairs	No. of discordant sibling pairs in analysisa	IOR (95% CI)
			Crudeb	Adjustedc
Preterm birth
<6	49 485	4450	1.43 (1.33-1.54)	1.40 (1.30-1.51)
6-11	174 675	11 872	1.30 (1.22-1.37)	1.27 (1.20-1.34)
12-17	181 172	11 677	1.11 (1.05-1.17)	1.10 (1.04-1.17)
18-23	122 866	7940	1 [Reference]	1 [Reference]
24-29	86 842	5823	1.00 (0.93-1.07)	1.00 (0.94-1.07)
30-35	55 364	3856	0.95 (0.88-1.03)	0.95 (0.87-1.02)
≥36	54 988	3992	1.12 (1.04-1.21)	1.10 (1.02-1.19)
Low birth weight
<6	49 485	4780	1.32 (1.23-1.42)	1.30 (1.21-1.40)
6-11	174 675	12 792	1.29 (1.22-1.37)	1.27 (1.20-1.35)
12-17	181 172	12 171	1.12 (1.06-1.18)	1.11 (1.05-1.18)
18-23	122 866	8339	1 [Reference]	1 [Reference]
24-29	86 842	5955	0.95 (0.89-1.01)	0.95 (0.89-1.02)
30-35	55 364	3743	0.98 (0.90-1.06)	0.98 (0.90-1.06)
≥36	549 88	3969	1.17 (1.09-1.27)	1.16 (1.07-1.26)
Small for gestational age
<6	49 444	11 143	1.17 (1.11-1.22)	1.16 (1.11-1.22)
6-11	174 611	35 835	1.20 (1.16-1.24)	1.19 (1.15-1.24)
12-17	181 121	34 971	1.06 (1.03-1.10)	1.05 (1.02-1.09)
18-23	122 844	22 455	1 [Reference]	1 [Reference]
24-29	86 829	15 333	0.95 (0.91-1.00)	0.95 (0.91-0.99)
30-35	55 350	9448	0.97 (0.93-1.02)	0.98 (0.93-1.03)
≥36	54 983	9427	0.90 (0.86-0.95)	0.90 (0.86-0.95)

Abbreviation: IOR, interaction odds ratio.

^a Sibling pairs with only 1 birth resulting in an adverse outcome under analysis.

^b Odds of second-born sibling being delivered with adverse outcome under analysis vs odds of first-born sibling being delivered with same adverse outcome.

^c Adjusted for maternal age, mode of delivery, neonate’s sex, and season of birth.