Table 2. Risks of Adverse Birth Outcomes in Unmatched Cohort Analysis by Interpregnancy Interval, Guangdong Province From 2014 to 2020.

Birth outcome and interpregnancy interval, mo	No. of sibling pairs	Second-born siblings with outcome, No. (%)	OR (95% CI)
			Crudea	Adjustedb
Preterm birth
<6	49 485	3135 (6.3)	1.71 (1.63-1.79)	1.96 (1.87-2.06)
6-11	174 675	7801 (4.5)	1.18 (1.14-1.23)	1.32 (1.27-1.37)
12-17	181 172	7116 (3.9)	1.03 (1.00-1.07)	1.1 (1.06-1.14)
18-23	122 866	4678 (3.8)	1 [Reference]	1 [Reference]
24-29	86 842	3421 (3.9)	1.04 (0.99-1.08)	0.99 (0.94-1.03)
30-35	55 364	2219 (4.0)	1.06 (1.00-1.11)	0.98 (0.93-1.03)
≥36	54 988	2519 (4.6)	1.21 (1.15-1.28)	1.08 (1.03-1.14)
Low birth weight
<6	49 485	2913 (5.9)	1.77 (1.69-1.86)	1.88 (1.79-1.98)
6-11	174 675	7346 (4.2)	1.24 (1.20-1.29)	1.32 (1.27-1.37)
12-17	181 172	6493 (3.6)	1.05 (1.01-1.10)	1.09 (1.05-1.13)
18-23	122 866	4190 (3.4)	1 [Reference]	1 [Reference]
24-29	86 842	2881 (3.3)	0.97 (0.93-1.02)	0.95 (0.90-1.00)
30-35	55 364	1894 (3.4)	1.00 (0.95-1.06)	0.97 (0.92-1.02)
≥36	54 988	2197 (4.0)	1.18 (1.12-1.24)	1.13 (1.07-1.19)
Small for gestational age
<6	49 444	7472 (15.1)	1.49 (1.44-1.53)	1.34 (1.30-1.38)
6-11	174 611	23 522 (13.5)	1.30 (1.27-1.33)	1.21 (1.18-1.23)
12-17	181 121	21 338 (11.8)	1.11 (1.09-1.14)	1.07 (1.05-1.10)
18-23	122 844	13 145 (10.7)	1 [Reference]	1 [Reference]
24-29	86 829	8770 (10.1)	0.94 (0.91-0.97)	0.97 (0.94-1.00)
30-35	55 350	5390 (9.7)	0.90 (0.87-0.93)	0.96 (0.93-0.99)
≥36	54 983	5175 (9.4)	0.87 (0.84-0.90)	0.96 (0.93-0.99)

Abbreviation: OR, odds ratio.

^a ORs and corresponding 95% CIs were based on logistic regression.

^b Adjusted for maternal age, maternal ethnicity, socioeconomic status, mode of delivery, neonate’s sex, and season of birth.