Disparities in the association between ambient temperature and preterm birth according to individual and regional characteristics: a nationwide time-stratified case-crossover study

Table 1 Baseline characteristics of the study population (preterm births, PTBs) during 2011–2019 in South Korea

	PTB (%)^a	Proportion of PTB (%)^b
Overall	160,067 (100)	4.83
Year
2011	19,046 (11.90)	4.42
2012	20,192 (12.61)	4.57
2013	18,426 (11.51)	4.64
2014	18,762 (11.72)	4.75
2015	19,311 (12.06)	4.84
2016	18,109 (11.31)	4.92
2017	16,819 (10.51)	5.22
2018	15,171 (9.48)	5.19
2019	14,231 (8.89)	5.30
Maternal age
<35 years	112,324 (70.2)	4.46
≥35 years	47,743 (29.8)	6.00
Parity
Primiparity	80,319 (50.2)	4.61
Multiparity	79,748 (49.8)	5.07
Maternal education level
≤ College	148,106 (92.53)	4.85
> College	10,970 (6.85)	4.48
Unknown	991 (0.62)	5.26
Classification of prematurity^c
Late	127,962 (79.9)	-
Moderate	14,708 (9.2)	-
Very	12,362 (7.7)	-
Extreme	5,035 (3.1)	-

^aColumn percent
^bProportion of PTB was calculated by dividing the number of PTB by total number of singleton births.
^cClassification of prematurity was specified based on gestational age: late (34 to 37 weeks), moderate (32 to 34 weeks), very (28 to 32 weeks), and extremely (20 to 28 weeks) preterm birth.

ISSN: 1476-069X