Table 1.

Characteristics of the study population in China Labor and Delivery Survey, 2015–2017 ($n=\mathrm{70,818}$).

Characteristics	Total births $n=\mathrm{70,818}$	Term births $n=\mathrm{65,853}$	Preterm births $n=\mathrm{4,965}$	$p$-Value
Maternal age [n (%)]	—	—	—	$<0.001$
15–19 y	1,021 (1.4)	887 (1.3)	134 (2.7)	—
20–24 y	10,496 (14.8)	9,783 (14.9)	713 (14.4)	—
25–29 y	33,026 (46.6)	31,056 (47.2)	1,970 (39.7)	—
30–34 y	18,301 (25.8)	16,958 (25.8)	1,343 (27.0)	—
35–49 y	7,974 (11.3)	7,169 (10.9)	805 (16.2)	—
Maternal ethnicity [n (%)]	—	—	—	0.019
Han	67,507 (95.6)	62,816 (95.6)	4,691 (94.9)	—
Minority	3,134 (4.4)	2,882 (4.4)	252 (5.1)	—
Unknown	177	155	22	—
Maternal education [n (%)]	—	—	—	$<0.001$
Less than middle school	15,976 (25.1)	14,495 (24.5)	1,481 (33.1)	—
High school	12,654 (19.9)	11,729 (19.8)	925 (20.7)	—
College ($<4$ y)	15,639 (24.6)	14,661 (24.8)	978 (21.9)	—
College graduate or higher ($\ge 4$ y)	19,319 (30.4)	18,232 (30.8)	1,087 (24.3)	—
Unknown	7,230	6,736	494	—
Insurance type [n (%)]	—	—	—	$<0.001$
Health insurance	41,422 (60.5)	38,794 (60.9)	2,628 (55.8)	—
Self-pay	27,016 (39.5)	24,938 (39.1)	2,078 (44.2)	—
Unknown	2,380	2,121	259	—
Child sex [n (%)]	—	—	—	$<0.001$
Male	37,826 (53.5)	34,963 (53.2)	2,863 (57.9)	—
Female	32,872 (46.5)	30,789 (46.8)	2,083 (42.1)	—
Unknown	120	101	19	—
Parity [n (%)]	—	—	—	$<0.001$
0	39,726 (56.2)	37,148 (56.6)	2,578 (52.2)	—
$\ge 1$	30,900 (43.8)	28,538 (43.4)	2,362 (47.8)	—
Unknown	192	167	25	—
GDP per capita [n (%)]	—	—	—	$<0.001$
Q1 ($\sim \mathrm{50,000}$ CNY)	12,729 (18.0)	11,874 (18.0)	855 (17.2)	—
Q2 ($\mathrm{5,000}\sim \mathrm{80,000}$ CNY)	19,826 (28.0)	18,242 (27.7)	1,584 (31.9)	—
Q3 ($\mathrm{8,000}\sim \mathrm{120,000}$ CNY)	16,837 (23.8)	15,654 (23.8)	1,183 (23.8)	—
Q4 ($\mathrm{120,000}\sim \text{CNY}$)	21,426 (30.3)	20,083 (30.5)	1,343 (27.0)	—
Geographic region [n (%)]	—	—	—	—
North	19,927 (28.1)	18,823 (28.6)	1,104 (22.2)	$<0.001$
East	23,640 (33.4)	22,120 (33.6)	1,520 (30.6)	—
South	13,192 (18.6)	12,152 (18.5)	1,040 (21.0)	—
West	14,059 (19.9)	12,758 (19.4)	1,301 (26.2)	—
Conception season [n (%)]	—	—	—	0.054
Spring	15,216 (21.5)	14,128 (21.5)	1,088 (21.9)	—
Summer	20,578 (29.1)	19,167 (29.1)	1,411 (28.4)	—
Autumn	16,712 (23.6)	15,475 (23.5)	1,237 (24.9)	—
Winter	18,312 (25.9)	17,083 (25.9)	1,229 (24.8)	—
Spontaneous PTB [n (%)]	2,454 (3.47)	—	—	—
PPROM [n (%)]	1,358 (1.92)	—	—	—
Iatrogenic PTB [n (%)]	1,153 (1.63)	—	—	—

Note: Term birth: gestational age $\ge 37$ wk. Preterm birth: gestational age $<37$ wk. Unknown refer to the missing value of each variable, which was included in all analyses as a missing category. Differences between groups were analyzed using the chi-square test. $p$-Value was derived from the chi-squared test and $p<0.05$ represents a significant difference between preterm and term births. —, no data; CNY, Chinese yuan; GDP, gross domestic product; PPROM, preterm premature rupture of the fetal membranes; PTB, preterm birth.