Table 3 Baseline demographics of the population of 442,268 twins born in China

N(%)

442,268(100.00%)

8145(1.75%)

-

31,408(7.70%)

-

3346(0.79%)

-

Area classification

   urban

345,102(78.03%)

6686(1.89%)

ref

25,833(7.54%)

ref

2601(0.77%)

ref

   rural

97,166(21.97%)

1459(1.51%)

0.97[0.88,1.08]

7575(7.99%)

1.05[1.01,1.10]

765(0.81%)

1.53[1.33,1.76]

Geographic location

   east

123,856(28.00%)

1862(1.49%)

0.81[0.74,0.88]

7879(6.61%)

0.94[0.90,0.97]

823(0.68%)

0.89[0.79,1.00]

   central

172,905(39.10%)

3158(1.74%)

ref

12,634(7.34%)

ref

1249(0.73%)

ref

   west

145,507(32.90%)

3125(2.11%)

1.17[1.08,1.27]

12,895(9.60%)

1.36[1.32,1.41]

1294(0.99%)

1.38[1.24,1.53]

Hospital level

   level 1

30,580(6.91%)

467(1.43%)

0.86[0.75,1.00]

2473(8.30%)

0.91[0.86,0.96]

190(0.69%)

0.76[0.62,0.92]

   level 2

123,667(27.96%)

1746(1.47%)

0.95[0.87,1.05]

9043(7.66%)

0.91[0.88,0.95]

853(0.74%)

0.86[0.76,0.99]

   level 3

288,021(65.12%)

5932(1.99%)

ref

21,892(7.65%)

ref

2323(0.82%)

ref

Year of delivery

   2012–2013

119,646(27.05%)

2727(2.16%)

ref

10,019(8.61%)

ref

1130(0.96%)

ref

   2014–2015

101,860(23.03%)

2008(1.86%)

0.85[0.78,0.93]

7913(7.96%)

0.98[0.94,1.02]

859(0.85%)

0.86[0.76,0.97]

   2016–2017

109,305(24.71%)

1724(1.50%)

0.68[0.61,0.74]

7872(7.33%)

0.95[0.91,0.99]

737(0.71%)

0.77[0.68,0.87]

   2018–2020

111,457(25.20%)

1686(1.43%)

0.60[0.54,0.66]

7604(6.78%)

0.92[0.88,0.95]

640(0.58%)

0.62[0.54,0.70]

Maternal age

   < 20

4506(1.02%)

182(3.66%)

1.50[1.19,1.89]

692(15.81%)

1.81[1.61,2.04]

82(1.73%)

1.51[1.08,2.11]

   20–25

47,481(10.74%)

1308(2.48%)

1.30[1.18,1.44]

4993(10.64%)

1.34[1.28,1.40]

535(1.14%)

1.28[1.12,1.47]

   25–30

151,693(34.30%)

2797(1.72%)

ref

11,478(7.69%)

ref

1096(0.73%)

ref

   30–35

153,274(34.66%)

2264(1.41%)

0.82[0.75,0.89]

10,012(6.51%)

0.84[0.82,0.87]

935(0.63%)

0.91[0.81,1.02]

   ≥ 35

72,852(16.47%)

1278(1.70%)

0.84[0.76,0.94]

5069(7.14%)

0.90[0.86,0.94]

549(0.77%)

0.95[0.82,1.09]

   unknown

12,462(2.82%)

316(2.56%)

-

1164(9.67%)

-

169(1.47%)

-

Maternal marriage

   Single/widower/divorced/cohabitation

4686(1.06%)

154(2.99%)

1.11[0.84,1.45]

511(11.44%)

1.14[0.99,1.30]

62(1.25%)

0.95[0.66,1.39]

   married

437,464(98.91%)

7990(1.74%)

ref

32,886(7.66%)

ref

3304(0.78%)

ref

   unknown

118(0.03%)

1(1.08%)

-

11(10.32%)

-

-

Maternal education

   college or above

193,505(43.75%)

2925(1.45%)

0.85[0.79,0.92]

12,884(6.64%)

0.86[0.84,0.89]

1056(0.55%)

0.80[0.72,0.89]

   secondary school

223,568(50.55%)

4509(1.86%)

ref

18,207(8.22%)

ref

1979(0.88%)

ref

   primary school or below

13,704(3.10%)

434(3.00%)

1.30[1.12,1.53]

1557(11.78%)

1.40[1.30,1.51]

234(1.75%)

1.50[1.24,1.82]

   unknown

11,491(2.60%)

277(2.36%)

-

760(7.18%)

-

97(0.96%)

-

Parity

   multipara

163,110(36.88%)

3444(1.96%)

1.34[1.24,1.44]

11,382(7.12%)

0.86[0.83,0.89]

1425(0.90%)

1.30[1.18,1.43]

   primipara

277,807(62.81%)

4642(1.60%)

ref

22,017(8.10%)

ref

1937(0.70%)

ref

   unknown

1351(0.31%)

59(4.32%)

-

9(5.85%)

-

4(2.62%)

-

Inadequate prenatal examination

   < 5 times

45,884(10.37%)

1691(3.40%)

1.50[1.38,1.64]

4562(10.28%)

1.27[1.22,1.33]

842(1.83%)

1.56[1.40,1.75]

   ≥ 5 times

372,236(84.17%)

5806(1.48%)

ref

27,131(7.36%)

ref

2306(0.63%)

ref

   unknown

24,148(5.46%)

648(2.63%)

-

1715(7.58%)

-

218(1.01%)

-

Maternal complications

   no

171,954(38.88%)

2806(1.56%)

ref

11,895(7.12%)

ref

1048(0.63%)

ref

   medical complication

77,773(17.59%)

1142(1.42%)

0.90[0.81,1.00]

5430(7.07%)

0.97[0.93,1.01]

651(0.87%)

1.37[1.20,1.57]

   antepartum complication

192,541(43.53%)

4197(2.05%)

1.07[0.99,1.15]

16,083(8.48%)

1.09[1.06,1.13]

1667(0.89%)

1.20[1.08,1.32]

Gestational week

   ≤ 31w

26,806(6.06%)

2520(9.48%)

12.13[10.87,13.54]

2048(8.58%)

0.98[0.92,1.04]

1444(6.47%)

29.25[25.14,34.03]

   32-33w

35,338(7.99%)

1324(3.78%)

4.45[3.95,5.01]

3361(9.88%)

1.18[1.12,1.24]

616(1.90%)

7.86[6.65,9.28]

   34-36w

173,638(39.26%)

2356(1.34%)

1.81[1.64,1.99]

13,704(8.02%)

1.13[1.10,1.17]

798(0.49%)

2.21[1.90,2.57]

   37-38w

181,673(41.08%)

1398(0.75%)

ref

11,784(6.65%)

ref

359(0.22%)

ref

   ≥ 39w

24,813(5.61%)

547(1.87%)

1.69[1.45,1.97]

2511(9.68%)

1.29[1.21,1.38]

149(0.59%)

1.81[1.42,2.31]

Infants weight discordance

   no

360,526(81.52%)

2809(0.78%)

ref

15,492(4.37%)

ref

2237(0.63%)

ref

   yes

74,604(16.87%)

3587(4.50%)

5.11[4.77,5.48]

17,772(24.95%)

7.32[7.13,7.53]

1077(1.49%)

1.90[1.73,2.10]

   unknown

7138(1.61%)

1749(24.58%)

-

144(6.64%)

-

52(2.49%)

-