Research ArticleOriginal Article
Open Access

Prevalence of preterm birth and risk factors associated with it at different gestational ages

A multicenter retrospective survey in China

Yi-Jie Zhang, Yan Zhu, Li Zhu, Cheng-Qiu Lu, Chao Chen and Lin Yuan
Saudi Medical Journal June 2022, 43 (6) 599-609; DOI: https://doi.org/10.15537/smj.2022.43.6.20220210

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Tables

  • Table 1 -

    General characteristics of the study group.

    CharacteristicsTotal group (n=215254)Preterm group (n=15833)Term group (n=199421)P-value
    Gestational age (weeks)*38±234±239±1<0.001
    Birth weight (g)*3262±5002419±5833329±4263<0.001
    Male †114084 (53.0)9212 (58.0)105094 (52.7)0.001
    Maternal age (years) †27.4±4.927.7±5.627.4±4.8<0.001
    Parity †#x00A0;#x00A0;#x00A0;<0.001
    1149951 (69.7)9949 (62.8)140002 (70.2)#x00A0;
    ≥265303 (30.3)5884 (37.2)59419 (29.8)#x00A0;
    Apgar score at 5 min of <7600 (2.8)335 (2.1)265 (1.3)<0.001

    • ↵* Mean±SD, p<0.05 was considered significant.

    • ↵† n (%), p<0.05 was considered significant.

  • Table 2

    - Univariate and multivariate analyses of risk factors for singleton preterm births.

    FactorsSingleton preterm(n=15833)Singleton term(n=199421)OR (95% CI)*P-valueaOR (95% CI) (Model A)†P-value
    Maternal age (years)#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    <20609 (3.8)4584 (2.3)1.796 (1.647–1.958)<0.0011.963 (1.796–2.145)<0.001
    20–3413124 (82.9)177395 (89.0)1.000#x00A0;1.000#x00A0;
    ≥352100 (13.3)17442 (8.7)1.627 (1.550–1.708)<0.0011.318 (1.250–1.390)<0.001
    Parity#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    19949 (62.8)140002 (70.2)1.000#x00A0;1.000#x00A0;
    ≥25884 (37.2)59419 (29.8)1.393 (1.347–1.441)<0.0011.423 (1.372–1.476)<0.001
    HDP#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes1007 (6.4)3486 (1.7)3.818 (3.552–4.103)<0.0013.978 (3.691–4.288)<0.001
    No14826 (93.6)195935 (98.3)1.000#x00A0;1.000#x00A0;
    GDM#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes723 (4.6)7020 (3.5)1.311 (1.213–1.418)<0.0011.216 (1.122–1.319)<0.001
    No15110 (95.4)192401 (96.5)1.000#x00A0;1.000#x00A0;
    ICP#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes347 (2.2)973 (0.5)4.570 (4.039–5.172)<0.0015.520 (4.860–6.270)<0.001
    No15486 (97.8)198448 (99.5)1.000#x00A0;1.000#x00A0;
    Anemia#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes288 (1.8)2407 (1.2)1.516 (1.341–1.71)<0.0011.261 (1.108–1.436)<0.001
    No15545 (98.2)199421 (98.2)1.000#x00A0;1.000#x00A0;
    Chorioamnionitis#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes196 (1.2)787 (0.4)3.164 (2.703–3.703)<0.0012.834 (2.406–3.340)<0.001
    No15637 (98.8)198634 (99.6)1.000#x00A0;1.000#x00A0;
    Placenta previa#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes284 (1.8)392 (0.2)9.27 (7.95–10.81)<0.00110.09 (8.60–11.83)<0.001
    No15549 (98.2)199029 (99.8)1.00#x00A0;1.00#x00A0;
    Placental abruption#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes96 (0.6)66 (0.0)18.42 (13.46–25.21)<0.00117.20 (12.35–23.95)<0.001
    No15737 (99.4)199355 (100.0)1.000#x00A0;1.000#x00A0;
    Fetal distress#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes832 (5.3)8018 (4.0)1.324 (1.230–1.425)<0.0011.249 (1.157–1.348)<0.001
    No15001 (94.7)191403 (96.0)1.000#x00A0;1.000#x00A0;

    Values are presented as number and percentages (%).

    • ↵* OR (odds ratio) value and 95% CI (confidence interval) of univariate logistic analysis model, p<0.05 was considered significant. †aOR (adjusted odds ratio) value and 95% CI after adjusting for other confounders in Model A (including maternal age, parity, HDP, GDM, ICP, anemia, chorioamnionitis, placenta previa, placental abruption, and fetal distress); p<0.05 was considered significant. HDP: hypertension disorders in pregnancy, ICP: intrahepatic cholestasis of pregnancy, GDM: gestational diabetes mellitus

  • Table 3 -

    Risk factors associated with each preterm subgroup compared to the term group in multinomial multivariate logistic regression analysis.

    #x00A0;<32 weeks#x00A0;32–33 weeks#x00A0;34–36 weeks#x00A0;
    FactorsaOR (95% CI)*P-valueaOR (95% CI)*P-valueaOR (95% CI)*P-value
    Maternal age (years)#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    <202.084 (1.650–2.632)<0.0012.197 (1.810–2.666)<0.0011.437 (1.289–1.601)<0.001
    20–341.000#x00A0;1.000#x00A0;1.000#x00A0;
    ≥351.396 (1.213–1.606)<0.0011.167 (1.024–1.330)0.0201.204 (1.132–1.280)<0.001
    Parity#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    11.000#x00A0;1.000#x00A0;1.000#x00A0;
    ≥21.369 (1.238–1.514)<0.0011.332 (1.220–1.455)<0.0011.218 (1.169–1.269)<0.001
    HDP#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes3.564 (2.930–4.335)<0.0014.418 (3.774–5.173)<0.0013.044 (2.786–3.326)<0.001
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    GDM#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes1.113 (0.886–1.399)0.3581.092 (0.893–1.336)0.3921.156 (1.054–1.267)<0.001
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    ICP#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes1.384 (0.766–2.499)0.2823.056 (2.147–4.348)<0.0015.763 (5.049–6.577)<0.001
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    Anemia#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes1.240 (0.896–1.715)0.1951.582 (1.207–2.074)0.0011.096 (0.939–1.279)0.246
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    Chorioamnionitis#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes11.06 (8.738–14.02)<0.0013.023 (2.119–4.314)<0.0012.247 (2.153–2.344)<0.001
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    Placenta previa#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes40.04 (32.00–50.09)<0.00113.85 (10.28–18.65)<0.0019.37 (7.77–11.29)<0.001
    No1.000#x00A0;1.000#x00A0;1.00#x00A0;
    Placental abruption#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes41.52 (25.89–66.58)<0.00132.06 (20.27–50.70)<0.00111.67 (7.99–17.02)<0.001
    No1.000#x00A0;1.00#x00A0;1.00#x00A0;
    Fetal distress#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes1.487 (1.230–1.799)<0.0011.477 (1.250–1.746)<0.0011.061 (0.969–1.161)0.201
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;

    • ↵* aOR (adjusted odds ration) value and 95% CI (confidence interval) after adjusting for other confounders in Model B (including maternal age, parity, HDP, GDM, ICP, anemia, chorioamnionitis, placenta previa, placental abruption, and fetal distress). The reference group was the term group. P<0.05 was considered significant. Bold values are significant. Bold numbers are significant. HDP: hypertension disorders in pregnancy, ICP: intrahepatic cholestasis of pregnancy, GDM: gestational diabetes mellitus

  • Table 4 -

    Risk factors associated with each spontaneous preterm subgroup compared to the term group in multinomial multivariate logistic regression analysis.

    #x00A0;<32 weeks#x00A0;32–33 weeks#x00A0;34–36 weeks#x00A0;
    FactorsaOR (95% CI)*P-valueaOR (95% CI)*P-valueaOR (95% CI)*P-value
    Maternal age (years)#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    <201.088 (0.757–1.564)0.6491.328 (1.004–1.756)0.0470.950 (0/868–1.163)0.950
    20–341.000#x00A0;1.000#x00A0;1.000#x00A0;
    ≥351.585 (1.357–1.851)<0.0011.299 (1.127–1.498)<0.0011.252 (1.166–1.343)<0.001
    Parity#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    11.000#x00A0;1.000#x00A0;1.000#x00A0;
    ≥21.391 (1.237–1.563)<0.0011.391 (1.258–1.453)<0.0011.218 (1.161–1.278)<0.001
    HDP#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes8.518 (7.099–10.221)<0.0017.924 (6.797–1.538)<0.0014.303(3.917–4.727)<0.001
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    GDM#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes1.185 (0.922–1.523)0.1841.293 (1.047–1.596)0.0171.286 (1.162–1.424)<0.001
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    ICP#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes1.384 (0.766–2.499)0.2823.875 (2.595–5.786)<0.00110.04(8.79–11.47)<0.001
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    Anemia#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes1.240 (0.896–1.715)0.1951.862 (1.392–2.489)0.0011.221 (1.027–1.452)0.240
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    Chorioamnionitis#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes33.30 (27.29–40.63)<0.0013.863 (2.620–5.694)<0.0011.835 (1.405–2.397)<0.001
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    Placenta previa#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes36.75 (26.02–51.92)<0.00122.71 (16.36–31.517)<0.00116.05 (13.29–19.36)<0.001
    No1.000#x00A0;1.000#x00A0;1.00#x00A0;
    Placental abruption#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes63.75 (37.49–108.4)<0.00149.59 (30.66–80.21)<0.00117.31 (11.76–25.48)<0.001
    No1.000#x00A0;1.00#x00A0;1.00#x00A0;
    Fetal distress#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes1.735 (1.414–2.130)<0.0011.654 (1.378–1.986)<0.0011.016 (1.050–1.286)0.004
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;

    • ↵* aOR (adjusted odds ration) value and 95% CI (confidence interval) after adjusting for other confounders in Model B (including maternal age, parity, HDP, GDM, ICP, anemia, chorioamnionitis, placenta previa, placental abruption, and fetal distress); the reference group was the term group. P<0.05 was considered significant. Bold values are significant. HDP: hypertension disorders in pregnancy, ICP: intrahepatic cholestasis of pregnancy, GDM: gestational diabetes mellitus

  • Table 5

    - Risk factors associated with each iatrogenic preterm subgroup compared to the term group in multinomial multivariate logistic regression analysis.

    #x00A0;<32 weeks#x00A0;32–33 weeks#x00A0;34–36 weeks#x00A0;
    FactorsaOR (95% CI)*P-valueaOR (95% CI)*P-valueaOR (95% CI)*P-value
    Maternal age (years)#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    <202.103 (1.663–2.660)<0.0012.221 (1.826–2.701)<0.0011.449 (1.299–1.615)<0.001
    20–341.000#x00A0;1.000#x00A0;1.000#x00A0;
    ≥351.390 (1.207–1.600)<0.0011.161 (1.018–1.324)0.0271.198 (1.126–1.274)0.001
    Parity#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    11.000#x00A0;1.000#x00A0;1.000#x00A0;
    ≥21.331 (1.238–1.514)<0.0011.287 (1.177–1.406)<0.0011.181(1.133–1.231)<0.001
    HDP#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes3.405 (2.799–4.143)<0.0014.176 (3.566–4.892)<0.0012.900 (2.655–3.168)<0.001
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    GDM#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes1.119 (0.889–1.408)0.3391.098 (0.896–1.347)0.3681.162 (1.059–1.274)0.001
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    ICP#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes1.280 (0.709–2.312)0.4132.775 (1.949–3.950)<0.0015.299 (4.646–6.044)<0.001
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    Anemia#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes1.235 (0.891–1.711)0.2041.575 (1.199–2.070)0.0011.092 (0.935–1.275)0.269
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    Chorioamnionitis#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes11.89 (9.391–15.07)<0.0013.317 (2.319–4.743)<0.0012.025 (1.630–2.516)<0.001
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;
    Placenta previa#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes3.786 (1.682–8.523)0.0011.228 (0.305–4.940)0.7732.276 (1.398–3.706)0.001
    No1.000#x00A0;1.000#x00A0;1.00#x00A0;
    Placental abruption#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes23.68 (10.21–654.9)<0.00111.236 (23.52–35.87)<0.0012.404(0.755–7.654)0.138
    No1.000#x00A0;1.00#x00A0;1.00#x00A0;
    Fetal distress#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;#x00A0;
    Yes1.487 (1.230–1.799)<0.0011.479 (1.263–1.779)<0.0011.071 (0.978–1.173)0.141
    No1.000#x00A0;1.000#x00A0;1.000#x00A0;

    • ↵* aOR (adjusted odds ration) value and 95% CI (confidence interval) after adjusting for other confounders in Model B (including maternal age, parity, HDP, GDM, ICP, anemia, chorioamnionitis, placenta previa, placental abruption, and fetal distress); the reference group was the term group. P<0.05 was considered significant. Bold values are significant. HDP: hypertension disorders in pregnancy, ICP: intrahepatic cholestasis of pregnancy, GDM: gestational diabetes mellitus.

  • Appendix 1 -

    Distribution of the risk factors in each group.

    Variables<32 weeks(n=1703)32–33 weeks(n=2345)34–36 weeks(n=11785)37–41 weeks(199421)P-value*
    Maternal age (years)#x00A0;#x00A0;#x00A0;#x00A0;<0.001
    <2083 (4.9)123 (5.2)403 (3.4)4584 (2.3)#x00A0;
    20–341345 (79.0)1915 (81.7)9864 (83.7)177395 (89.0)#x00A0;
    ≥35275 (16.1)307 (13.1)1518 (12.9)17442 (8.7)#x00A0;
    Parity ≥ 2704 (41.3)905 (38.6)4275 (36.3)59419 (29.8)<0.001
    HDP119 (7.0)182 (7.8)706 (6.0)3486 (1.7)<0.001
    GDM76 (4.5)100 (4.3)547 (4.6)7020 (3.5)<0.001
    ICP12 (0.7)38 (1.6)297 (2.5)1973 (0.5)<0.001
    Anemia37 (2.2)61 (2.6)190 (1.6)2407 (1.2)<0.001
    Chorioamnionitis48 (2.8)35 (1.5)113 (1.0)787 (0.4)<0.001
    Placenta previa47 (2.8)46 (2.0)191 (1.6)6392 (0.2)<0.001
    Placental abruption24 (1.4)26 (1.1)46 (0.4)66 (0.0)<0.001
    Fetal distress124 (7.3)166 (7.1)542 (4.6)80718 (4.0)<0.001

    Values are presented as number and percentages (%).

    • ↵* P-value indicates results of χ2 test comparing proportions of the distribution of risk factors for preterm subgroups. HDP: hypertension disorders in pregnancy, ICP: intrahepatic cholestasis of pregnancy, GDM: gestational diabetes mellitus

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