[1] Li PC, Liao ZJ, Wang C, Zhou YB, Li HT, Chen DJ, et al. Maternal characteristics and birth outcomes under different family planning policy. Chin J Reprod Health 2017;28(6):501 − 5. http://d.wanfangdata.com.cn/periodical/zgsyjkzz201706001. (In Chinese). http://d.wanfangdata.com.cn/periodical/zgsyjkzz201706001
[2] Li HT, Xue M, Hellerstein S, Cai Y, Gao YQ, Zhang YL, et al. Association of China’s universal two child policy with changes in births and birth related health factors: national, descriptive comparative study. BMJ 2019;366:l4680. http://dx.doi.org/10.1136/bmj.l4680CrossRef
[3] Zhang SS, Huang T. Value of improved risk early warning and evaluation management on improving perinatal outcome of high-risk pregnant women. Chin Med Rec 2019;20(2):91 − 4. https://kns.cnki.net/KCMS/detail/detail.aspx?FileName=ZGBN201902032&DbName=CJFQ2019. (In Chinese). https://kns.cnki.net/KCMS/detail/detail.aspx?FileName=ZGBN201902032&DbName=CJFQ2019
[4] National Health Commission of the People’s Republic of China. Regulations on pregnancy risk assessment and management (2017 No.35). http://www.nhc.gov.cn/fys/s3581/201711/9c3dc9b4a8494d9a94c02f890e5085b1.shtml. [2020-12-8]. (In Chinese). http://www.nhc.gov.cn/fys/s3581/201711/9c3dc9b4a8494d9a94c02f890e5085b1.shtml
[5] National Health Commission of the People’s Republic of China. Notice on strengthening the work of safeguarding maternal and infant safety (2017 No.42). https://www.chinafpa.org.cn/zcfg/xgfg/201901/t20190125_42951.html. [2020-12-8]. (In Chinese). https://www.chinafpa.org.cn/zcfg/xgfg/201901/t20190125_42951.html
[6] Yan S, Huang AQ, Xu T, Hu HQ, Zhao W, Ma WX, et al. Analysis of obstetricians’ workload of institutions of maternal and children’s healthcare in some Chinese large and medium-sized cities. Chin J Reprod Health 2008;29(4):319 − 23. https://kns.cnki.net/KCMS/detail/detail.aspx?FileName=SYJK201804004&DbName=CJFQ2018. (In Chinese). https://kns.cnki.net/KCMS/detail/detail.aspx?FileName=SYJK201804004&DbName=CJFQ2018
[7] Huang AQ, Liu YH, Jin X. The impact of new adjustments to the family planning policy on the number of live births in four developed urban areas — China, 2013−2019. China CDC Wkly 2020;2(28):530 − 3. http://weekly.chinacdc.cn/en/article/doi/10.46234/ccdcw2020.143.http://weekly.chinacdc.cn/en/article/doi/10.46234/ccdcw2020.143
[8] Care AS, Bourque SL, Morton JS, Hjartarson EP, Davidge ST. Effect of advanced maternal age on pregnancy outcomes and vascular function in the rat. Hypertension 2015;65(6):1324 − 30. http://dx.doi.org/10.1161/HYPERTENSIONAHA.115.05167CrossRef
[9] Zhao YY, Yuan PB, Chen L. Problems faced by advanced maternal age in the age of two children. Chin J Pract Gynecol Obstet 2020;36(2):97 − 100. https://kns.cnki.net/KCMS/detail/detail.aspx?FileName=ZGSF202002001&DbName=CJFQ2020. (In Chinese). https://kns.cnki.net/KCMS/detail/detail.aspx?FileName=ZGSF202002001&DbName=CJFQ2020
[10] Department of Maternal and Child Health of the National Health Commission. Analysis report on the national maternal and child health information for 2020. [2020-12-8]. (In Chinese). 
[11] Knight HE, Self A, Kennedy SH. Why are women dying when they reach hospital on time? a systematic review of the ‘third delay’. PLoS One 2013;8(5):e63846. http://dx.doi.org/10.1371/journal.pone.0063846CrossRef
[12] Wu YT, Chen Q, Liao HZ, Wu L, Wang ZQ, Xu HL, et al. Matching degree between the classification of maternal pregnancy risk and the level of midwifery institutions, Guangdong. Mod Prev Med 2020;47(1):84 − 7. http://d.wanfangdata.com.cn/periodical/xdyfyx202001020. (In Chinese). http://d.wanfangdata.com.cn/periodical/xdyfyx202001020