1. Blencowe, H, Cousens, S, Oestergaard, MZ, et al. (2012) National, regional, and worldwide estimates of preterm birth rates in the year 2010 with time trends since 1990 for selected countries: a systematic analysis and implications. Lancet 379, 2162–2172.
3. Liu, L, Oza, S, Hogan, D, et al. (2015) Global, regional, and national causes of child mortality in 2000–13, with projections to inform post-2015 priorities: an updated systematic analysis. Lancet 385, 430–440.
4. GBD 2016 Causes of Death Collaborators (2017) Global, regional, and national age-sex specific mortality for 264 causes of death, 1980–2016: a systematic analysis for the Global Burden of Disease Study 2016. Lancet 390, 1151–1210.
5. Goldenberg, RL, Culhane, JF, Iams, JD, et al. (2008) Epidemiology and causes of preterm birth. Lancet 371, 75–84.
6. Levy, A, Fraser, D, Katz, M, et al. (2005) Maternal anemia during pregnancy is an independent risk factor for low birthweight and preterm delivery. Eur J Obstet Gynecol Reprod Biol 122, 182–186.
7. Ren, A, Wang, J, Ye, RW, et al. (2007) Low first-trimester hemoglobin and low birth weight, preterm birth and small for gestational age newborns. Int J Gynaecol Obstet 98, 124–128.
8. Zhang, Q, Ananth, CV, Li, Z, et al. (2009) Maternal anaemia and preterm birth: a prospective cohort study. Int J Epidemiol 38, 1380–1389.
9. Scanlon, KS, Yip, R, Schieve, LA, et al. (2000) High and low hemoglobin levels during pregnancy: differential risks for preterm birth and small for gestational age. Obstet Gynecol 96, 741–748.
10. Gonzales, GF, Tapia, V, Gasco, M, et al. (2012) Maternal hemoglobin concentration and adverse pregnancy outcomes at low and moderate altitudes in Peru. J Matern Fetal Neonatal Med 25, 1105–1110.
11. Abeysena, C, Jayawardana, P & de A Seneviratne, R (2010) Maternal haemoglobin level at booking visit and its effect on adverse pregnancy outcome. Aust N Z J Obstet Gynaecol 50, 423–427.
12. Sukrat, B, Wilasrusmee, C, Siribumrungwong, B, et al. (2013) Hemoglobin concentration and pregnancy outcomes: a systematic review and meta-analysis. Biomed Res Int 2013, 769057.
13. Dewey, KG & Oaks, BM (2017) U-shaped curve for risk associated with maternal hemoglobin, iron status, or iron supplementation. Am J Clin Nutr 106, 1694S–1702S.
14. Zhang, SK, Wang, QM & Sheng, HP (2015) Design of the National Free Preconception Health Examination Project in China. Zhonghua Yi Xue Za Zhi 95, 162–165 (in Chinese).
15. Yang, Y, He, Y, Li, Q, et al. (2015) Preconception blood pressure and risk of preterm birth: a large historical cohort study in a Chinese rural population. Fertil Steril 104, 124–130.
16. Wang, QM, Zhang, M, Zhang, SK, et al. (2015) Establishment of quality assurance system of the National Free Preconception Health Examination Project in China. Zhonghua Yi Xue Za Zhi 95, 166–168 (in Chinese).
17. Centers for Disease Control (1989) CDC criteria for anemia in children and childbearing-aged women. MMWR Morb Mortal Wkly Rep 38, 400–404.
18. World Health Organization (2011) Haemoglobin concentrations for the diagnosis of anemia and assessment of severity. Vitamin and Mineral Nutrition Information System. Geneva: WHO (WHO/NMH/NHD/MNM/11.1). http://www.who.int/vmnis/indicators/haemoglobin (accessed January 2018).
19. Yip, R (2000) Significance of an abnormally low or high hemoglobin concentration during pregnancy: special consideration of iron nutrition. Am J Clin Nutr 72, 272S–279S.
20. Ge, JB & Xu, YJ (2013) Internal Medicine, 8th ed. Beijing: People’s Medical Publishing House (in Chinese).
21. Hu, J, Xia, W, Pan, X, et al. (2017) Association of adverse birth outcomes with prenatal exposure to vanadium: a population-based cohort study. Lancet Planet Health 1, e230–e241.
22. Yi, SW, Han, YJ & Ohrr, H (2013) Anemia before pregnancy and risk of preterm birth, low birth weight and small-for-gestational-age birth in Korean women. Eur J Clin Nutr 67, 337–342.
23. Ronnenberg, AG, Wood, RJ, Wang, X, et al. (2004) Preconception hemoglobin and ferritin concentrations are associated with pregnancy outcome in a prospective cohort of Chinese women. J Nutr 134, 2586–2591.
24. Chang, SC, O’Brien, KO, Nathanson, MS, et al. (2003) Hemoglobin concentrations influence birth outcomes in pregnant African-American adolescents. J Nutr 133, 2348–2355.
25. Lu, ZM, Goldenberg, RL, Cliver, SP, et al. (1991) The relationship between maternal hematocrit and pregnancy outcome. Obstet Gynecol 77, 190–194.
26. Steer, PJ (2000) Maternal hemoglobin concentration and birth weight. Am J Clin Nutr 71, 1285S–1287S.
27. Murphy, JF, O’Riordan, J, Newcombe, RG, et al. (1986) Relation of haemoglobin levels in first and second trimesters to outcome of pregnancy. Lancet i, 992–995.
28. Steer, P, Alam, MA, Wadsworth, J, et al. (1995) Relation between maternal haemoglobin concentration and birth weight in different ethnic groups. BMJ 310, 489–491.
29. Gonzales, GF, Steenland, K & Tapia, V (2009) Maternal hemoglobin level and fetal outcome at low and high altitudes. Am J Physiol Regul Integr Comp Physiol 297, R1477–R1485.
30. Zhou, LM, Yang, WW, Hua, JZ, et al. (1998) Relation of hemoglobin measured at different times in pregnancy to preterm birth and low birth weight in Shanghai, China. Am J Epidemiol 148, 998–1006.
31. Mohamed, MA, Ahmad, T, Macri, C, et al. (2012) Racial disparities in maternal hemoglobin concentrations and pregnancy outcomes. J Perinat Med 40, 141–149.
32. Casanueva, E, Viteri, FE, Mares-Galindo, M, et al. (2006) Weekly iron as a safe alternative to daily supplementation for nonanemic pregnant women. Arch Med Res 37, 674–682.
33. Iqbal, S & Ekmekcioglu, C (2017) Maternal and neonatal outcomes related to iron supplementation or iron status: a summary of meta-analyses. J Matern Fetal Neonatal Med 5, 1–13.
34. Penaloza, D & Ariasstella, J (2007) The heart and pulmonary circulation at high altitudes: healthy highlanders and chronic mountain sickness. Circulation 115, 1132–1146.
35. Laflamme, EM (2011) Maternal hemoglobin concentration and pregnancy outcome: a study of the effects of elevation in el alto, Bolivia. Mcgill J Med 13, 47.