Official reprint from UpToDate®
www.uptodate.com ©2015 UpToDate®

Risk factors for preterm labor and delivery

Julian N Robinson, MD
Errol R Norwitz, MD, PhD
Section Editor
Charles J Lockwood, MD, MHCM
Deputy Editor
Vanessa A Barss, MD, FACOG


Multiple genetic, environmental, and immunological factors contribute to a woman’s likelihood of preterm delivery; interactions between genetic

susceptibility and environmental stimuli must also play a role. Preterm labor likely results from local changes that prematurely stimulate the cascade of events resulting in spontaneous labor or prematurely withdraw suppressive factors that maintain uterine quiescence and thus inhibit this cascade. The four major pathogenic pathways leading to preterm labor are intrauterine infection, decidual hemorrhage, excessive uterine stretch, and maternal or fetal stress.

There are many risk factors for preterm labor and delivery (table 1). Increasing evidence suggests that these risk factors are markers of dysfunctional immunological defense within the tissues of the uterus and this dysfunction is the root cause preterm labor [1].

Some risk factors are reversible, others are permanent. Identification of risk factors for spontaneous preterm birth (PTB) before conception or early in pregnancy ideally would lead to interventions that could help prevent this complication. However, this goal has been elusive for several reasons:

The causality of purported risk factors has been difficult to prove. Some obstetrical complications resulting in PTB require cofactors to exert their effect, further complicating the chain of causality


Subscribers log in here

To continue reading this article, you must log in with your personal, hospital, or group practice subscription. For more information or to purchase a personal subscription, click below on the option that best describes you:
Literature review current through: Jun 2015. | This topic last updated: Jun 12, 2015.
The content on the UpToDate website is not intended nor recommended as a substitute for medical advice, diagnosis, or treatment. Always seek the advice of your own physician or other qualified health care professional regarding any medical questions or conditions. The use of this website is governed by the UpToDate Terms of Use ©2015 UpToDate, Inc.
  1. Norwitz ER, Bonney EA, Snegovskikh VV, et al. Molecular Regulation of Parturition: The Role of the Decidual Clock. Cold Spring Harb Perspect Med 2015.
  2. Bloom SL, Yost NP, McIntire DD, Leveno KJ. Recurrence of preterm birth in singleton and twin pregnancies. Obstet Gynecol 2001; 98:379.
  3. Esplin MS, O'Brien E, Fraser A, et al. Estimating recurrence of spontaneous preterm delivery. Obstet Gynecol 2008; 112:516.
  4. Bhattacharya S, Raja EA, Mirazo ER, et al. Inherited predisposition to spontaneous preterm delivery. Obstet Gynecol 2010; 115:1125.
  5. Adams MM, Elam-Evans LD, Wilson HG, Gilbertz DA. Rates of and factors associated with recurrence of preterm delivery. JAMA 2000; 283:1591.
  6. Mercer BM, Goldenberg RL, Moawad AH, et al. The preterm prediction study: effect of gestational age and cause of preterm birth on subsequent obstetric outcome. National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network. Am J Obstet Gynecol 1999; 181:1216.
  7. Gardosi J, Francis A. Early pregnancy predictors of preterm birth: the role of a prolonged menstruation-conception interval. BJOG 2000; 107:228.
  8. Ananth CV, Getahun D, Peltier MR, et al. Recurrence of spontaneous versus medically indicated preterm birth. Am J Obstet Gynecol 2006; 195:643.
  9. Bakketeig LS, Hoffman HJ, Harley EE. The tendency to repeat gestational age and birth weight in successive births. Am J Obstet Gynecol 1979; 135:1086.
  10. Carr-Hill RA, Hall MH. The repetition of spontaneous preterm labour. Br J Obstet Gynaecol 1985; 92:921.
  11. McManemy J, Cooke E, Amon E, Leet T. Recurrence risk for preterm delivery. Am J Obstet Gynecol 2007; 196:576.e1.
  12. Lykke JA, Paidas MJ, Langhoff-Roos J. Recurring complications in second pregnancy. Obstet Gynecol 2009; 113:1217.
  13. Kristensen J, Langhoff-Roos J, Kristensen FB. Implications of idiopathic preterm delivery for previous and subsequent pregnancies. Obstet Gynecol 1995; 86:800.
  14. Laughon SK, Albert PS, Leishear K, Mendola P. The NICHD Consecutive Pregnancies Study: recurrent preterm delivery by subtype. Am J Obstet Gynecol 2014; 210:131.e1.
  15. Mercer B, Milluzzi C, Collin M. Periviable birth at 20 to 26 weeks of gestation: proximate causes, previous obstetric history and recurrence risk. Am J Obstet Gynecol 2005; 193:1175.
  16. Menard MK, Newman RB, Keenan A, Ebeling M. Prognostic significance of prior preterm twin delivery on subsequent singleton pregnancy. Am J Obstet Gynecol 1996; 174:1429.
  17. Rydhstroem H. Gestational duration in the pregnancy after a preterm twin delivery. Am J Obstet Gynecol 1998; 178:136.
  18. Facco FL, Nash K, Grobman WA. Are women who have had a preterm twin delivery at greater risk of preterm birth in a subsequent singleton pregnancy? Am J Obstet Gynecol 2007; 197:253.e1.
  19. Rafael TJ, Hoffman MK, Leiby BE, Berghella V. Gestational age of previous twin preterm birth as a predictor for subsequent singleton preterm birth. Am J Obstet Gynecol 2012; 206:156.e1.
  20. Schaaf JM, Hof MH, Mol BW, et al. Recurrence risk of preterm birth in subsequent singleton pregnancy after preterm twin delivery. Am J Obstet Gynecol 2012; 207:279.e1.
  21. Schaaf JM, Hof MH, Mol BW, et al. Recurrence risk of preterm birth in subsequent twin pregnancy after preterm singleton delivery. BJOG 2012; 119:1624.
  22. Shah PS, Zao J, Knowledge Synthesis Group of Determinants of preterm/LBW births. Induced termination of pregnancy and low birthweight and preterm birth: a systematic review and meta-analyses. BJOG 2009; 116:1425.
  23. Kiely JL. What is the population-based risk of preterm birth among twins and other multiples? Clin Obstet Gynecol 1998; 41:3.
  24. TambyRaja RL, Ratnam SS. Plasma steroid changes in twin pregnancies. Prog Clin Biol Res 1981; 69A:189.
  25. Muechler EK, Huang KE. Plasma estrogen and progesterone in quintuplet pregnancy induced with menotropins. Am J Obstet Gynecol 1983; 147:105.
  26. Weiss G, Goldsmith LT, Sachdev R, et al. Elevated first-trimester serum relaxin concentrations in pregnant women following ovarian stimulation predict prematurity risk and preterm delivery. Obstet Gynecol 1993; 82:821.
  27. Williams MA, Mittendorf R, Lieberman E, Monson RR. Adverse infant outcomes associated with first-trimester vaginal bleeding. Obstet Gynecol 1991; 78:14.
  28. Harger JH, Hsing AW, Tuomala RE, et al. Risk factors for preterm premature rupture of fetal membranes: a multicenter case-control study. Am J Obstet Gynecol 1990; 163:130.
  29. Ekwo EE, Gosselink CA, Moawad A. Unfavorable outcome in penultimate pregnancy and premature rupture of membranes in successive pregnancy. Obstet Gynecol 1992; 80:166.
  30. Norwitz ER, Robinson JN, Challis JR. The control of labor. N Engl J Med 1999; 341:660.
  31. Blanc WA. Pathology of the placenta, membranes, and umbilical cord in bacterial, fungal, and viral infections in man. Monogr Pathol 1981; :67.
  32. Naeye RL. Acute bacterial chorioamnionitis. Prog Clin Biol Res 1988; 281:73.
  33. Salafia CM, Vogel CA, Vintzileos AM, et al. Placental pathologic findings in preterm birth. Am J Obstet Gynecol 1991; 165:934.
  34. Klein LL, Gibbs RS. Use of microbial cultures and antibiotics in the prevention of infection-associated preterm birth. Am J Obstet Gynecol 2004; 190:1493.
  35. Williams MC, O'Brien WF, Nelson RN, Spellacy WN. Histologic chorioamnionitis is associated with fetal growth restriction in term and preterm infants. Am J Obstet Gynecol 2000; 183:1094.
  36. Faro S. Screening for bacterial vaginosis. Infect Dis Obstet Gynecol 1998; 6:235.
  37. Bejar R, Curbelo V, Davis C, Gluck L. Premature labor. II. Bacterial sources of phospholipase. Obstet Gynecol 1981; 57:479.
  38. Simhan HN, Caritis SN, Krohn MA, Hillier SL. Elevated vaginal pH and neutrophils are associated strongly with early spontaneous preterm birth. Am J Obstet Gynecol 2003; 189:1150.
  39. Donders GG, Bosmans E, Dekeersmaecker A, et al. Pathogenesis of abnormal vaginal bacterial flora. Am J Obstet Gynecol 2000; 182:872.
  40. Wen A, Srinivasan U, Goldberg D, et al. Selected vaginal bacteria and risk of preterm birth: an ecological perspective. J Infect Dis 2014; 209:1087.
  41. Schnarr J, Smaill F. Asymptomatic bacteriuria and symptomatic urinary tract infections in pregnancy. Eur J Clin Invest 2008; 38 Suppl 2:50.
  42. Meis PJ, Michielutte R, Peters TJ, et al. Factors associated with preterm birth in Cardiff, Wales. I. Univariable and multivariable analysis. Am J Obstet Gynecol 1995; 173:590.
  43. Meis PJ, Michielutte R, Peters TJ, et al. Factors associated with preterm birth in Cardiff, Wales. II. Indicated and spontaneous preterm birth. Am J Obstet Gynecol 1995; 173:597.
  44. Smaill F, Vazquez JC. Antibiotics for asymptomatic bacteriuria in pregnancy. Cochrane Database Syst Rev 2007; :CD000490.
  45. Villar J, Gülmezoglu AM, de Onis M. Nutritional and antimicrobial interventions to prevent preterm birth: an overview of randomized controlled trials. Obstet Gynecol Surv 1998; 53:575.
  46. Romero R, Oyarzun E, Mazor M, et al. Meta-analysis of the relationship between asymptomatic bacteriuria and preterm delivery/low birth weight. Obstet Gynecol 1989; 73:576.
  47. Smaill F. Antibiotics for asymptomatic bacteriuria in pregnancy. Cochrane Database Syst Rev 2001; :CD000490.
  48. Offenbacher S, Katz V, Fertik G, et al. Periodontal infection as a possible risk factor for preterm low birth weight. J Periodontol 1996; 67:1103.
  49. Offenbacher S, Lieff S, Boggess KA, et al. Maternal periodontitis and prematurity. Part I: Obstetric outcome of prematurity and growth restriction. Ann Periodontol 2001; 6:164.
  50. Boggess KA, Moss K, Murtha A, et al. Antepartum vaginal bleeding, fetal exposure to oral pathogens, and risk for preterm birth at <35 weeks of gestation. Am J Obstet Gynecol 2006; 194:954.
  51. Dasanayake AP. Poor periodontal health of the pregnant woman as a risk factor for low birth weight. Ann Periodontol 1998; 3:206.
  52. Jeffcoat MK, Geurs NC, Reddy MS, et al. Periodontal infection and preterm birth: results of a prospective study. J Am Dent Assoc 2001; 132:875.
  53. Goepfert AR, Jeffcoat MK, Andrews WW, et al. Periodontal disease and upper genital tract inflammation in early spontaneous preterm birth. Obstet Gynecol 2004; 104:777.
  54. Jeffcoat MK, Hauth JC, Geurs NC, et al. Periodontal disease and preterm birth: results of a pilot intervention study. J Periodontol 2003; 74:1214.
  55. López NJ, Smith PC, Gutierrez J. Periodontal therapy may reduce the risk of preterm low birth weight in women with periodontal disease: a randomized controlled trial. J Periodontol 2002; 73:911.
  56. Jarjoura K, Devine PC, Perez-Delboy A, et al. Markers of periodontal infection and preterm birth. Am J Obstet Gynecol 2005; 192:513.
  57. Offenbacher S, Boggess KA, Murtha AP, et al. Progressive periodontal disease and risk of very preterm delivery. Obstet Gynecol 2006; 107:29.
  58. Mitchell-Lewis D, Engebretson SP, Chen J, et al. Periodontal infections and pre-term birth: early findings from a cohort of young minority women in New York. Eur J Oral Sci 2001; 109:34.
  59. Offenbacher S, Jared HL, O'Reilly PG, et al. Potential pathogenic mechanisms of periodontitis associated pregnancy complications. Ann Periodontol 1998; 3:233.
  60. Newnham JP, Shub A, Jobe AH, et al. The effects of intra-amniotic injection of periodontopathic lipopolysaccharides in sheep. Am J Obstet Gynecol 2005; 193:313.
  61. Han YW, Redline RW, Li M, et al. Fusobacterium nucleatum induces premature and term stillbirths in pregnant mice: implication of oral bacteria in preterm birth. Infect Immun 2004; 72:2272.
  62. Xiong X, Buekens P, Fraser WD, et al. Periodontal disease and adverse pregnancy outcomes: a systematic review. BJOG 2006; 113:135.
  63. Vergnes JN, Sixou M. Preterm low birth weight and maternal periodontal status: a meta-analysis. Am J Obstet Gynecol 2007; 196:135.e1.
  64. Kornman KS, di Giovine FS. Genetic variations in cytokine expression: a risk factor for severity of adult periodontitis. Ann Periodontol 1998; 3:327.
  65. Galbraith GM, Steed RB, Sanders JJ, Pandey JP. Tumor necrosis factor alpha production by oral leukocytes: influence of tumor necrosis factor genotype. J Periodontol 1998; 69:428.
  66. Kornman KS, Crane A, Wang HY, et al. The interleukin-1 genotype as a severity factor in adult periodontal disease. J Clin Periodontol 1997; 24:72.
  67. Boggess KA, Moss K, Madianos P, et al. Fetal immune response to oral pathogens and risk of preterm birth. Am J Obstet Gynecol 2005; 193:1121.
  68. McGaw T. Periodontal disease and preterm delivery of low-birth-weight infants. J Can Dent Assoc 2002; 68:165.
  69. Stamilio DM, Chang JJ, Macones GA. Periodontal disease and preterm birth: do the data have enough teeth to recommend screening and preventive treatment? Am J Obstet Gynecol 2007; 196:93.
  70. Aidoo M, McElroy PD, Kolczak MS, et al. Tumor necrosis factor-alpha promoter variant 2 (TNF2) is associated with pre-term delivery, infant mortality, and malaria morbidity in western Kenya: Asembo Bay Cohort Project IX. Genet Epidemiol 2001; 21:201.
  71. Valkenburg-van den Berg AW, Sprij AJ, Dekker FW, et al. Association between colonization with Group B Streptococcus and preterm delivery: a systematic review. Acta Obstet Gynecol Scand 2009; 88:958.
  72. Martin DH, Koutsky L, Eschenbach DA, et al. Prematurity and perinatal mortality in pregnancies complicated by maternal Chlamydia trachomatis infections. JAMA 1982; 247:1585.
  73. Andrews WW, Goldenberg RL, Mercer B, et al. The Preterm Prediction Study: association of second-trimester genitourinary chlamydia infection with subsequent spontaneous preterm birth. Am J Obstet Gynecol 2000; 183:662.
  74. Ryan GM Jr, Abdella TN, McNeeley SG, et al. Chlamydia trachomatis infection in pregnancy and effect of treatment on outcome. Am J Obstet Gynecol 1990; 162:34.
  75. Cohen I, Veille JC, Calkins BM. Improved pregnancy outcome following successful treatment of chlamydial infection. JAMA 1990; 263:3160.
  76. Hillier SL, Nugent RP, Eschenbach DA, et al. Association between bacterial vaginosis and preterm delivery of a low-birth-weight infant. The Vaginal Infections and Prematurity Study Group. N Engl J Med 1995; 333:1737.
  77. Meis PJ, Goldenberg RL, Mercer B, et al. The preterm prediction study: significance of vaginal infections. National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network. Am J Obstet Gynecol 1995; 173:1231.
  78. Bretelle F, Rozenberg P, Pascal A, et al. High Atopobium vaginae and Gardnerella vaginalis vaginal loads are associated with preterm birth. Clin Infect Dis 2015; 60:860.
  79. Edwards LE, Barrada MI, Hamann AA, Hakanson EY. Gonorrhea in pregnancy. Am J Obstet Gynecol 1978; 132:637.
  80. Watson-Jones D, Changalucha J, Gumodoka B, et al. Syphilis in pregnancy in Tanzania. I. Impact of maternal syphilis on outcome of pregnancy. J Infect Dis 2002; 186:940.
  81. Silver BJ, Guy RJ, Kaldor JM, et al. Trichomonas vaginalis as a cause of perinatal morbidity: a systematic review and meta-analysis. Sex Transm Dis 2014; 41:369.
  82. Collins S, Ramsay M, Slack MP, et al. Risk of invasive Haemophilus influenzae infection during pregnancy and association with adverse fetal outcomes. JAMA 2014; 311:1125.
  83. Andrews WW, Klebanoff MA, Thom EA, et al. Midpregnancy genitourinary tract infection with Chlamydia trachomatis: association with subsequent preterm delivery in women with bacterial vaginosis and Trichomonas vaginalis. Am J Obstet Gynecol 2006; 194:493.
  84. Mancuso MS, Figueroa D, Szychowski JM, et al. Midtrimester bacterial vaginosis and cervical length in women at risk for preterm birth. Am J Obstet Gynecol 2011; 204:342.e1.
  85. Mass SB, Brennan JP, Silverman N, van Hoeven KH. Association between a shift in vaginal flora on Papanicolaou smear and acute chorioamnionitis and preterm delivery. Diagn Cytopathol 1999; 21:7.
  86. Luki N, Lebel P, Boucher M, et al. Comparison of polymerase chain reaction assay with culture for detection of genital mycoplasmas in perinatal infections. Eur J Clin Microbiol Infect Dis 1998; 17:255.
  87. Milman IA, Freĭmanis IaF. [Microbiological synthesis and transformation of prostaglandins]. Prikl Biokhim Mikrobiol 1986; 22:595.
  88. Ward K, Argyle V, Meade M, Nelson L. The heritability of preterm delivery. Obstet Gynecol 2005; 106:1235.
  89. Clausson B, Lichtenstein P, Cnattingius S. Genetic influence on birthweight and gestational length determined by studies in offspring of twins. BJOG 2000; 107:375.
  90. Hao K, Wang X, Niu T, et al. A candidate gene association study on preterm delivery: application of high-throughput genotyping technology and advanced statistical methods. Hum Mol Genet 2004; 13:683.
  91. Treloar SA, Macones GA, Mitchell LE, Martin NG. Genetic influences on premature parturition in an Australian twin sample. Twin Res 2000; 3:80.
  92. Annells MF, Hart PH, Mullighan CG, et al. Interleukins-1, -4, -6, -10, tumor necrosis factor, transforming growth factor-beta, FAS, and mannose-binding protein C gene polymorphisms in Australian women: Risk of preterm birth. Am J Obstet Gynecol 2004; 191:2056.
  93. Winkvist A, Mogren I, Högberg U. Familial patterns in birth characteristics: impact on individual and population risks. Int J Epidemiol 1998; 27:248.
  94. Macones GA, Parry S, Elkousy M, et al. A polymorphism in the promoter region of TNF and bacterial vaginosis: preliminary evidence of gene-environment interaction in the etiology of spontaneous preterm birth. Am J Obstet Gynecol 2004; 190:1504.
  95. Kistka ZA, DeFranco EA, Ligthart L, et al. Heritability of parturition timing: an extended twin design analysis. Am J Obstet Gynecol 2008; 199:43.e1.
  96. Velez DR, Fortunato S, Thorsen P, et al. Spontaneous preterm birth in African Americans is associated with infection and inflammatory response gene variants. Am J Obstet Gynecol 2009; 200:209.e1.
  97. Svensson AC, Sandin S, Cnattingius S, et al. Maternal effects for preterm birth: a genetic epidemiologic study of 630,000 families. Am J Epidemiol 2009; 170:1365.
  98. Wilcox AJ, Skjaerven R, Lie RT. Familial patterns of preterm delivery: maternal and fetal contributions. Am J Epidemiol 2008; 167:474.
  99. Boivin A, Luo ZC, Audibert F, et al. Risk for preterm and very preterm delivery in women who were born preterm. Obstet Gynecol 2015; 125:1177.
  100. Manuck TA, Lai Y, Meis PJ, et al. Admixture mapping to identify spontaneous preterm birth susceptibility loci in African Americans. Obstet Gynecol 2011; 117:1078.
  101. Genc MR, Onderdonk A. Endogenous bacterial flora in pregnant women and the influence of maternal genetic variation. BJOG 2011; 118:154.
  102. Tsai HJ, Hong X, Chen J, et al. Role of African ancestry and gene-environment interactions in predicting preterm birth. Obstet Gynecol 2011; 118:1081.
  103. Genc MR, Witkin SS, Delaney ML, et al. A disproportionate increase in IL-1beta over IL-1ra in the cervicovaginal secretions of pregnant women with altered vaginal microflora correlates with preterm birth. Am J Obstet Gynecol 2004; 190:1191.
  104. Simhan HN, Caritis SN, Krohn MA, et al. Decreased cervical proinflammatory cytokines permit subsequent upper genital tract infection during pregnancy. Am J Obstet Gynecol 2003; 189:560.
  105. Simhan HN, Krohn MA. First-trimester cervical inflammatory milieu and subsequent early preterm birth. Am J Obstet Gynecol 2009; 200:377.e1.
  106. Genc MR, Onderdonk AB, Vardhana S, et al. Polymorphism in intron 2 of the interleukin-1 receptor antagonist gene, local midtrimester cytokine response to vaginal flora, and subsequent preterm birth. Am J Obstet Gynecol 2004; 191:1324.
  107. Preterm Birth www.cdc.gov (Accessed on January 31, 2012).
  108. Srinivasjois RM, Shah S, Shah PS, Knowledge Synthesis Group on Determinants Of Preterm/LBW Births. Biracial couples and adverse birth outcomes: a systematic review and meta-analyses. Acta Obstet Gynecol Scand 2012; 91:1134.
  109. Iams JD, Goldenberg RL, Mercer BM, et al. The Preterm Prediction Study: recurrence risk of spontaneous preterm birth. National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network. Am J Obstet Gynecol 1998; 178:1035.
  110. Henriksen TB, Hedegaard M, Secher NJ, Wilcox AJ. Standing at work and preterm delivery. Br J Obstet Gynaecol 1995; 102:198.
  111. Newman RB, Goldenberg RL, Moawad AH, et al. Occupational fatigue and preterm premature rupture of membranes. National Institute of Child Health and Human Development Maternal-Fetal Medicine, Units Network. Am J Obstet Gynecol 2001; 184:438.
  112. McDonald AD. The 'retrait préventif': an evaluation. Can J Public Health 1994; 85:136.
  113. Launer LJ, Villar J, Kestler E, de Onis M. The effect of maternal work on fetal growth and duration of pregnancy: a prospective study. Br J Obstet Gynaecol 1990; 97:62.
  114. Luke B, Mamelle N, Keith L, et al. The association between occupational factors and preterm birth: a United States nurses' study. Research Committee of the Association of Women's Health, Obstetric, and Neonatal Nurses. Am J Obstet Gynecol 1995; 173:849.
  115. Homer CJ, Beresford SA, James SA, et al. Work-related physical exertion and risk of preterm, low birthweight delivery. Paediatr Perinat Epidemiol 1990; 4:161.
  116. Mamelle N, Laumon B, Lazar P. Prematurity and occupational activity during pregnancy. Am J Epidemiol 1984; 119:309.
  117. Ramirez G, Grimes RM, Annegers JF, et al. Occupational physical activity and other risk factors for preterm birth among US Army primigravidas. Am J Public Health 1990; 80:728.
  118. Siega-Riz AM, Herrmann TS, Savitz DA, Thorp JM. Frequency of eating during pregnancy and its effect on preterm delivery. Am J Epidemiol 2001; 153:647.
  119. Berkowitz GS, Kelsey JL, Holford TR, Berkowitz RL. Physical activity and the risk of spontaneous preterm delivery. J Reprod Med 1983; 28:581.
  120. Murphy JF, Dauncey M, Newcombe R, et al. Employment in pregnancy: prevalence, maternal characteristics, perinatal outcome. Lancet 1984; 1:1163.
  121. Henriksen TB, Savitz DA, Hedegaard M, Secher NJ. Employment during pregnancy in relation to risk factors and pregnancy outcome. Br J Obstet Gynaecol 1994; 101:858.
  122. Magann EF, Evans SF, Newnham JP. Employment, exertion, and pregnancy outcome: assessment by kilocalories expended each day. Am J Obstet Gynecol 1996; 175:182.
  123. Wildschut HI, Nas T, Golding J. Are sociodemographic factors predictive of preterm birth? A reappraisal of the 1958 British Perinatal Mortality Survey. Br J Obstet Gynaecol 1997; 104:57.
  124. Lawson CC, Whelan EA, Hibert EN, et al. Occupational factors and risk of preterm birth in nurses. Am J Obstet Gynecol 2009; 200:51.e1.
  125. Juhl M, Andersen PK, Olsen J, et al. Physical exercise during pregnancy and the risk of preterm birth: a study within the Danish National Birth Cohort. Am J Epidemiol 2008; 167:859.
  126. Barakat R, Stirling JR, Lucia A. Does exercise training during pregnancy affect gestational age? A randomised controlled trial. Br J Sports Med 2008; 42:674.
  127. Hegaard HK, Hedegaard M, Damm P, et al. Leisure time physical activity is associated with a reduced risk of preterm delivery. Am J Obstet Gynecol 2008; 198:180.e1.
  128. Owe KM, Nystad W, Skjaerven R, et al. Exercise during pregnancy and the gestational age distribution: a cohort study. Med Sci Sports Exerc 2012; 44:1067.
  129. Berghella V, Klebanoff M, McPherson C, et al. Sexual intercourse association with asymptomatic bacterial vaginosis and Trichomonas vaginalis treatment in relationship to preterm birth. Am J Obstet Gynecol 2002; 187:1277.
  130. Read JS, Klebanoff MA. Sexual intercourse during pregnancy and preterm delivery: effects of vaginal microorganisms. The Vaginal Infections and Prematurity Study Group. Am J Obstet Gynecol 1993; 168:514.
  131. Mills JL, Harlap S, Harley EE. Should coitus late in pregnancy be discouraged? Lancet 1981; 2:136.
  132. Yost NP, Owen J, Berghella V, et al. Effect of coitus on recurrent preterm birth. Obstet Gynecol 2006; 107:793.
  133. Ota E, Tobe-Gai R, Mori R, Farrar D. Antenatal dietary advice and supplementation to increase energy and protein intake. Cochrane Database Syst Rev 2012; 9:CD000032.
  134. Kramer MS. Isocaloric balanced protein supplementation in pregnancy. Cochrane Database Syst Rev 2000; :CD000118.
  135. Olsen SF, Secher NJ. Low consumption of seafood in early pregnancy as a risk factor for preterm delivery: prospective cohort study. BMJ 2002; 324:447.
  136. Olsen SF, Secher NJ, Tabor A, et al. Randomised clinical trials of fish oil supplementation in high risk pregnancies. Fish Oil Trials In Pregnancy (FOTIP) Team. BJOG 2000; 107:382.
  137. Onwude JL, Lilford RJ, Hjartardottir H, et al. A randomised double blind placebo controlled trial of fish oil in high risk pregnancy. Br J Obstet Gynaecol 1995; 102:95.
  138. Bloomfield FH, Oliver MH, Hawkins P, et al. Periconceptional undernutrition in sheep accelerates maturation of the fetal hypothalamic-pituitary-adrenal axis in late gestation. Endocrinology 2004; 145:4278.
  139. Han Z, Mulla S, Beyene J, et al. Maternal underweight and the risk of preterm birth and low birth weight: a systematic review and meta-analyses. Int J Epidemiol 2011; 40:65.
  140. McDonald SD, Han Z, Mulla S, et al. Overweight and obesity in mothers and risk of preterm birth and low birth weight infants: systematic review and meta-analyses. BMJ 2010; 341:c3428.
  141. Honest H, Bachmann LM, Ngai C, et al. The accuracy of maternal anthropometry measurements as predictor for spontaneous preterm birth--a systematic review. Eur J Obstet Gynecol Reprod Biol 2005; 119:11.
  142. Carmichael SL, Abrams B. A critical review of the relationship between gestational weight gain and preterm delivery. Obstet Gynecol 1997; 89:865.
  143. Dietz PM, Callaghan WM, Cogswell ME, et al. Combined effects of prepregnancy body mass index and weight gain during pregnancy on the risk of preterm delivery. Epidemiology 2006; 17:170.
  144. Hickey CA, Cliver SP, McNeal SF, et al. Prenatal weight gain patterns and spontaneous preterm birth among nonobese black and white women. Obstet Gynecol 1995; 85:909.
  145. Schieve LA, Cogswell ME, Scanlon KS. An empiric evaluation of the Institute of Medicine's pregnancy weight gain guidelines by race. Obstet Gynecol 1998; 91:878.
  146. Stotland NE, Caughey AB, Lahiff M, Abrams B. Weight gain and spontaneous preterm birth: the role of race or ethnicity and previous preterm birth. Obstet Gynecol 2006; 108:1448.
  147. SIMPSON WJ. A preliminary report on cigarette smoking and the incidence of prematurity. Am J Obstet Gynecol 1957; 73:807.
  148. Meyer MB, Tonascia JA. Maternal smoking, pregnancy complications, and perinatal mortality. Am J Obstet Gynecol 1977; 128:494.
  149. Shiono PH, Klebanoff MA, Rhoads GG. Smoking and drinking during pregnancy. Their effects on preterm birth. JAMA 1986; 255:82.
  150. Berkowitz GS, Papiernik E. Epidemiology of preterm birth. Epidemiol Rev 1993; 15:414.
  151. Cnattingius S, Forman MR, Berendes HW, et al. Effect of age, parity, and smoking on pregnancy outcome: a population-based study. Am J Obstet Gynecol 1993; 168:16.
  152. Harlow BL, Frigoletto FD, Cramer DW, et al. Determinants of preterm delivery in low-risk pregnancies. The RADIUS Study Group. J Clin Epidemiol 1996; 49:441.
  153. Kyrklund-Blomberg NB, Cnattingius S. Preterm birth and maternal smoking: risks related to gestational age and onset of delivery. Am J Obstet Gynecol 1998; 179:1051.
  154. Nicholson W, Croughan-Minihane M, Posner S, et al. Preterm delivery in patients admitted with preterm labor: a prediction study. J Matern Fetal Med 2001; 10:102.
  155. Ney JA, Dooley SL, Keith LG, et al. The prevalence of substance abuse in patients with suspected preterm labor. Am J Obstet Gynecol 1990; 162:1562.
  156. Spence MR, Williams R, DiGregorio GJ, et al. The relationship between recent cocaine use and pregnancy outcome. Obstet Gynecol 1991; 78:326.
  157. Borges G, Lopez-Cervantes M, Medina-Mora ME, et al. Alcohol consumption, low birth weight, and preterm delivery in the National Addiction Survey (Mexico). Int J Addict 1993; 28:355.
  158. Wilkins-Haug L, Gabow PA. Toluene abuse during pregnancy: obstetric complications and perinatal outcomes. Obstet Gynecol 1991; 77:504.
  159. Boer K, Smit BJ, van Huis AM, Hogerzeil HV. Substance use in pregnancy: do we care? Acta Paediatr Suppl 1994; 404:65.
  160. Almario CV, Seligman NS, Dysart KC, et al. Risk factors for preterm birth among opiate-addicted gravid women in a methadone treatment program. Am J Obstet Gynecol 2009; 201:326.e1.
  161. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6409a3.htm?s_cid=mm6409a3_e.
  162. Lockwood CJ. Stress-associated preterm delivery: the role of corticotropin-releasing hormone. Am J Obstet Gynecol 1999; 180:S264.
  163. Petraglia F, Hatch MC, Lapinski R, et al. Lack of effect of psychosocial stress on maternal corticotropin-releasing factor and catecholamine levels at 28 weeks' gestation. J Soc Gynecol Investig 2001; 8:83.
  164. Lu MC, Chen B. Racial and ethnic disparities in preterm birth: the role of stressful life events. Am J Obstet Gynecol 2004; 191:691.
  165. Kramer MS, Lydon J, Séguin L, et al. Stress pathways to spontaneous preterm birth: the role of stressors, psychological distress, and stress hormones. Am J Epidemiol 2009; 169:1319.
  166. Hedegaard M, Henriksen TB, Sabroe S, Secher NJ. Psychological distress in pregnancy and preterm delivery. BMJ 1993; 307:234.
  167. Peacock JL, Bland JM, Anderson HR. Preterm delivery: effects of socioeconomic factors, psychological stress, smoking, alcohol, and caffeine. BMJ 1995; 311:531.
  168. Hedegaard M, Henriksen TB, Sabroe S, Secher NJ. The relationship between psychological distress during pregnancy and birth weight for gestational age. Acta Obstet Gynecol Scand 1996; 75:32.
  169. Dole N, Savitz DA, Hertz-Picciotto I, et al. Maternal stress and preterm birth. Am J Epidemiol 2003; 157:14.
  170. Shaw JG, Asch SM, Kimerling R, et al. Posttraumatic stress disorder and risk of spontaneous preterm birth. Obstet Gynecol 2014; 124:1111.
  171. Chen MJ, Grobman WA, Gollan JK, Borders AE. The use of psychosocial stress scales in preterm birth research. Am J Obstet Gynecol 2011; 205:402.
  172. Hodnett ED, Fredericks S. Support during pregnancy for women at increased risk of low birthweight babies. Cochrane Database Syst Rev 2003; :CD000198.
  173. Newman RB, Goldenberg RL, Iams JD, et al. Preterm prediction study: comparison of the cervical score and Bishop score for prediction of spontaneous preterm delivery. Obstet Gynecol 2008; 112:508.
  174. Koike T, Minakami H, Kosuge S, et al. Uterine leiomyoma in pregnancy: its influence on obstetric performance. J Obstet Gynaecol Res 1999; 25:309.
  175. Davis JL, Ray-Mazumder S, Hobel CJ, et al. Uterine leiomyomas in pregnancy: a prospective study. Obstet Gynecol 1990; 75:41.
  176. Rice JP, Kay HH, Mahony BS. The clinical significance of uterine leiomyomas in pregnancy. Am J Obstet Gynecol 1989; 160:1212.
  177. Heinonen PK. Unicornuate uterus and rudimentary horn. Fertil Steril 1997; 68:224.
  178. Cooney MJ, Benson CB, Doubilet PM. Outcome of pregnancies in women with uterine duplication anomalies. J Clin Ultrasound 1998; 26:3.
  179. Propst AM, Hill JA 3rd. Anatomic factors associated with recurrent pregnancy loss. Semin Reprod Med 2000; 18:341.
  180. Bortoli A, Saibeni S, Tatarella M, et al. Pregnancy before and after the diagnosis of inflammatory bowel diseases: retrospective case-control study. J Gastroenterol Hepatol 2007; 22:542.
  181. Thangaratinam S, Tan A, Knox E, et al. Association between thyroid autoantibodies and miscarriage and preterm birth: meta-analysis of evidence. BMJ 2011; 342:d2616.
  182. Gleicher N. Does the immune system induce labor? Lessons from preterm deliveries in women with autoimmune diseases. Clin Rev Allergy Immunol 2010; 39:194.
  183. Scanlon KS, Yip R, Schieve LA, Cogswell ME. High and low hemoglobin levels during pregnancy: differential risks for preterm birth and small for gestational age. Obstet Gynecol 2000; 96:741.
  184. Xiong X, Buekens P, Alexander S, et al. Anemia during pregnancy and birth outcome: a meta-analysis. Am J Perinatol 2000; 17:137.
  185. Zeisel SH, Allen LH, Coburn SP, et al. Nutrition: a reservoir for integrative science. J Nutr 2001; 131:1319.
  186. Scholl TO, Reilly T. Anemia, iron and pregnancy outcome. J Nutr 2000; 130:443S.
  187. Kongnyuy, E, van den Broek, N. Effects of routine oral supplementation with or without folic acid for women during pregnancy: RHL commentary (last revised: 4 January 2007). The WHO Reproductive Health Library; Geneva: World Health Organization.
  188. Lackman F, Capewell V, Richardson B, et al. The risks of spontaneous preterm delivery and perinatal mortality in relation to size at birth according to fetal versus neonatal growth standards. Am J Obstet Gynecol 2001; 184:946.
  189. Morken NH, Källen K, Jacobsson B. Fetal growth and onset of delivery: a nationwide population-based study of preterm infants. Am J Obstet Gynecol 2006; 195:154.
  190. Ott WJ. Intrauterine growth retardation and preterm delivery. Am J Obstet Gynecol 1993; 168:1710.
  191. Zeitlin J, Ancel PY, Saurel-Cubizolles MJ, Papiernik E. The relationship between intrauterine growth restriction and preterm delivery: an empirical approach using data from a European case-control study. BJOG 2000; 107:750.
  192. Gardosi JO. Prematurity and fetal growth restriction. Early Hum Dev 2005; 81:43.
  193. Wadhwa PD, Garite TJ, Porto M, et al. Placental corticotropin-releasing hormone (CRH), spontaneous preterm birth, and fetal growth restriction: a prospective investigation. Am J Obstet Gynecol 2004; 191:1063.
  194. Dolan SM, Gross SJ, Merkatz IR, et al. The contribution of birth defects to preterm birth and low birth weight. Obstet Gynecol 2007; 110:318.
  195. Purisch SE, DeFranco EA, Muglia LJ, et al. Preterm birth in pregnancies complicated by major congenital malformations: a population-based study. Am J Obstet Gynecol 2008; 199:287.e1.
  196. Zeitlin J, Saurel-Cubizolles MJ, De Mouzon J, et al. Fetal sex and preterm birth: are males at greater risk? Hum Reprod 2002; 17:2762.
  197. Ingemarsson I. Gender aspects of preterm birth. BJOG 2003; 110 Suppl 20:34.
  198. Zeitlin J, Ancel PY, Larroque B, et al. Fetal sex and indicated very preterm birth: results of the EPIPAGE study. Am J Obstet Gynecol 2004; 190:1322.
  199. McGregor JA, Leff M, Orleans M, Baron A. Fetal gender differences in preterm birth: findings in a North American cohort. Am J Perinatol 1992; 9:43.
  200. Ghidini A, Salafia CM. Gender differences of placental dysfunction in severe prematurity. BJOG 2005; 112:140.
  201. Goldenberg RL, Andrews WW, Faye-Petersen OM, et al. The Alabama Preterm Birth Study: intrauterine infection and placental histologic findings in preterm births of males and females less than 32 weeks. Am J Obstet Gynecol 2006; 195:1533.
  202. Basso O, Olsen J, Christensen K. Low birthweight and prematurity in relation to paternal factors: a study of recurrence. Int J Epidemiol 1999; 28:695.
  203. Boyd HA, Poulsen G, Wohlfahrt J, et al. Maternal contributions to preterm delivery. Am J Epidemiol 2009; 170:1358.
  204. Smith GC, Wood AM, Pell JP, Dobbie R. Sudden infant death syndrome and complications in other pregnancies. Lancet 2005; 366:2107.
  205. Conde-Agudelo A, Papageorghiou AT, Kennedy SH, Villar J. Novel biomarkers for the prediction of the spontaneous preterm birth phenotype: a systematic review and meta-analysis. BJOG 2011; 118:1042.