title: Placenta mediates the association between maternal second-hand smoke exposure during pregnancy and small for gestational age.
abstract: BACKGROUND
  The causal relationship between maternal second-hand smoke (SHS) exposure during pregnancy and small for gestational-age (SGA) has been affirmed, but the mechanism is still unclear. Previous studies have found that the placenta remarkably affects fetal intrauterine growth and that SHS exposure during pregnancy impairs placental growth and decreases placental weight. Therefore, the placenta may mediate the association between maternal SHS exposure during pregnancy and SGA. This study explores whether and to what extent the association between maternal SHS exposure during pregnancy and SGA is mediated by the placenta.
  
  
  METHODS
  We investigated 562 pregnant women delivering SGA newborns (cases) and 1581 delivering appropriate-for-gestational-age newborns (controls) in this case-control study. Information on maternal SHS exposure during pregnancy, socio-demographic characteristics and obstetric conditions, including placental weight, were collected at the Maternity and Child Health Care Hospitals of Shenzhen and Foshan in Guangdong, China. Linear and hierarchical logistic regression models were fitted to examine the mediation effects of placental weight on the association between maternal SHS exposure during pregnancy and SGA.
  
  
  RESULTS
  After controlling for ethnicity, maternal age, educational level, family income, pre-pregnancy body mass index (BMI), parity, gestational age and newborn gender, maternal SHS exposure during pregnancy was associated with a higher SGA risk (adjusted odds ratio (OR) = 1.26; 95% confidence interval (CI) = 1.03-1.55) and lower placental weight (standard deviation (SD) = -0.15, SE = 0.04). Regression models illustrated that placental weight partially mediated (49.6%; 95% CI = 35.9-63.3%) the association between SHS exposure during pregnancy and SGA.
  
  
  CONCLUSIONS
  Our findings suggest that the placenta plays an intermediary role in how maternal prenatal SHS exposure affects fetal growth.
authors.affiliation: Department of Biostatistics and Epidemiology, School of Public Health, Sun Yat-sen University, Guangzhou, China.
authors.forename: Z
authors.initials: Z
authors.lastname: Niu
authors.affiliation: Department of Biostatistics and Epidemiology, School of Public Health, Sun Yat-sen University, Guangzhou, China.
authors.forename: C
authors.initials: C
authors.lastname: Xie
authors.affiliation: Division of Behavioral Medicine, Department of Pediatrics, School of Medicine and Biomedical Sciences, State University of New York at Buffalo, Buffalo, NY, USA.
authors.forename: X
authors.initials: X
authors.lastname: Wen
authors.affiliation: Department of Biostatistics and Epidemiology, School of Public Health, Sun Yat-sen University, Guangzhou, China.
authors.forename: F
authors.initials: F
authors.lastname: Tian
authors.affiliation: Department of Biostatistics and Epidemiology, School of Public Health, Sun Yat-sen University, Guangzhou, China.
authors.forename: P
authors.initials: P
authors.lastname: Ding
authors.affiliation: Department of Biostatistics and Epidemiology, School of Public Health, Sun Yat-sen University, Guangzhou, China.
authors.forename: Y
authors.initials: Y
authors.lastname: He
authors.affiliation: Department of Science and Education, Shenzhen Women and Children's Hospital, Shenzhen, China.
authors.forename: J
authors.initials: J
authors.lastname: Lin
authors.affiliation: Department of Science and Education, Shenzhen Women and Children's Hospital, Shenzhen, China.
authors.forename: S
authors.initials: S
authors.lastname: Yuan
authors.affiliation: Department of Obstetrical, Foshan Women and Children's Hospital, Foshan, China.
authors.forename: X
authors.initials: X
authors.lastname: Guo
authors.affiliation: Department of Obstetrical, Foshan Women and Children's Hospital, Foshan, China.
authors.forename: D
authors.initials: D
authors.lastname: Jia
authors.affiliation: Department of Biostatistics and Epidemiology, School of Public Health, Sun Yat-sen University, Guangzhou, China. Electronic address: chenwq@mail.sysu.edu.cn.
authors.forename: W-Q
authors.initials: WQ
authors.lastname: Chen
mesh_terms: D001724:Birth Weight; D016022:Case-Control Studies; D005260:Female; D047109:Fetal Development / Q000502:physiology*; D005317:Fetal Growth Retardation / Q000209:etiology* / Q000378:metabolism; D006801:Humans; D007231:Infant, Newborn; D007236:Infant, Small for Gestational Age / Q000378:metabolism*; D018811:Maternal Exposure / Q000009:adverse effects*; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D014028:Tobacco Smoke Pollution / Q000009:adverse effects*
chemical_list: D014028:Tobacco Smoke Pollution
keywords: Mediation; Placental weight; Second-hand smoke; Small for gestational age