title: A retrospective cohort study of factors relating to the longitudinal change in birth weight.
abstract: BACKGROUND
  Recent reports have shown a decrease in birth weight, a change from prior steady increases. Therefore we sought to describe the demographic and anthropometric changes in singleton term fetal growth.
  
  
  METHODS
  This was a retrospective cohort analysis of term singleton deliveries (37-42 weeks) from January 1, 1995 to January 1, 2010 at a single tertiary obstetric unit. We included all 43,217 neonates from term, singleton, non-anomalous pregnancies. Data were grouped into five 3-year intervals. Mean and median birth weight (BW), birth length (BL), and Ponderal Index (PI) were estimated by year, race and gestational age. Our primary outcome was change in BW over time. The secondary outcomes were changes in BL and PI over time.
  
  
  RESULTS
  Mean and median BW decreased by 72 and 70 g respectively (p < 0.0001) over the 15 year period while BL also significantly decreased by 1.0 cm (P < 0.001). This contributed to an increase in the neonatal PI by 0.11 kg/m(3) (P < 0.001). Mean gestational age at delivery decreased while maternal BMI at delivery, hypertension, diabetes, and African American race increased. Adjusting for gestational age, race, infant sex, maternal BMI, smoking, diabetes, hypertension, and parity, year of birth contributed 0.1 % to the variance (-1.7 g/year; 26 g) of BW, 1.8% (-0.06 cm/year; 0.9 cm) of BL, and 0.7% (+0.008 kg/m(3)/year; 0.12 kg/m(3)) of PI. These findings were independent of the proportional change in race or gestational age.
  
  
  CONCLUSIONS
  We observed a crude decrease in mean BW of 72 g and BL of 1 cm over 15 years. Furthermore, once controlling for gestational age, race, infant sex, maternal BMI, smoking, diabetes, hypertension, and parity, we identified that increasing year of birth was associated with a decrease in BW of 1.7 g/year. The significant increase in PI, despite the decrease in BW emphasizes the limitation of using birth weight alone to define changes in fetal growth.
authors.affiliation: Department of Obstetrics and Gynecology, MetroHealth Medical Center, Case Western Reserve University School of Medicine, 2500 Metrohealth Drive, Cleveland, OH, 44109, USA. kgibson@metrohealth.org.
authors.forename: Kelly S
authors.initials: KS
authors.lastname: Gibson
authors.affiliation: Loyola University Medical Center, 2160 S 1st Ave, Maywood, IL, 60153, USA. tedwatersmd@gmail.com.
authors.forename: Thaddeus P
authors.initials: TP
authors.lastname: Waters
authors.affiliation: Center for Health Care Research & Policy, Case Western Reserve University School of Medicine, 2500 Metrohealth Drive, Cleveland, OH, 44109, USA. dgunzler@metrohealth.org.
authors.forename: Douglas D
authors.initials: DD
authors.lastname: Gunzler
authors.affiliation: Department of Obstetrics and Gynecology, MetroHealth Medical Center, Case Western Reserve University School of Medicine, 2500 Metrohealth Drive, Cleveland, OH, 44109, USA. pcatalano@metrohealth.org.
authors.forename: Patrick M
authors.initials: PM
authors.lastname: Catalano
mesh_terms: D000328:Adult; D001724:Birth Weight; D001827:Body Height; D015992:Body Mass Index; D036861:Delivery, Obstetric; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D008423:Maternal Age; D011247:Pregnancy; D011248:Pregnancy Complications; D044469:Racial Groups; D012189:Retrospective Studies; D047929:Term Birth; D014481:United States; D055815:Young Adult