title: Ultrasound estimated fetal weight slightly below the median is associated with increased risk of spontaneous preterm birth.
abstract: BACKGROUND
  This study investigates the possible relationship between ultrasound estimated fetal weight (EFW) at third trimester and the risk of preterm birth following spontaneous preterm labor in otherwise uncomplicated pregnancies.
  
  
  METHODS
  We performed a nested case-control study including 281 cases of spontaneous preterm labor with preterm delivery in the third trimester and 3372 matched controls within a cohort of 6207 consecutive pregnant women. Pregnancies with fetal growth restriction (birth weight <10th centile of population-based normograms) or fetal anomalies were not included. EFW was calculated by using Hadlock's formula and converted to fetal gender adjusted multiples of median (MoM) for each gestational age.
  
  
  RESULTS
  EFW correlated with birth weight (r = 0.959, p < 0.0001) and was lower in preterm than in control fetuses (p < 0.0001). The odds ratios (95% confidence intervals) for preterm birth for fetuses below 0.9 MoM, 0.85 MoM, 0.80 MoM, and 0.75 MoM of EFW were, respectively, 4.6 (3.6-5.9), 5.7 (4.3-7.5), 8.5 (5.9-12.1), and 11.2 (6.8-18.3). The independent relationship between preterm birth and lower EFW was confirmed in multivariate analysis with adjustment for potential confounders, such as maternal age, parity, and fetal gender.
  
  
  CONCLUSIONS
  In asymptomatic women between 28 and 36 weeks of gestation, an EFW lower than 0.90 MoM increases by 4.6 times the risk of spontaneous preterm birth, and the risk increases proportionally to the degree of weight reduction.
authors.affiliation: Department of Pediatrics, Obstetrics and Reproductive Medicine, Section of Obstetrics and Gynaecology, University of Siena, Policlinico Santa Maria alle Scotte, Viale Bracci, 53100, Siena, Italy. severi@unisi.it
authors.forename: Filiberto M
authors.initials: FM
authors.lastname: Severi
authors.forename: Caterina
authors.initials: C
authors.lastname: Bocchi
authors.forename: Alberto
authors.initials: A
authors.lastname: Imperatore
authors.forename: Carlotta
authors.initials: C
authors.lastname: Boni
authors.forename: Chiara
authors.initials: C
authors.lastname: Ferrata
authors.forename: Pasquale
authors.initials: P
authors.lastname: Florio
authors.forename: Fernando M
authors.initials: FM
authors.lastname: Reis
authors.forename: Felice
authors.initials: F
authors.lastname: Petraglia
mesh_terms: D000328:Adult; D001724:Birth Weight; D016022:Case-Control Studies; D005260:Female; D020567:Fetal Weight; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D016017:Odds Ratio; D011247:Pregnancy; D047928:Premature Birth / Q000000981:diagnostic imaging*; D012307:Risk Factors; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal