title: Gestational age-specific reference ranges for amniotic fluid assessment in monochorionic diamniotic twin pregnancies.
abstract: OBJECTIVE
  To establish gestational age-specific reference ranges for amniotic fluid measurements in monochorionic diamniotic twin pregnancies, to compare them with previously reported singleton and twin reference ranges and to examine the rationale for using a gestational age-dependent cut-off to define polyhydramnios in twin-twin transfusion syndrome, as is the practice in most European centers.
  
  
  METHODS
  We retrospectively evaluated amniotic fluid volume in 32 monochorionic diamniotic twin pregnancies that were followed longitudinally at 2-week intervals from the first trimester until birth. Amniotic fluid volume was assessed by measuring the deepest vertical pocket in both amniotic sacs. We used multilevel modeling to estimate the gestational age-specific reference ranges for deepest vertical pocket measurements.
  
  
  RESULTS
  Based on 429 observations in 64 fetuses, we constructed gestational age-specific reference ranges from 11 weeks until term. The deepest pocket increased from the first trimester to reach a maximum at 26 weeks, followed by a gradual decrease towards term. Measurements between 18 and 28 weeks were comparable to those in singleton pregnancies. However, before 18 weeks values were higher, whereas after 28 weeks they were lower, as compared to singleton references.
  
  
  CONCLUSIONS
  In monochorionic twin pregnancies, the deepest vertical pocket is a gestational age-dependent measurement. Therefore, a gestational age-dependent definition of polyhydramnios in twin-twin transfusion syndrome, as used by most European centers, seems a logical approach.
authors.affiliation: Fetal Medicine Unit, Department of Obstetrics and Gynecology, University Hospitals Leuven.
authors.forename: P
authors.initials: P
authors.lastname: Dekoninck
authors.forename: J
authors.initials: J
authors.lastname: Deprest
authors.forename: P
authors.initials: P
authors.lastname: Lewi
authors.forename: J
authors.initials: J
authors.lastname: Richter
authors.forename: S
authors.initials: S
authors.lastname: Galjaard
authors.forename: J
authors.initials: J
authors.lastname: Van Keirsbilck
authors.forename: K
authors.initials: K
authors.lastname: Van Calsteren
authors.forename: L
authors.initials: L
authors.lastname: Lewi
mesh_terms: D000328:Adult; D000650:Amnion / Q000000981:diagnostic imaging; D000653:Amniotic Fluid / Q000000981:diagnostic imaging*; D000704:Analysis of Variance; D005260:Female; D005330:Fetofetal Transfusion / Q000000981:diagnostic imaging*; D005865:Gestational Age; D006801:Humans; D006831:Polyhydramnios / Q000000981:diagnostic imaging*; D011247:Pregnancy; D059285:Pregnancy, Twin; D012016:Reference Values; D014430:Twins, Monozygotic; D014463:Ultrasonography; D055815:Young Adult