title: Hemoglobin concentration in multiple versus singleton pregnancies--retrospective evidence for physiology not pathology.
abstract: OBJECTIVE
  To describe hemoglobin (Hb) levels and percentiles for multiple pregnancies and to compare them with reference ranges of singleton pregnancies as published by the Center for Disease Control.
  
  
  METHODS
  Maternal hemoglobin and red cell indices were compared: (a) between multiple and singleton pregnancies of our hospital and (b) with trimester-specific cut-offs for singleton pregnancies obtained from literature.
  
  
  RESULTS
  While early pregnancy hemoglobin values were similar in multiple and singleton pregnancies, second trimester values decreased much faster in multiple pregnancies. Nadir in multiple pregnancies was reached in weeks 24-28 of gestation. In the third trimester mean hemoglobin values were higher in multiple pregnancies, while 5th and 10th percentile were still lower compared to singleton pregnancies at term. The mean whole-pregnancy hemoglobin exceeded the Center for Disease Control (CDC) cut-off for anemia in singleton pregnancies (fifth percentile) and red cell indices remained constant.
  
  
  CONCLUSIONS
  We conclude that with a significantly lower fifth percentile than introduced by the CDC in all trimesters, there should be different cut-offs for anemia in multiple pregnancies considered. A more pronounced decrease in multiple pregnancy hemoglobin levels seems to be physiological.
authors.affiliation: Clinic of Obstetrics, Department of Obstetrics and Gynaecology, Division of Perinatal Physiology, University Hospital Zurich, Frauenklinikstrasse 10, CH 8091, Zurich, Switzerland. alexander.krafft@fhk.usz.ch
authors.forename: A
authors.initials: A
authors.lastname: Krafft
authors.forename: C
authors.initials: C
authors.lastname: Breymann
authors.forename: J
authors.initials: J
authors.lastname: Streich
authors.forename: R
authors.initials: R
authors.lastname: Huch
authors.forename: A
authors.initials: A
authors.lastname: Huch
mesh_terms: D000328:Adult; D004909:Erythrocyte Indices; D005260:Female; D005865:Gestational Age; D006400:Hematocrit; D006454:Hemoglobins / Q000032:analysis*; D006801:Humans; D008875:Middle Aged; D011247:Pregnancy; D011272:Pregnancy, Multiple / Q000097:blood*; D012016:Reference Values; D012189:Retrospective Studies; D015430:Weight Gain
chemical_list: D006454:Hemoglobins