title: New population-based references for birth weight, length, and head circumference in singletons and twins from 23 to 43 gestation weeks.
abstract: OBJECTIVE
  Birth size curves are needed for clinical and epidemiological purposes. We constructed birth weight (BW), length (BL), and head circumference (BHC) references, assessed effects of twinness and parity, and defined cut-off points for small, appropriate, and large for gestational age.
  
  
  METHODS
  Birth register data of all 753,036 infants born in 1996-2008 in Finland were cleaned to create references reflecting optimal intrauterine growth. The final data included 533,666 singletons and 15,033 twins (median gestation weeks (gws) 40.0 and 37.1, respectively, 41.6% primiparous). Sex-specific BW, BL, and BHC references were constructed from 23 to 43 gws separately for singletons and twins born to primiparous or multiparous mothers. GAMLSS method was used for modelling.
  
  
  RESULTS
  In singletons from 36 gws onwards, increased BW and BL were observed in comparison to previous reference from 1979-1983. Twins diverged from singletons from 30 gws onwards. At 37.0 gws, mean BW was 400 g lower and mean BL 1.2 cm shorter than in singletons. From 30 gws onwards, birth size was larger in infants of multiparous than primiparous mothers.
  
  
  CONCLUSIONS
  Population-based birth size references are available for the evaluation of birth size. Accounting for plurality and parity improves the accuracy of birth size evaluation.
authors.affiliation: Department of Pediatrics, Kuopio University Hospital, Finland. ulla.sankilampi@kuh.fi
authors.forename: Ulla
authors.initials: U
authors.lastname: Sankilampi
authors.forename: Marja-Leena
authors.initials: ML
authors.lastname: Hannila
authors.forename: Antti
authors.initials: A
authors.lastname: Saari
authors.forename: Mika
authors.initials: M
authors.lastname: Gissler
authors.forename: Leo
authors.initials: L
authors.lastname: Dunkel
mesh_terms: D001724:Birth Weight; D001827:Body Height; D005260:Female; D047109:Fetal Development / Q000502:physiology*; D005387:Finland; D005865:Gestational Age; D006257:Head / Q000033:anatomy & histology*; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D015233:Models, Statistical; D010298:Parity; D011247:Pregnancy; D012016:Reference Values; D012042:Registries; D014427:Twins