title: The heavier the better? Birthweight and perinatal mortality in different ethnic groups.
abstract: BACKGROUND
  Mother's ethnicity is associated with her baby's birthweight and risk of perinatal mortality. Given the close relation between birthweight and perinatal mortality, we explored whether ethnic differences in birthweight explain ethnic differences in perinatal mortality.
  
  
  METHODS
  Data on all births to mothers born in Norway (808 658), Pakistan (6854), Vietnam (3283) and North Africa (1461) from 1980 to 1995 were obtained from the Medical Birth Registry of Norway. The associations between birthweight and perinatal mortality among ethnic groups were analysed using univariate and multivariate methods.
  
  
  RESULTS
  Mean birthweights were low for Vietnamese and Pakistani mothers (3202 g, 3244 g) and high for Norwegian and North African mothers (3530 g, 3559 g). Mean birthweights were largely unrelated to perinatal mortality, which was lowest for Vietnamese (8.2/1000, 95% CI: 5.1-11.3) and highest for Pakistanis (14.9/1000, 95% CI: 12.0-17.7). Intermediate perinatal mortality rates were found among Norwegians (9.5/1000, 95% CI: 9.3-9.7) and North Africans (9.6/1000, 95% CI: 4.6-14.6). Further comparison of weight-specific mortality rates between the two largest ethnic groups showed the low birthweight paradox, where among low-weight births, perinatal mortality was lower among Pakistani than among Norwegian babies. However, adjustment to a relative birthweight scale (units of standard deviations from population-specific mean value) revealed higher rates of weight-specific mortality among Pakistanis across the entire range of birthweights. Multivariate adjustment for relative birthweight and other factors did not change these results.
  
  
  CONCLUSIONS
  Differences in perinatal mortality between the ethnic groups were not explained by differences in mean birthweight. Paradoxical differences in birthweight-specific mortality rates could be resolved by adjustment to a relative scale.
authors.affiliation: Department of Obstetrics and Gynaecology, the National Hospital, N-0027 Oslo, Norway. siri.vangen@folkehelsa.no
authors.forename: Siri
authors.initials: S
authors.lastname: Vangen
authors.forename: Camilla
authors.initials: C
authors.lastname: Stoltenberg
authors.forename: Rolv
authors.initials: R
authors.lastname: Skjaerven
authors.forename: Per
authors.initials: P
authors.lastname: Magnus
authors.forename: Jennifer R
authors.initials: JR
authors.lastname: Harris
authors.forename: Babill
authors.initials: B
authors.lastname: Stray-Pedersen
mesh_terms: D000328:Adult; D000352:Africa, Northern / Q000208:ethnology; D000704:Analysis of Variance; D001208:Asia / Q000208:ethnology; D001724:Birth Weight; D005006:Ethnicity / Q000706:statistics & numerical data*; D005260:Female; D006801:Humans; D007226:Infant Mortality; D007231:Infant, Newborn; D015999:Multivariate Analysis; D009664:Norway / Q000453:epidemiology; D011247:Pregnancy; D012306:Risk