title: The contribution of maternal race/ethnicity and fetal sex to prematurity in twins.
abstract: OBJECTIVE
  The contribution of male fetal sex to twin preterm birth (PTB) rates was evaluated in the United States.
  
  
  METHODS
  The 2002 National Center for Health Statistics Natality database was analyzed for fetal sex and twin gestations based on birth sex for MM and FF pairs (M, male; F, female). Multivariable Cox proportional hazards regression was applied to estimate the hazard ratio for PTB with adjustments for known risks.
  
  
  RESULTS
  Thirty-three thousand nine hundred twenty-six pairs were analyzed. PTBs between 20-36 completed weeks occurred in 59.1% MM pairs and 57.5% FF pairs. MM sex was an independent risk factor for PTB. This effect was greatest for deliveries between 20-29 weeks (hazard ratio, 1.224; 95% CI, 1.113-1.346). The effect was most pronounced in non-Hispanic white MM pairs.
  
  
  CONCLUSIONS
  In the United States, MM pairs are at greater risk for PTB. Race and ethnicity modulate sex effects. Further studies are needed to understand potential mechanisms.
authors.affiliation: Division of Maternal-Fetal Medicine, the Warren Alpert Medical School of Brown University, Women & Infants' Hospital of Rhode Island, Brown University, Providence, RI, USA.
authors.forename: Tanya L
authors.initials: TL
authors.lastname: Dailey
authors.forename: Asha
authors.initials: A
authors.lastname: Jayakrishnan
authors.forename: Maureen
authors.initials: M
authors.lastname: Phipps
authors.forename: Christina A
authors.initials: CA
authors.lastname: Raker
authors.forename: Edward K
authors.initials: EK
authors.lastname: Chien
mesh_terms: D005006:Ethnicity / Q000706:statistics & numerical data*; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D007234:Infant, Premature; D008297:Male; D011247:Pregnancy; D011272:Pregnancy, Multiple / Q000208:ethnology*; D047928:Premature Birth / Q000453:epidemiology* / Q000208:ethnology; D016016:Proportional Hazards Models; D012737:Sex Factors; D014427:Twins; D014481:United States / Q000453:epidemiology