title: Lower perinatal mortality in preterm born twins than in singletons: a nationwide study from The Netherlands.
abstract: BACKGROUND
  Twin pregnancies are at increased risk for perinatal morbidity and death because of many factors that include a high incidence of preterm delivery. Compared with singleton pregnancies, overall perinatal risk of death is higher in twin pregnancies; however, for the preterm period, the perinatal mortality rate has been reported to be lower in twins.
  
  
  OBJECTIVE
  The purpose of this study was to compare perinatal mortality rates in relation to gestational age at birth between singleton and twin pregnancies, taking into account socioeconomic status, fetal sex, and parity.
  
  
  METHODS
  We studied perinatal mortality rates according to gestational age at birth in 1,502,120 singletons pregnancies and 51,658 twin pregnancies without congenital malformations who were delivered between 2002 and 2010 after 28 weeks of gestation. Data were collected from the nationwide Netherlands Perinatal Registry.
  
  
  RESULTS
  Overall the perinatal mortality rate in twin pregnancies (6.6/1000 infants) was higher than in singleton pregnancies (4.1/1000 infants). However, in the preterm period, the perinatal mortality rate in twin pregnancies was substantially lower than in singleton pregnancies (10.4 per 1000 infants as compared with 34.5 per 1000 infants, respectively) for infants who were born at <37 weeks of gestation; this held especially for antepartum deaths. After 39 weeks of gestation, the perinatal mortality rate was higher in twin pregnancies. Differences in parity, fetal sex, and socioeconomic status did not explain the observed differences in outcome.
  
  
  CONCLUSIONS
  Overall the perinatal mortality rate was higher in twin pregnancies than in singleton pregnancies, which is most likely caused by the high preterm birth rate in twins and not by a higher mortality rate for gestation, apart from term pregnancies. During the preterm period, the antepartum mortality rate was much lower in twin pregnancies than in singleton pregnancies. We suggest that this might be partially due to a closer monitoring of twin pregnancies, which indirectly suggests a need for closer surveillance of singleton pregnancies.
authors.affiliation: Division Woman and Baby, University Medical Center Utrecht, The Netherlands. Electronic address: b.vasak@umcutrecht.nl.
authors.forename: Blanka
authors.initials: B
authors.lastname: Vasak
authors.affiliation: Division Woman and Baby, University Medical Center Utrecht, The Netherlands.
authors.forename: Jessica J
authors.initials: JJ
authors.lastname: Verhagen
authors.affiliation: Division Woman and Baby, University Medical Center Utrecht, The Netherlands.
authors.forename: Steven V
authors.initials: SV
authors.lastname: Koenen
authors.affiliation: Division Woman and Baby, University Medical Center Utrecht, The Netherlands.
authors.forename: Maria P H
authors.initials: MP
authors.lastname: Koster
authors.affiliation: Independent midwife, retired.
authors.forename: Paul A O M
authors.initials: PA
authors.lastname: de Reu
authors.affiliation: Division Woman and Baby, University Medical Center Utrecht, The Netherlands.
authors.forename: Arie
authors.initials: A
authors.lastname: Franx
authors.affiliation: Department of Obstetrics and Gynaecology, GROW-School for Oncology and Developmental Biology, Maastricht University Medical Centre, Maastricht, The Netherlands.
authors.forename: Jan G
authors.initials: JG
authors.lastname: Nijhuis
authors.affiliation: Division Woman and Baby, University Medical Center Utrecht, The Netherlands.
authors.forename: Gouke J
authors.initials: GJ
authors.lastname: Bonsel
authors.affiliation: Division Woman and Baby, University Medical Center Utrecht, The Netherlands.
authors.forename: Gerard H A
authors.initials: GH
authors.lastname: Visser
mesh_terms: D016022:Case-Control Studies; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D016015:Logistic Models; D008297:Male; D015999:Multivariate Analysis; D009426:Netherlands; D054238:Perinatal Mortality; D011247:Pregnancy; D059285:Pregnancy, Twin / Q000706:statistics & numerical data*; D047928:Premature Birth / Q000453:epidemiology*; D012042:Registries; D012189:Retrospective Studies
keywords: gestational age; perinatal mortality; preterm mortality; singleton pregnancies; twin pregnancies