title: Is gestational hypertension protective against perinatal mortality in twin pregnancies?
abstract: BACKGROUND
  Pregnancy-induced or gestational hypertension is a common pregnancy complication. Paradoxically, gestational hypertension has been associated with a protective effect against perinatal mortality in twin pregnancies in analytic models (logistic regression) without accounting for survival time. Whether this effect is real remains uncertain. This study aimed to validate the impact of gestational hypertension on perinatal mortality in twin pregnancies using a survival analysis approach.
  
  
  METHODS
  This was a retrospective cohort study of 278,821 twin pregnancies, using the U.S. 1995-2000 matched multiple birth dataset (the largest dataset available for multiple births). Cox proportional hazard models were applied to estimate the adjusted hazard ratios (aHR) of perinatal death (stillbirth and neonatal death) comparing gestational hypertensive vs. non-hypertensive pregnancies controlling for maternal characteristics and twin cluster-level dependence.
  
  
  RESULTS
  Comparing births in gestational hypertensive vs. non-hypertensive twin pregnancies, perinatal mortality rates were significantly lower (1.20% vs. 3.38%), so were neonatal mortality (0.72% vs. 2.30%) and stillbirth (0.48% vs. 1.10%) rates. The aHRs (95% confidence intervals) were 0.34 (0.31-0.38) for perinatal death, 0.31 (0.27-0.34) for neonatal death, and 0.45 (0.38-0.53) for stillbirth, respectively. The protective effect of gestational hypertension against perinatal death became weaker over advancing gestational age; the aHRs in very preterm (<32 weeks), mild preterm (32-36 weeks) and term (37+ weeks) births were 0.29, 0.48 and 0.76, respectively. The largest risk reductions in neonatal mortality were observed for infections and immaturity-related conditions.
  
  
  CONCLUSIONS
  Gestational hypertension appears to be beneficial for fetal survival in twin pregnancies, especially in those ending more prematurely or for deaths due to infections and immaturity-related conditions. Prospective studies are required to rule out the possibility of unmeasured confounders.
authors.affiliation: Department of Obstetrics and Gynecology, Sainte-Justine Hospital, University of Montreal, Montreal, Canada; Department of Statistics and Actuarial Science, University of Waterloo, Waterloo, Canada.
authors.forename: Qi-Guang
authors.initials: QG
authors.lastname: Luo
authors.affiliation: Department of Obstetrics and Gynecology, Sainte-Justine Hospital, University of Montreal, Montreal, Canada.
authors.forename: Ji-Yan
authors.initials: JY
authors.lastname: Zhang
authors.affiliation: Department of Obstetrics and Gynecology, International Peace Maternity and Child Health Hospital, Shanghai Jiao-Tong University School of Medicine, Shanghai, China.
authors.forename: Wei-Wei
authors.initials: WW
authors.lastname: Cheng
authors.affiliation: Department of Obstetrics and Gynecology, Sainte-Justine Hospital, University of Montreal, Montreal, Canada.
authors.forename: Francois
authors.initials: F
authors.lastname: Audibert
authors.affiliation: Department of Obstetrics and Gynecology, Sainte-Justine Hospital, University of Montreal, Montreal, Canada; Ministry of Education-Shanghai Key Laboratory of Children's Environmental Health, Xinhua Hospital, Shanghai Jiao-Tong University School of Medicine, Shanghai, China.
authors.forename: Zhong-Cheng
authors.initials: ZC
authors.lastname: Luo
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D046110:Hypertension, Pregnancy-Induced / Q000453:epidemiology*; D007231:Infant, Newborn; D054238:Perinatal Mortality; D011247:Pregnancy; D059285:Pregnancy, Twin / Q000706:statistics & numerical data*; D011336:Probability; D050497:Stillbirth / Q000453:epidemiology; D014481:United States / Q000453:epidemiology; D055815:Young Adult
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