title: Effect of advanced maternal age on perinatal outcomes in twins: the impact of chorionicity.
abstract: OBJECTIVE
  In contrast to singletons, twins born to older mothers have lower rates of perinatal mortality than twins born to younger mothers. We examined whether differences in chorionicity could explain this unexpected maternal age effect.
  
  
  METHODS
  We used population-based data on twins born to mothers aged 20-29 (n = 3702) and ≥35 years (n = 1880) in the North of England, UK, 1998-2007. We calculated rate ratios (RR) and 95% confidence intervals (CIs) to estimate the effect of maternal age; adjusted RRs (ARRs) were estimated by the use of generalized estimating equations for Poisson regression controlling for chorionicity and other confounders.
  
  
  RESULTS
  Older mothers had a lower proportion of monochorionic twins (17.6% vs. 24.3%, P < .01); lower neonatal (RR, 0.57; 95% CI, 0.34-0.95) and perinatal mortality (RR, 0.74; 95% CI, 0.53-1.04). Adjustment for chorionicity attenuated these associations (ARR, 0.59; 95% CI, 0.35-0.98 and ARR, 0.80; 95% CI, 0.57-1.12, respectively) and after further adjustment for additional factors, both associations became nonsignificant. Older mothers had greater rates of small-for-gestational-age infants (ARR, 1.59; 95% CI, 1.24-2.05), and cesarean delivery (ARR, 1.31; 95% CI, 1.16-1.48).
  
  
  CONCLUSIONS
  Perinatal death rates were lower but not statistically different for twins born to older versus younger mothers. This association was attenuated by adjustment for chorionicity.
authors.affiliation: Department of Obstetrics & Gynaecology, University of British Columbia and the Children's and Women's Hospital of British Columbia, Vancouver, Canada. slisonkova@cfri.ca
authors.forename: Sarka
authors.initials: S
authors.lastname: Lisonkova
authors.forename: K S
authors.initials: KS
authors.lastname: Joseph
authors.forename: Ruth
authors.initials: R
authors.lastname: Bell
authors.forename: Svetlana V
authors.initials: SV
authors.lastname: Glinianaia
mesh_terms: D000328:Adult; D000367:Age Factors; D002823:Chorion / Q000502:physiology*; D015331:Cohort Studies; D004739:England / Q000453:epidemiology; D005260:Female; D005865:Gestational Age; D006801:Humans; D007223:Infant; D008423:Maternal Age; D054238:Perinatal Mortality; D011159:Population Surveillance; D011247:Pregnancy; D011256:Pregnancy Outcome / Q000453:epidemiology*; D059285:Pregnancy, Twin / Q000706:statistics & numerical data*; D012044:Regression Analysis; D012307:Risk Factors; D014427:Twins / Q000706:statistics & numerical data; D055815:Young Adult