title: Time trends in the natural dizygotic twinning rate.
abstract: BACKGROUND
  The natural dizygotic (DZ) twinning rate has been proposed as a reliable and useful measure of human fecundity, if adjusted for maternal age at twin birth. The aim of this study was to analyze age-adjusted trends in natural DZ twinning rates over the past 40 years using data from the 'East Flanders Prospective Twin Survey (EFPTS)'.
  
  
  METHODS
  This study involved 4835 naturally conceived twin pregnancies between 1969 and 2009 from the population-based Belgian 'EFPTS'. Age-adjusted trends in the incidence of natural DZ twin pregnancies were calculated using a generalized linear model with Poisson distribution.
  
  
  RESULTS
  Both the natural DZ twinning rates and maternal age at twin birth increased in a linear fashion from 1969 to 2009. When age-adjusted, we found that the trend in the natural DZ twinning rate was stable during the whole time period.
  
  
  CONCLUSIONS
  According to our population-based data and after age-adjustment, a stable natural DZ twinning rate could be observed over the last four decades. Under the assumption that the spontaneous DZ twinning rate is a sensor of fecundity, this indicates a stable 'high' fecundity for this population.
authors.affiliation: Department for Human Genetics, University Hospital Leuven, Herestraat 49, B-3000 Leuven, Belgium. catherine.derom@uz.kuleuven.ac.be
authors.forename: Catherine
authors.initials: C
authors.lastname: Derom
authors.forename: Marij
authors.initials: M
authors.lastname: Gielen
authors.forename: Hilde
authors.initials: H
authors.lastname: Peeters
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Frijns
authors.forename: Maurice P A
authors.initials: MP
authors.lastname: Zeegers
mesh_terms: D001530:Belgium / Q000453:epidemiology; D001723:Birth Rate / Q000639:trends*; D005260:Female; D005298:Fertility; D006801:Humans; D008423:Maternal Age; D011247:Pregnancy; D011272:Pregnancy, Multiple / Q000706:statistics & numerical data*; D014429:Twins, Dizygotic