title: Secular trends in gestational age and birthweight in twins.
abstract: BACKGROUND
  In recent decades, the overall rate of preterm births has increased. The aim of the present study was to examine whether this trend is also seen for multiple gestations. More specifically, we examined if there has been a decrease in gestational age for live born monozygotic (MZ) and dizygotic (DZ) twins and if there has been a simultaneous change in birthweight. The contributions of fertility treatments and Caesarean sections were taken into consideration. All analyses were carried out in two large European twin cohorts.
  
  
  METHODS
  Cross-sectional study of 6310 live born twin pairs, born between 1964-2007, from the Belgian East Flanders Prospective Twin Survey and 14,712 twin pairs, born between 1990-2006, from the Netherlands Twin Register. Multiple regression analyses were performed with gestational age as outcome variable, and multilevel analysis with birthweight as outcome variable. All analyses were performed with and without adjustment for zygosity, parity, maternal age, mode of conception and delivery and, for the analyses of birthweight, gestational age.
  
  
  RESULTS
  Gestational age decreased in a linear fashion from 1964 to 2007 with a decrease of 0.25 days per year in a similar way for MZ and DZ twins. Changes in birthweight depended on gestational age: up to 32 weeks, birthweight decreased and after 32 weeks birthweight increased. The frequency of infertility treatment and Caesarean sections, primiparity and advanced maternal age increased over the years, but none of these factors influenced the secular trends in gestational age and birthweight.
  
  
  CONCLUSIONS
  The decrease in gestational age and change in birthweight in twins are sources of concern, especially for very preterm twins, for whom birthweight decreased. For twins born after 32 weeks, an increase in birthweight was observed and this is very likely the explanation for the decrease in gestational age.
authors.affiliation: Department of Complex Genetics, Cluster of Genetics and Cell Biology, NUTRIM School for Nutrition, Toxicology and Metabolism, Maastricht University Medical Centre, Universiteitssingel 50 Box 616, Maastricht 6200 MD, The Netherlands. marij.gielen@gen.unimaas.nl
authors.forename: M
authors.initials: M
authors.lastname: Gielen
authors.forename: C E M
authors.initials: CE
authors.lastname: van Beijsterveldt
authors.forename: C
authors.initials: C
authors.lastname: Derom
authors.forename: R
authors.initials: R
authors.lastname: Vlietinck
authors.forename: J G
authors.initials: JG
authors.lastname: Nijhuis
authors.forename: M P A
authors.initials: MP
authors.lastname: Zeegers
authors.forename: D I
authors.initials: DI
authors.lastname: Boomsma
mesh_terms: D001530:Belgium; D001724:Birth Weight; D015331:Cohort Studies; D003430:Cross-Sectional Studies; D005260:Female; D005865:Gestational Age; D006306:Health Surveys; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D009426:Netherlands; D012042:Registries; D013223:Statistics as Topic; D014427:Twins; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic