title: Defining women who are prone to have twins in in vitro fertilization--a necessary step towards single embryo transfer.
abstract: OBJECTIVE
  The aim of the present study was to identify subsets of women undergoing two-embryo transfer in in vitro fertilization (IVF) who are prone to give birth to twins.
  
  
  METHODS
  During 1990 to 2002 4580 day 2 or day 3 two-embryo transfers were conducted and these constituted the study population.
  
  
  RESULTS
  By selecting combinations of factors, some subsets of patients where single-embryo transfer might be conducted would reduce the estimated twin rate by 60%, but requiring single-embryo transfer in only approximately one third of the patients. Examples of such selected groups were patients less than 33 years of age with two top quality embryos, patients less than 39 years of age with two top quality embryos and conducting their first IVF cycle, and patients less than 39 years of age with two top quality embryos and two optimal cleavage cells.
  
  
  CONCLUSIONS
  By combining factors, subsets of patients with both a high birth and twin rate that could be recommended single embryo transfer were identified.
authors.affiliation: Centre for Clinical Research, Falun Hospital, Falun, Sweden. dan.hellberg@ltdalarna.se
authors.forename: Dan
authors.initials: D
authors.lastname: Hellberg
authors.forename: Maria
authors.initials: M
authors.lastname: Blennborn
authors.forename: Staffan
authors.initials: S
authors.lastname: Nilsson
mesh_terms: D000328:Adult; D004624:Embryo Transfer; D005260:Female; D005307:Fertilization in Vitro / Q000379:methods*; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D008423:Maternal Age; D010062:Ovulation Induction; D011247:Pregnancy; D011272:Pregnancy, Multiple / Q000502:physiology*; D014427:Twins