title: Folate supplementation and twin pregnancies.
abstract: BACKGROUND
  Women in many countries are advised to increase their folate intake to lower the risk of neural tube defects. For this purpose several countries add folate to the flour. Therefore, it is important to monitor possible adverse effects of this B vitamin. We have assessed the effect of folate on twin pregnancies.
  
  
  METHODS
  We conducted a retrospective, population-register based study of 176,042 women who gave birth from December 1998 through the end of 2001. Use of folate and multivitamin supplements was recorded on the mandatory birth notification form of the Medical Birth Registry of Norway. Pregnancies after in vitro fertilization (IVF) were reported separately.
  
  
  RESULTS
  With adjustment for maternal age and parity, we observed an increased risk of twin pregnancies associated with preconceptional use of folate (odds ratio = 1.59; 95% confidence interval = 1.41-1.78). This association was largely explained by confounding with IVF pregnancies, which were strongly associated both with twin pregnancies and folate use. After exclusion of known IVF pregnancies, and accounting for underreporting of both IVF pregnancies and folate use, the risk was no longer elevated (1.02; 0.85-1.24). Weak associations with twin pregnancies were observed for use of multivitamins and folate during pregnancy, but could be due to increased use of vitamins after a recognized twin pregnancy.
  
  
  CONCLUSIONS
  The association between preconceptional folate use and twin pregnancies was strongly confounded by IVF. After accounting for IVF pregnancies and underreporting, we found no evidence for an association between preconceptional folate supplements and twinning.
authors.affiliation: Department of Public Health and Primary Health Care, Section for Epidemiology and Medical Statistics, Locus for Registry Based Epidemiology, University of Bergen, Bergen, Norway. vollset@uib.no
authors.forename: Stein Emil
authors.initials: SE
authors.lastname: Vollset
authors.forename: Håkon K
authors.initials: HK
authors.lastname: Gjessing
authors.forename: Anne
authors.initials: A
authors.lastname: Tandberg
authors.forename: Thorbjørn
authors.initials: T
authors.lastname: Rønning
authors.forename: Lorentz M
authors.initials: LM
authors.lastname: Irgens
authors.forename: Valborg
authors.initials: V
authors.lastname: Baste
authors.forename: Roy M
authors.initials: RM
authors.lastname: Nilsen
authors.forename: Anne Kjersti
authors.initials: AK
authors.lastname: Daltveit
mesh_terms: D000328:Adult; D015986:Confounding Factors, Epidemiologic; D019587:Dietary Supplements; D005260:Female; D005307:Fertilization in Vitro; D005492:Folic Acid / Q000009:adverse effects* / Q000627:therapeutic use*; D006801:Humans; D009664:Norway / Q000453:epidemiology; D011247:Pregnancy; D012042:Registries / Q000706:statistics & numerical data*; D012189:Retrospective Studies; D012307:Risk Factors; D014427:Twins
chemical_list: D005492:Folic Acid