title: Trends in twinning rates in metropolitan Atlanta before and after folic acid fortification.
abstract: OBJECTIVE
  To examine whether trends in rates of twinning in metropolitan Atlanta increased after folic acid fortification.
  
  
  METHODS
  Live births to residents of 5 metropolitan Atlanta counties during the period 1990-2001 were identified from the state of Georgia's vital records. Rates of twinning and rate ratios were computed for the periods before and after fortification with folic acid.
  
  
  RESULTS
  Of the 510,000 singleton and twin births in metropolitan Atlanta during the study period, 7168 (1.43%) represented twin pregnancies. Overall, the rate of twinning increased 18% (P < .001)from the prefortification to postfortification period. As compared with the rate of twinning during the prefortification period, that during the postfortification period increased by 23% (OR 1.23, 95% CI 1.7, 1.28) among women over 30 years but showed no increase among women <30 years of age (OR 1.02, CI 0.94, 1.10). Among women >30, there was an increasing rate of twinning throughout the prefortification period (4.9%). There was no upward trend in twinning rates among women younger than 30 years prior to fortification.
  
  
  CONCLUSIONS
  Increasing trends of twinning were observed only in women older than 30 years, but these trends began prior to folic acid fortification and reached a plateau in recent years. Further elucidation of the possible relationship between folic acid and twinning will need to account for the use of fertility treatments by older women.
authors.affiliation: Division of Birth Defects and Developmental Disabilities, National Center on Birth Defects and Developmental Disabilities, Centers for Disease Control and Prevention, Atlanta, Georgia 30333, USA. jkucik@cdc.gov
authors.forename: James
authors.initials: J
authors.lastname: Kucik
authors.forename: Adolfo
authors.initials: A
authors.lastname: Correa
mesh_terms: D000367:Age Factors; D001723:Birth Rate / Q000639:trends*; D019587:Dietary Supplements / Q000706:statistics & numerical data; D005260:Female; D005492:Folic Acid / Q000627:therapeutic use*; D005845:Georgia / Q000453:epidemiology; D006397:Hematinics / Q000627:therapeutic use*; D006801:Humans; D007231:Infant, Newborn; D008423:Maternal Age; D016742:Preconception Care / Q000379:methods*; D011247:Pregnancy; D011256:Pregnancy Outcome / Q000453:epidemiology*; D012189:Retrospective Studies; D013997:Time Factors; D014427:Twins / Q000187:drug effects*
chemical_list: D006397:Hematinics; D005492:Folic Acid