title: [Difficulties of the methods for studying environmental exposure and neural tube defects].
abstract: OBJECTIVE
  To discuss the attitudes in the assessment of environmental exposures as risk factors associated with neural tube defects, and to present the main risk factors studied to date.
  
  
  RESULTS
  Environmental exposures have been suggested to have a roll in the genesis of birth defects. However, studies conducted in human populations have found difficulties in the design and conduction to show such an association for neural tube defects (anencephaly, espina bifida and encephalocele) because of problems raised from: a) the frequency measures used to compare time trends and communities, b) the classification of heterogeneous malformations, c) the inclusion of maternal, paternal and fetal factors as an integrated process and, d) the assessment of environmental exposures.
  
  
  CONCLUSIONS
  Hypothetically both maternal and paternal environmental exposures can produce damage before and after conception by direct action on the embryo and the fetus-placenta complex. Therefore, in the assessment of environmental exposures we need to take into account: a) both paternal and maternal exposures; b) the critical exposure period, three months before conception for paternal exposures and one month around the conceptional period for maternal exposures; c) quantitatively evaluate environmental exposures when possible, avoiding a dichotomous classification; d) the use of biological markers of exposure is highly recommended as well as markers of genetic susceptibility.
authors.affiliation: Centro Nacional de Salud Ambiental, México, D.F., México. vborja@toluca.podernet.com.mx
authors.forename: V H
authors.initials: VH
authors.lastname: Borja-Aburto
authors.forename: O
authors.initials: O
authors.lastname: Bermúdez-Castro
authors.forename: M
authors.initials: M
authors.lastname: Lacasaña-Navarro
authors.forename: P
authors.initials: P
authors.lastname: Kuri
authors.forename: P
authors.initials: P
authors.lastname: Bustamante-Montes
authors.forename: V
authors.initials: V
authors.lastname: Torres-Meza
mesh_terms: D004781:Environmental Exposure; D004812:Epidemiologic Methods; D006801:Humans; D009436:Neural Tube Defects / Q000139:chemically induced* / Q000145:classification / Q000453:epidemiology; D012307:Risk Factors
vernacular_title: Dificultades en los métodos de estudio de exposiciones ambientales y defectos del tubo neural.