title: Is maternal parity an independent risk factor for birth defects?
abstract: BACKGROUND
  Although associations between maternal parity and birth defects have been observed previously, few studies have focused on the possibility that parity is an independent risk factor for birth defects. We investigated the relation between levels of parity and a range of birth defects, adjusting each defect group for the same covariates.
  
  
  METHODS
  We included infants who had an estimated delivery date between 1997 and 2007 and participated in the National Birth Defects Prevention Study, a multisite case-control study. Cases included infants or fetuses belonging to 38 phenotypes of birth defects (n = 17,908), and controls included infants who were unaffected by a major birth defect (n = 7173). Odds ratios (ORs) were adjusted for 12 covariates using logistic regression.
  
  
  RESULTS
  Compared with primiparous mothers, nulliparous mothers were more likely to have infants with amniotic band sequence, hydrocephaly, esophageal atresia, hypospadias, limb reduction deficiencies, diaphragmatic hernia, omphalocele, gastroschisis, tetralogy of Fallot, and septal cardiac defects, with significant ORs (1.2 to 2.3). Compared with primiparous mothers, multiparous mothers had a significantly increased risk of omphalocele, with an OR of 1.5, but had significantly decreased risk of hypospadias and limb reduction deficiencies, with ORs of 0.77 and 0.77.
  
  
  CONCLUSIONS
  Nulliparity was associated with an increased risk of specific phenotypes of birth defects. Most of the phenotypes associated with nulliparity in this study were consistent with those identified by previous studies. Research into biologic or environmental factors that are associated with nulliparity may be helpful in explaining some or all of these associations.
authors.affiliation: The University of Texas, Houston Health Science Center, School of Public Health, USA.
authors.forename: Hao T
authors.initials: HT
authors.lastname: Duong
authors.forename: Adrienne T
authors.initials: AT
authors.lastname: Hoyt
authors.forename: Suzan L
authors.initials: SL
authors.lastname: Carmichael
authors.forename: Suzanne M
authors.initials: SM
authors.lastname: Gilboa
authors.forename: Mark A
authors.initials: MA
authors.lastname: Canfield
authors.forename: Amy
authors.initials: A
authors.lastname: Case
authors.forename: Melanie L
authors.initials: ML
authors.lastname: McNeese
authors.forename: Dorothy Kim
authors.initials: DK
authors.lastname: Waller
mesh_terms: D000328:Adult; D016022:Case-Control Studies; D000013:Congenital Abnormalities / Q000145:classification / Q000453:epidemiology*; D005260:Female; D006330:Heart Defects, Congenital / Q000145:classification / Q000453:epidemiology; D006554:Hernia, Umbilical / Q000453:epidemiology; D006801:Humans; D016017:Odds Ratio; D010298:Parity; D010641:Phenotype; D011247:Pregnancy; D011248:Pregnancy Complications / Q000453:epidemiology; D012307:Risk Factors; D055815:Young Adult