title: Primary tooth size asymmetry in twins and singletons.
abstract: OBJECTIVE
  To explore asymmetry values of antimeric deciduous tooth crown dimensions in three types of twins: monozygotic (MZ), dizygotic same-sex (DZ) and opposite-sex (OS) vs. single-born controls.
  
  
  METHODS
  Mesiodistal and labio-lingual crown dimensions of second deciduous molars and mesiodistal canine and first molar crown dimensions of 2159 children at 6-12 years of age were evaluated, originating from the US cross-sectional Collaborative Perinatal Study from the 1970s, including altogether MZ (n = 28), DZ same-sex (n = 33) and OS (n = 39) pairs. Single born (n = 1959) were used as controls.
  
  
  METHODS
  Dental casts were measured for comparison of variance relationships calculated from antimeric teeth, exhibiting fluctuating (FA), and directional (DA) asymmetry using anova.
  
  
  RESULTS
  Significant differences appeared in MZ and OS girls in DA of deciduous canines, which gain size in the first and second trimester, and deciduous second molars, which finally stop crown growth during the early post-natal period. Significantly, increased FA values appeared for lower deciduous canines and second molars, indicating greatest environmental stress in OS girls, MZ girls and DZ boys. Twin girls had more fluctuating and directional crown asymmetry than twin boys, but in some dimensions, the twins were more symmetric than controls.
  
  
  CONCLUSIONS
  Transmembrane hormonal influence between opposite-sex twins, and late gestational stress factors, caused by placental malfunction and/or monochorionicity, may be involved in asymmetric growth of antimers, during critical periods of crown size gain.
authors.affiliation: Oral Development and Orthodontics, Unit of Oral Health Sciences, University of Oulu, Oulu, Finland.|Medical Research Center, Oulu, Oulu, Finland.|Oulu University Hospital, Oulu, Finland.
authors.forename: T
authors.initials: T
authors.lastname: Heikkinen
authors.affiliation: Oral Development and Orthodontics, Unit of Oral Health Sciences, University of Oulu, Oulu, Finland.|Medical Research Center, Oulu, Oulu, Finland.|Oulu University Hospital, Oulu, Finland.
authors.forename: V
authors.initials: V
authors.lastname: Harila
authors.affiliation: Oral Development and Orthodontics, Unit of Oral Health Sciences, University of Oulu, Oulu, Finland.|Medical Research Center, Oulu, Oulu, Finland.|Oulu University Hospital, Oulu, Finland.
authors.forename: A
authors.initials: A
authors.lastname: Ollikkala
authors.affiliation: Oral Development and Orthodontics, Unit of Oral Health Sciences, University of Oulu, Oulu, Finland.|Medical Research Center, Oulu, Oulu, Finland.|Oulu University Hospital, Oulu, Finland.
authors.forename: L
authors.initials: L
authors.lastname: Alvesalo
mesh_terms: D002648:Child; D003430:Cross-Sectional Studies; D005260:Female; D047109:Fetal Development / Q000502:physiology*; D006801:Humans; D008297:Male; D011247:Pregnancy; D059285:Pregnancy, Twin / Q000502:physiology*; D012737:Sex Factors; D019228:Tooth Crown / Q000196:embryology / Q000254:growth & development; D014094:Tooth, Deciduous / Q000196:embryology / Q000254:growth & development* / Q000473:pathology; D014427:Twins
keywords: asymmetry; dimorphism; monochorionicity; odontometry; prenatal stress; twins