×ArticlesLogInCasesSignUpCoursesQuizDonateAbout×MenuSearchADVERTISEMENT:Radiopaediaisfreethankstooursupportersandadvertisers.BecomeaGoldSupporterandseenoads.ArticlesCasesCoursesLogInLoginSignup{"url":"/signup-modal-props.json?lang=us\u0026email="}ArticlesCasesCoursesQuizAboutRecentEditsGoad-freeSearchTwinpregnancyAssocProfCraigHacking◉◈andAssocProfFrankGaillard◉◈etal.Twinpregnanciesarethemostcommonmultifetalpregnancies. Onthispage:Article:EpidemiologyClassificationComplicationsRadiographicfeaturesCalculatingduedate(EDD)RelatedarticlesReferencesImages:CasesandfiguresEpidemiologyMultifetalpregnanciesaccountfor~1%ofallpregnanciesbutareseeninmuchhighernumbersinpopulationswhereinvitro fertilisation(IVF)isacommonpractice.ClassificationAtwinpregnancycanbebroadlycategorizedinto:dizygotictwins:70% monozygotictwins:30%DizygotictwinsAlsoknownasfraternaltwinsresultingfromindependentfertilizationoftwoova,andalwaysresultindichorionic- diamnioticpregnancies.Thisfeaturealonecannotbeusedtodistinguishdizygoticfrommonozygoticpregnanciesasapproximately20%ofmonozygoticpregnancieswillalsobedichorionic-diamniotic(resultingin~80%ofalltwinpregnanciesbeingdichorionic-diamniotic). Themostreliablesignofdizygosityonultrasoundisdocumentationofdifferentsexfetuses.Riskfactorsfordizygoticpregnancy:invitrofertilisation(IVF):particularlyuseofovulationinductionagentsadvancedmaternalageadvancedparitymaternalfamilyhistoryethnicity(e.g.Nigerian1in25inNigeriato1in155inJapan) 4 MonozygotictwinsAlsoknownasidenticaltwins areduetofertilisationofasingleovumthatthenseparatesintotwo. Unlikedizygoticpregnancy,monozygoticpregnanciesdonothavematernalage,familyhistory,orethnicpredisposition. Thetimeatwhichthisseparationoccursdeterminesthechorionicityandamnionicityofthepregnancy.Divisionat:1-4days(morula)resultsindichorionic-diamniotictwins(di-di)(DCDA): 20-30%ofmonozygotictwinpregnancies76%ofalltwinpregnancies 4-8days(blastocyst)resultsinmonochorionic-diamniotictwins(mono-di)(MCDA): 70-75%ofmonozygotictwinpregnancie