出生缺陷

Birthdefectsaredefinedasabnormalitiesofstructure,function,orbodymetabolismthatarepresentatbirth.1.GeneralOneofthemostcommonworriesforexpectantparentsisthatsomethingwillbewrongwiththeirbaby.

However,mostbabiesarebornwithoutproblems,whilemostofthosewhoarecanhavetreatmentthatwillmakethemmuchbetter,orevenallowthemtoleadanormallife.Understandingthenatureandcausesofbirthdefectscanhelpreducethemystery,fearandguiltoftenassociatedwiththem.Aninternationalmonitoringprogrammeshowsthat,acrossEurope,justover2percentofbabiesarediscoveredatbirthtohaveadefectthatwillaffecttheirabilitytosurviveorfunctionnormally.Somedefectsareoftennotdiscovereduntillater(egdeafnessandproblemsinwalking).

Thereisconsiderableregionalvariationintherateofdefectsseenatbirth.GlasgowandDublinhavetraditionallybeenblackspots,withratesover3percent,butthesefiguresarefalling.Thenumberofbabiesactuallybornwithdefectsisalsomarkedlyaffectedbytheavailabilityofprenatalscreening,andtheavailabilityofvoluntaryterminationofpregnancyinthecaseofsevereabnormalities.

2.FormsofBirthDefectsDefectsoflimbs,heartandspinalcordrepresentabouthalfofallabnormalities.

Themostcommonkindsofdefectarethoseaffectingthelimbs(armsandlegs).Theseincludemissingorextrafingersortoes,deficienciesinlimblength,andabnormalitiesinpositioning,suchasclubfoot.

Heartabnormalitiesrepresentthenextmostcommoncategoryofdefect.Commonheartdefectsinclude'holesintheheart'wherebloodcanpassfromonesideofthehearttotheother.Again,thesemaynotallbedetectedatbirth.

Thethirdmostcommonkindofdefectaffectsthespinalcord,suchasspinabifida.

Otherdefectscommonlyobservedincludethoseaffectingtheface(suchascleftlipandpalate),problemswiththedevelopmentoftheintestinesandstomach,andproblemsaffectingthesexualorgans.

MajorchromosomalproblemssuchasDown'ssyndrome(Trisomy21)arefoundinabout0.15percentofbirths(aboutthreebabiesinevery2000).

3.ThecauseofbirthdefectsA.GeneticsGeneticsplayaroleinsomebirthdefects.Onemissingorfaultygenecancauseabirthdefect.

Inancienttimes,abnormalitieswereseenaswarningsorpunishmentsfromthegods.Evenquiterecently,itwasbelievedthatspecificeventsduringthepregnancy-suchasbeingfrightenedbyamouse-wouldleadtospecificdefectsinthebaby,egamouseshapedbirthmark.Suchfolkbeliefsstillpersistinplaces.However,mostdefectsthatcanbetracedtoasinglecausehaveageneticorigin.Thisdoesnotnecessarilymeanthattheparentssufferfromthesamedefect.Itmaybethattheyarecarriersofthecondition,orthatthegeneticproblemoccurredforthefirsttimeinthecellsthatgaverisetothebaby.Knowngeneticcausesaccountforabout25percentofallabnormalities.

B.Environment

Environmentalcauses(includingexposuretodrugs,radiationandillnesses)canbeidentifiedinabout10percentofbirthdefects.Theriskofexposuretosuchenvironmentalfactorsoftencausesgreatconcern,althoughgeneticcausesaremorecommon.Therearethreemainkindsoffactorthatcancausedefects:chemical,biologicalandphysical.

Chemicalfactors,includingdrugs

Thalidomideisperhapsthemostnotoriousexampleofadrugthatcauseddefects.Itwasbroughtontothemarketafterinadequatetestingandspecificallyprescribedforpregnantwomen.

Followingthis,muchstricterrulesaboutdrugtestingwereintroduced,andinternationalmonitoringprogrammeswereputinplace.Doctorsarealsomuchmorecautiousaboutprescribingdrugstowomenofchild-bearingage,soitisunlikelythatadisasteronthisscalewilloccuragainintheUK.Severalmedicaltreatmentsposetheriskofbirthabnormalities.Tetracyclineantibioticsaffecthardtissueformationduringthefoetalperiod,affectingthelongbonesandteeth.Someanticonvulsantmedicinesusedforepilepsymayalsocauseproblems.Anyoneonlong-termmedicationwhoisplanningtobecomepregnant,orwhohasbecomepregnant,shoulddiscusstheirsituationwiththeirdoctor.Moderatealcoholuseinpregnancyisaparticularlytrickyproblem.Smokingisbadforthedevelopmentofthebabyinanumberofways.Pregnantwomenshouldnotsmokeunderanycircumstances.

Lackofsomethingmaybeasdamagingasthepresenceofsomething.Extensivetrialshaveconfirmedthattakingfolicacidbeforepregnancycanreducethenumberofspinalcorddefectssuchasspinabifida.

Biologicalfactors,includingillnesses

Therubellavirus,whichcausesGermanmeasles,posesaserioushazardtothedevelopingbaby.Itcancrosstheplacentaandattackthebabytocausearangeofdefects.Theproportionofmothersinfectedwiththerubellavirusduringthefirst12weeksofpregnancywhohaveanabnormalchildcanreach20percent.

Otherviruses,suchascytomegalovirusandherpessimplex,canalsocauseproblems.Bacteriasuchassalmonella,presentinundercookedmeat(especiallychicken),andlisteria(foundinsomecheeses),cancauseproblemsforthecontinuationofthepregnancy(ratherthanbirthdefects).

Physicalfactors(includingradiation)

X-raysandradiationgivenascancertherapycanaffectthebaby.YourmedicalcarerswillbeverycautiousaboutgivingyouanabdominalX-rayifyouarepregnantorpotentiallypregnant(thismeansalmosteverywomanofchild-bearingage!).C.Otherfactors

Theexactcausesoftheremaining65percentofdefectsarecurrentlyunclear.Manyofthesedefectsarisefromaninterplayofgeneticandenvironmentalfactors.Forinstance,somebabiesmayhaveageneticsusceptibilitytocertainenvironmentalfactors.Ifexposedtosuchfactorsduringdevelopment,theymaydevelopanabnormality.Iftheyarenotexposedtothefactor,theywillbeperfectlynormal.Anotherbabyexposedtothesameenvironmentalfactor,butwhodoesn'thavethegeneticsusceptibility,maybeentirelynormal.Thiscanmakeitdifficulttopindowntheexactcauseofadefect.

TheHumanGenomeProjecthasworkedoutthenormalsequenceofallgenesfoundinpeople.Inthefuture,thiswillbeinvaluableinteasingouttherelativerolesofgeneticsandtheenvironmentincausingbirthdefects.

4.Whenababymostvulnerable

Thetimeatwhichthedevelopingbabyisexposedtoapossiblydamagingenvironmentalfactorismoreimportantindeterminingthenatureofthelikelyabnormalitythanthenatureofthefactoritself.Infact,ifbabiesareexposedtothesamedamagingfactoratdifferenttimesintheirdevelopment,thenatureofthedamageislikelytobedifferent.Conversely,ifbabiesareexposedtodifferentdamagingfactorsatthesamestageindevelopment,thedamagecausedmaywellbesimilar.Forinstance,exposuretotherubellavirusatthreeweeksafterfertilisationmaycauseheartdefects,andatsixweeksafterfertilisation,itmaycausedeafness.Exposureto,say,radiationatthreeweeksmaycausesimilardamagetorubella.Thedevelopingbabyisparticularlyvulnerabletoenvironmentalfactorsduringafive-weekperiodstartingaboutthreeweeksafterfertilisation,andlastingtillabouttheendoftheeighthweekafterfertilisation.Pregnancyisusuallytimedfromthelastmenstrualperiod,whichisgenerallyaboutafortnightbeforefertilisation.'Threeweeksafterfertilisation'thereforecorrespondstothe'fifthweekofpregnancy'and'eightweeksafterfertilisation'tothe'10thweekofpregnancy'.

Thisfive-weekintervalisoftencalledthe'sensitiveperiod'asitiscrucialtonormaldevelopment.Evenwithinthisperiod,thebodyorgansformatdifferenttimes.Sometimesitispossibletoestimateretrospectivelyfromthenatureofthedefectwhenthebabymighthavebeenexposedtoapossibleenvironmentalfactor.Themother'smedicalhistorycanthenbeexaminedtoseewhatshewasexposedtooverthisperiod.

5.HowdetectedinadvanceDiagnosisofbirthdefectshasbeenrevolutionisedbythewidespreaduseofultrasound.Ultrasoundscanningseemstobeverysafeinnormaluse.

Bloodtestsforexpectantmothersarealsovaluable.The'tripletest'iswidelyused.Inthis,levelsofalpha-fetoprotein,humanchorionic

gonadotrophinandoestriolinthemother'sbloodaremeasured.Variousabnormalitiesareassociatedwithalterationsintheirlevels.Acommonfurthertestisamniocentesis,whenasampleofthefluidsurroundingthebabyisremoved,andthecellsinthefluidarechecked.Thisprocedureposesasmallriskofinducingaterminationofpregnancy.

6.SpinaBifida

Spinabifidaisabirthdefectthatinvolvestheincompletedevelopmentofthespinalcordoritscoverings.ThetermspinabifidacomesfromLatinandliterallymeans"split"or"open"spine.

Spinabifidaoccursattheendofthefirstmonthofpregnancywhenthetwosidesofthe

embryo'sspinefailtojointogether,leavinganopenarea.Insomecases,thespinalcordorothermembranesmaypushthroughthisopeningintheembryo'sback.Theconditioncantypicallybedetectedbeforeababyisbornandtreatedrightaway.A.TypesofSpinaBifidaThecausesofspinabifidaarelargelyunknown.Someevidencesuggeststhatgenesmaybeinvolved.Ahighfeverduringpregnancymayincreaseawoman'schancesofhavingababywithspinabifida.Womenwithepilepsywhohavetakenthedrugvalproicacidtocontrolseizuresmayhaveanincreasedriskofhavingababywithspinabifida.Therearetwoformsofspinabifida—

spinabifidaoccultaandspinabifidamanifesta.7.Congenitalheartdefects

Acongenitalheartdefectisastructuralproblem(ordefect)intheheartthatispresentatbirth.Ababy'sheartbeginstodevelopshortlyafterconception.Duringdevelopment,structuraldefectscanoccur.Thesedefectscaninvolvethewallsoftheheart,thevalvesoftheheart,andthearteriesandveinsneartheheart.Congenitalheartdefectscandisruptthenormalflowofbloodthroughtheheart.Multiplegeneticandenvironmentalfactorsinteracttoalterthedevelopmentoftheheartduringtheearlystagesofafetus'development(thefirst8to9weeksduringpregnancy).TypesofCongenitalHeartDefectsTherearemanytypesofcongenitalheartdefects.Theyinclude:AbnormalpassagesintheheartorbetweenbloodvesselsProblemswiththeheartvalvesProblemswiththeplacementordevelopmentofbloodvesselsneartheheartProblemswithdevelopmentoftheheartitselfAbnormalpassagesintheheartorbetweenbloodvesselsAtrial

septaldefect(ASD)Ventricularseptaldefect(VSD)Atrioventricular

septaldefect(AVSD)Patentductus

arteriosus(PDA)

Atrial

septaldefect(ASD)isaholeinthewallthatseparatestheupperchambers(atria(AY-tree-uh))oftheheart.Thiscausesbloodtoleakfromoneatriumtotheother.

Ventricularseptaldefect(VSD)isaholeinthewallthatseparatesthelowerchambers(ventricles(VEN-trih-kuls))oftheheart.Thiscausesbloodtoleakfromoneventricletotheother.

Patentductus

arteriosus(PDA)isapersistentconnectionbetweentheaortaandthepulmonary(PULL-mun-ary)artery.Thisconnectioniscalledtheductus

arteriosusandisnormallypresentbeforebirth.Inmostbabies,thevesselcloseswithinafewhoursordaysafterbirth.Insomechildren,thevesselfailstoclose,resultinginPDA.B.ProblemswiththeheartvalvesCongenitalheartdefectscaninvolveanyofthevalvesandincludethefollowingtypesofpro