Page 848 - TNFlipTest

OB8 Obstetrics
Antepartum Care
Toronto Notes 2019
Compared to CVS, amniocentesis has a higher accuracy of prenatal cytogenetic diagnosis (99.8% vs. 97.5%) and lower risk of spontaneous abortion (0.5% vs. 1-2%)
Risk Factors for Neural Tube Defects
GRIMM
Genetics: family history of NTD (risk of having second child with NTD is increased to 2-5%), consanguinity, chromosomal (characteristic of Trisomy 13, 18, and 21)
Race: European Caucasians > African Americans, 3-fold higher in Hispanics Insufficient vitamins: zinc and folate
Maternal chronic disease (e.g. DM)
Maternal use of antiepileptic drugs
General population risk for NTD is 0.1%
Diagnostic Tests
• Diagnostictestsavailable:
■ amniocentesis
■ chorionic villous Sampling
Indications
• age>35yr(increasedriskofchromosomalanomalies) ■ risk factors in current pregnancy
■ abnormal U/S
• abnormalprenatalscreen(IPS,FTS,orMSS) • pasthistory/familyhistoryof:
■ chromosomal anomaly or genetic disease
■ either parent a known carrier of a genetic disorder or balanced translocation ■ consanguinity
■ >3 spontaneous abortions
AMNIOCENTESIS
• UU/S-guidedtransabdominalextractionofamnioticfluidperformedasearlyas15weeksGA
Indications
• identificationofgeneticandchromosomalanomalies(15-16wkgestation)asperindicationsabove • confirmationofpositiveNIPTtesting
• positiveFTS/IPS
• assessmentoffetallungmaturity(T3)viatheL/Sratio(lecithin:sphingomyelin)
■ if >2:1, RDS is less likely to occur
Advantages
• alsoscreensforONTD(acetylcholinesteraseandamnioticAFP)–96%accurate
• inwomen>35yr,theriskofchromosomalanomaly(1/180)isgreaterthantheriskofmiscarriagefrom
the procedure
• moreaccurategenetictestingthanCVS
Disadvantages
• 1/200riskofprocedurerelatedpregnancyloss,1/100formultiplegestations
• resultstake14-28d;QF-PCRorFISHcanbedoneonchromosomesX,Y,21,22,13,18togive
preliminary results in 48 h
CHORIONIC VILLUS SAMPLING
• biopsyoffetal-derivedchorionusingatransabdominalneedleortranscervicalcatheterat10-12wk
Advantages
• enablespregnancytobeterminatedearlierthanwithamniocentesis
• rapidkaryotypingandbiochemicalassaywithin48h,includingFISHanalysis • highsensitivityandspecificity
Disadvantages
• 1-2%riskofprocedurerelatedpregnancyloss
• doesnotscreenforONTD
• 1-2%incidenceofgeneticmosaicism“falsenegative”results
ISOIMMUNIZATION SCREENING
Definition
• isoimmunization:antibodies(Ab)producedagainstaspecificRBCantigen(Ag)asaresultofantigenic stimulation with RBC of another individual
Etiology
• maternal-fetalcirculationnormallyseparatedbyplacentalbarrier,butsensitizationcanoccurandcan affect the current pregnancy, or more commonly, future pregnancies
• anti-RhAbproducedbyasensitizedRh-negativemothercanleadtofetalhemolyticanemia
• riskofisoimmunizationofanRh-negativemotherwithanRh-positiveABO-compatibleinfantis16%
• sensitizationroutes
■ incompatible blood transfusions
■ previous fetal-maternal transplacental hemorrhage (e.g. ectopic pregnancy, abruption) ■ invasive procedures in pregnancy (e.g. prenatal diagnosis, cerclage, D&C)
■ any type of abortion
■ labour and delivery
Rh Antibody Titre
A positive titre (≥1:16) indicates an increased risk of fetal hemolytic anemia