Antenatal Screening Flashcards
What are risk factors for congenital abnormality?
Previously affected fetus or child
Pre-existing diabetes
Epilepsy
What is nuchal translucency? When is it carried out? When is it increased?
Early pregnancy scan for dating, diagnosing multiple pregnancy and chorionicity
11-13 weeks
Increased in: Down's Congenital heart defects Abdominal wall defects Turner's
How is pregnancy dated
Crown-rump length between 6 and 12 weeks
After 14 weeks, biparietal diameter is most accurate
In the 3rd trimester, two growth scans. 2 weeks apart
When is the anomaly scan undertaken? What is it for?
Detailed US at 18-22 weeks to detect structural malformations
What are anomaly scan requirements?
Skull shape and internal structures including the cerebellum, ventricular size and nuchal fold
Spine - longitudinal and transverse
Abdomen - shape and content at level of stomach, kidneys, umbilicus, bladder
Arms and legs for three bones and a hand or foot
Heart in four chamber view and lungs
Face and lips
What are lethal anomalies?
Anencephaly (absence of skull and cerebral cortex)
Bilateral renal agenesis
Cardiac abnormalities
Trisomy 13 and 18
When is fetal echocardiography offered?
To those at high risk of fetal cardiac abnormality: FHx or personal Hx NT of >3.5mm Suspected abnormality Drugs in pregnancy Pre-existing diabetes
What are soft markers? Examples?
Findings on the anomaly scan that are of little significance but are slightly more common in chromosomal abnormal foetuses
Choroid plexus cyst are seen in 1% of 20 week scans and have a weak association with trisomy 18
2 vessel umbilical cord
What are fetal growth scans? When are these done?
Head circumference and abdominal circumference are used to calculate estimated fetal weight
Along with liquor volume (single deepest vertical pocket or amniotic fluid index) fetal weight is used to determine pattern of growth.
Scans should be 2 weeks apart and used when there is increased risk of growth abnormality - previous growth restriction, pre-eclampsia, measuring small for dates
SGA fetus should prompt umbilical artery Doppler
What does doppler US measure? What can this indicate?
Measures blood flow in the uterus placenta and fetus
Uterine artery Doppler measures resistance within the placenta
Carried out at 23 weeks
High resistance increases risk of maternal pre-eclampsia and fetal growth restriction and requires extra surveillance
Umbilical artery Doppler measure resistance in the placenta
High resistance indicates placental failure
What are features of trisomy 21? Trisomy 18? Trisomy 13?
Trisomy 21 - Down’s
- Increased rate of spontaneous miscarriage
- Prevalence increases with maternal age
- Occur secondary to non-disjunction of chromosome 21 at meiosis
- Congenital cardiac malformations
- Learning disability
- Duodenal atresia
Trisomy 18 - Edwards Most die soon after birth Small chin Low set ears Rocker-bottom feed VSD
Trisomy 13 - Patau's Die soon after birth Microcephaly Holoprosencephaly Exomphalos Cleft lip/palate
What is the most used screening test for aneuploidy?
Combined test
What does the combined test involve? When is it used?
Nuchal translucency measurement - US
Serum B-hCG
Pregnancy associated plasma protein A PAPP-A
Woman’s age
Doen beween 11 - 13+6 weeks
Result available in first trimester allowing for termination
What is the integrated test?
Better screening test that the combined test but expensive and rare
NT + PAPP-A in 1st trimester + the quadruple test in the 2nd trimester
What is the quadruple test? When is this used?
Blood test at 16 weeks and uses a dating scan+ maternal alpha-fetoprotein + unconjugated estriol + free beta-hCG + inhibin-A + the woman’s age in the 2nd trimester
Use between 15+0 and 20+0 weeks
So used for women presenting int he 2nd trimester
What is pregnancy associated plasma protein A? Alpha-fetoprotein? What are they associated with?
PAPP-A is a glycoprotein produced by the placenta that ay have functions including matrix mineralisation and angiogenesis
Low levels in 1st trimester screening are associated with trisomy 18,21, pre-eclampsia, growth restriction, preterm delivery, maternal DM
AFP is a glycoprotein synthesised by fetal liver and GI tract
In 10% with high AFP there is fetal malformation such as dural tube defects (meningocele, anencephaly), abdo wall defects (omphalocoele,gastroschisis)
What is preimplantation genetic diagnosis?
Early form of prenatal diagnosis in which embryos created in vitro are analysed for well-defined genetic diseases such as CF or beta-thalassaemia or chromosome structural abnormalities
FISH used for analysis of chromosomes and PCR for analysis in monogenic diseases
PGD selection f embryos by HLA type so a child born after can be used as a stem cell donor to save a sibling
What happens if screening suggests increased risk of aneuploidy?
Invasive testing is offered
DNA can be analysed for single gene disorders
What are methods of invasive testing? When are these used?
Chorionic villus sampling - 10-13 weeks
Amniocentesis from 16 weeks onwards
Describe chorionic villus sampling.
Placenta is sampled by trans abdominal or transcervical approach under continuous US control
Karyotyping takes 2 days and enzyme gene probe analysis 3 weeks so termination for abnormality is ealier, sager and done before pregnancy is apparent
What are risks of chorionic villus sampling?
Excess miscarriage rate
Increased transmission of blood borne viruses
Rarely contamination by maternal cells
False positives/negatives from placental mosacsm -rare
Non recommended in dichorionic multiple pregnancy
What is amniocentesis?
Aspiration of fluid containing fetal cells shed from skin and gut
Small needle passed transabdominally under continuous US, preferably not transplacentally
Anti-D is needed in all Rh negative women
What is the advantage of amniocentesis?
Can diagnose fetal infections such as CMV and the excess miscarriage rate is lower than for chorionic villus sampling
Full cell culture for karyotyping may take as long as 3 weeks but rapid results are possible by FISH and PCR
What is cell free fetal DNA? How can this be used?
Method of non-invasive prenatal testing for chromosomal abnormality
Fetal DNA is produced by placenta and released into maternal circulation and is cleared rapidly following delivery
Detection of this DNA with focus on certain sequences can be used to guide anti-D use in Rh negative women and the need for invasive testing in x-linked diseases.
Dichorionic twins, maternal obesity, gestation < 10weeks and placental mosaicism are known to affect results due to lower cffDNA in maternal blood.
What are expected results for trisomy 21 pregnancy on combined test?
Low AFP Low oestriol High hCG Low PAPP A Thickened nuchal translucency
What are causes of hyperechogenic bowel?
Cystic fibrosis
Down’s
CMV infection
