Antenatal Screening

Question 1

What are risk factors for congenital abnormality?

Answer 1

Previously affected fetus or child
Pre-existing diabetes
Epilepsy

Question 2

What is nuchal translucency? When is it carried out? When is it increased?

Answer 2

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

Question 3

How is pregnancy dated

Answer 3

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

Question 4

When is the anomaly scan undertaken? What is it for?

Answer 4

Detailed US at 18-22 weeks to detect structural malformations

Question 5

What are anomaly scan requirements?

Answer 5

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

Question 6

What are lethal anomalies?

Answer 6

Anencephaly (absence of skull and cerebral cortex)
Bilateral renal agenesis
Cardiac abnormalities
Trisomy 13 and 18

Question 7

When is fetal echocardiography offered?

Answer 7

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

Question 8

What are soft markers? Examples?

Answer 8

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

Question 9

What are fetal growth scans? When are these done?

Answer 9

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

Question 10

What does doppler US measure? What can this indicate?

Answer 10

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

Question 11

What are features of trisomy 21? Trisomy 18? Trisomy 13?

Answer 11

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

Question 12

What is the most used screening test for aneuploidy?

Answer 12

Combined test

Question 13

What does the combined test involve? When is it used?

Answer 13

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

Question 14

What is the integrated test?

Answer 14

Better screening test that the combined test but expensive and rare

NT + PAPP-A in 1st trimester + the quadruple test in the 2nd trimester

Question 15

What is the quadruple test? When is this used?

Answer 15

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

Question 16

What is pregnancy associated plasma protein A? Alpha-fetoprotein? What are they associated with?

Answer 16

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)

Question 17

What is preimplantation genetic diagnosis?

Answer 17

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

Question 18

What happens if screening suggests increased risk of aneuploidy?

Answer 18

Invasive testing is offered

DNA can be analysed for single gene disorders

Question 19

What are methods of invasive testing? When are these used?

Answer 19

Chorionic villus sampling - 10-13 weeks

Amniocentesis from 16 weeks onwards

Question 20

Describe chorionic villus sampling.

Answer 20

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

Question 21

What are risks of chorionic villus sampling?

Answer 21

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

Question 22

What is amniocentesis?

Answer 22

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

Question 23

What is the advantage of amniocentesis?

Answer 23

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

Question 24

What is cell free fetal DNA? How can this be used?

Answer 24

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.

Question 25

What are expected results for trisomy 21 pregnancy on combined test?

Answer 25

Low AFP
Low oestriol
High hCG
Low PAPP A
Thickened nuchal translucency

Question 26

What are causes of hyperechogenic bowel?

Answer 26

Cystic fibrosis
Down’s
CMV infection