[10] Diagnostic Tests for Fetal Abnormality Flashcards
What is the advantage of ultrasound in the diagnostic testing for fetal abnormalty?
They are non-invasive, and so there is no risk to the baby
Why is ultrasound a versatile investigation?
They may be offered to diagnose a wide range of conditions in the baby
When is the first ultrasound scan in pregnancy usually performed?
Between 10-13 weeks
What is the purpose of the 10-13 week ultrasound?
- Diagnose pregnancy
- Accurately determine gestational age
- Determine viability
- Determine fetal number
- Detect gross fetal abnormality
Why is it important to accurately determine gestational age?
- To allow intervention in post-maturity
- To allow accurate serum screening for Down’s
How is viability determined on 10-13 week ultrasound?
Confirm heartbeat, and exclude ectopic pregnancy and molar pregnancy
What is further determined if multiple pregnancy is found on 10-13 week ultrasound?
The chorionicity/amnionicity
How is gestational age determined on 10-13 week ultrasound?
Measurement of crown-rump length
When can the measurement of crown-rump length be used to determine gestational age?
Before 13 weeks
What are the alternative ways of estimating gestational age?
- Bi-parietal diameter and/or head circumference
- Femur length
How can risk of Down’s syndrome be tested for on ultrasound?
Measure nuchal translucency
When is scanning for nuchal translucency best done?
10-14 weeks
When is the anomaly scan offered to pregnant women?
Ideally 18-20 weeks
What is the main purpose of the anomaly scan?
- Reassure mother that the baby appears to have no gross structural abnormalities
- Determine placental morphology and localisation
- Confirm fetal growth is appropriate
What % of significant abnormalities will be detected by the 20-week screening scan?
50%
What should you do if a problem with the fetus is found on the anomaly scan?
Provide parents with options, e.g. termination, preparation
What is considered to be abnormal placental localisation on the anomaly scan?
If the placenta extends within 20mm of the internal cervical os
What should be done if the placenta extends within 20mm of the internal cervical os on the anomaly scan?
Another scan at 32 weeks should be offered
What will be done during the anomaly scan?
- Assessment of growth
- Look at head shape and internal structures
- Look at other systems
What parameters are used for the assessment of growth in the anomaly scan?
- Bi-parietal diameter
- Head circumference
- Femur length
- Abdominal circumference
What other systems are looked at during the anomaly scan?
- Spine
- Abdomen
- Thorax
- Arms and legs
What is looked at with regard to the abdomen at the anomaly scan?
- Shape
- Content at level of stomach, kidneys, and umbilicus
What is looked at with regard to the thorax at the anomaly scan?
- Four-chamber cardiac view
- Aortic arch
What is looked at with regard to the arms and legs at the anomaly scan?
- Three bones
- Hands
- Orientation of the feet
Are aneuploidy scans routinely performed?
No
Why are aneuploidy scans not routinely performed?
As many normal pregnancies may have some of these features, resulting in a high false positive rate
What is aneuploidy?
Abnormal chromosome number
What will pregnancies affected by aneuploidy have on ultrasound?
Sonographic markers
Other than ultrasound, how can aneuploidy be identified?
Triple test, maternal age, and NT measurements
What % of cases of aneuploidy will be identified by the triple test, maternal age, and NT measurements?
50-80%
What are the indications for an aneuploidy marker scan?
- Family history of abnormalities, e.g. neural tube defects
- Multiple pregnancy
- Maternal diabetes or epilepsy
- Recurrent miscarriage
- Polyhydraminos
- AFP abnormal
- Oligohydraminos
What does Doppler ultrasound do?
Uses high-intensity sound waves to detect the blood circulation in the baby, uterus, and placenta
What is Doppler ultrasound used for?
High risk pregnancies where there is concern about the baby’s wellbeing
Give 3 examples of where Doppler ultrasound may be used because there is concern about the babies wellbeing?
- IUGR
- Hypertensive disorders of pregnancy
- Distinguish normal small fetus to the ‘sick’ small fetus
Is Dopper ultrasound used routinely?
No
What is the current role of Doppler ultrasound?
Optimising management, particularly timing of delivery
What is chorionic villus sampling (CVS)?
An invasive diagnostic test
What does CVS involve?
Sampling of the developing placenta late into the first trimester of pregnancy
What is the purpose of CVS?
To allow examination of the fetal karyotype and/or genotype
How can CVS be performed?
- Transabdominally (most common)
- Transcervically (prior to 13 weeks)
When is CVS used?
Usually reserved for women at increased risk of carrying a fetus with a chromosomal or genetic abnormality
What is the advantage of CVS, compared to amniocentesis?
It can be performed earlier
What needs to be considered when deciding between CVS and amniocentesis?
Any benefit of earlier diagnosis with CVS must be carefully balanced against the slightly greater risks of pregnancy loss compared with second trimester amniocentesis
Are there significant differences in long-term health outcomes between children who have had CVS and amniocentesis?
No
What are the indications for CVS?
- Positive antenatal screening test
- History of genetic or chromosomal abnormality
- Familial chromosomal rearrangement
- Biochemical or molecular diagnosis of familial genetic disorder
Give an example of a positive antenatal screening test that would be an indication for CVS
Combined test for trisomy
What can CVS not be used for?
To screen for structural problems such as neural tube defects
Why can CVS not be used to screen for structural problems such as NTD?
Because they have no known metabolic or molecular basis
What are the contraindications to CVS?
- Active vaginal bleeding
- Infection
What is the trans-cervical route of CVS contraindicated by?
- Cervical polyps
- Fibroids
- Fundal placenta
- Retroverted uterus with posterior placement of placenta
When is CVS usually performed?
11 - 13+6 weeks
What medication should be given before CVS?
Rhesus immunoprophylaxis, where appropriate
How is the placental sample obtained in CVS?
Either by ultrasound-guided trans-abdominal needle, or ultrasound-guided cannula aspiration
How long does it take for results to be obtained with CVS?
Within 7-14 days, although newer tests can reduce reporting time to 24-48 hours
What are the risks and complications of CVS?
- Sampling failure
- Miscarriage risk
- Amniotic fluid leakage
- Vaginal bleeding
- Sepsis
What might cause sampling failure in CVS?
- Lab failure
- Mosacism
- Ambiguous results
- Insufficient sample
- Maternal cell contamination
What is amniocentesis?
An invasive, diagnostic antenatla test
What does amniocentesis involve?
Taking a sample of amniotic fluid to examine fetal cells found in the fluid
Who is amniocentesis used in?
Usually reserved for those women considered at high risk of carrying a fetus with a chromosomal or genetic abnormality
What does the amniotic fluid extracted in amniocentesis contain?
Cells from the amnion and fetal skin, lungs, and urinary tract
What can be done with the cells obtained in amniocentesis?
They can undergo chromosomal, genetic, biochemical, and molecular biological analysis
Is amniocentesis used as a screening test?
No
Why is amniocentesis not used as a screening test?
As it is an invasive test posing risk to fetus and mother
What can the timing of amniocentesis be divided into?
- Early
- Mid-trimester
- Third trimester
What is considered to be early amniocentesis?
12+0 - 14+6 weeks
Is early amniocentesis recommended?
No
Why is early amniocentesis not recommended?
As it is associated with an increased risk of miscarriage and higher incidence of talipes
What is considered to be mid-trimester amniocentesis?
15-18 weeks
What is the most common timing for amniocentesis?
Mid-trimester
Why might third trimester amniocentesis be carried out?
- Late karyotyping
- Detection of fetal infection in P-PROM
What are the most common indications for amniocentesis?
- Positive antenatal screening test
- Previous child with chromosomal or congenital abnormality
- High risk features on history
- Analysis to detect specific conditions
Give 2 examples of positive antenatal screening tests that may be indications for amniocentesis?
- Combined test for trisomy abnormalities
- Abnormal fetal anomaly scan
What high risk features on history may be an indication for amniocentesis?
- Parent carrying balanced chromosomal translocation
- Risk of recessively inherited metabolic disorder
- Mother carrying X-linked disorder
Give 3 examples of specific conditions that can be detected by amniocentesis?
- CF
- Tay-Sachs
- Congenital adrenal hyperplasia
What medication should be given before amniocentesis?
Rhesus immunoprophylaxis, where appropriate
Describe the procedure for amniocentesis
It should be performed under continuous ultrasound guidance. A spinal needle is inserted through the maternal abdominal and uterine walls into the pocket of amniotic fluid within the amniotic sac
How much fluid is obtained in amniocentesus?
1ml per week of gestation
What are the risks and complications of amniocentesis?
- Discomfort (uterine cramping)
- Vaginal bleeding
- Amniotic fluid leakage
- Maternal rhesus sensitisation in susceptible pregnancies
- Amnionitis
- Miscarriage risk
What is the risk of miscarriage in amniocentesis?
Increased risk compared to background is 0.5-1%
What should be considered when thinking about the miscarriage risk with amniocentesis?
The risk of miscarriage may be due to the pregnancy characteristics of women undergoing the procedure
