Lecture 6 Flashcards
What are the stages that take place following a positive pregnancy test?
Booked into antenatal care - see the midwife
Nuchal Scan - 10-14 weeks - different tests dependent on the NHS trust (ultrasound)
A Mid-Trimester anomaly scan
When are the two ultrasounds offered to pregnant women?
11-14 weeks
20-22 weeks
What is the purpose of the nuchal scan and when does it take place?
Uses of the Nuchal Scan:
- Date the pregnancy
- Multiple Pregnancies
- Major Foetal Abnormalities
- Early Miscarriage
- Assess risks of Down Syndrome and other chromosomal abnormalities
Done by looking at Nuchal Translucency and taking into account: maternal age, hormone levels, nasal bone, blood floor through the foetal heart and foetal abnormalities.
IT IS A SCREENING TEST NOT DIAGNOSTIC
11-14 weeks
What is normal and abnormal NT and what does abnormla suggest?
Normal NT is <3.5mm
>3mm means chromosomal abnormalities, cardiac anomalies, pulmonary defects, renal defects, abdominal wall defects, skeletal dysplasia
When is prenatal testing arranged?
- Following abnormal findings at nuchal scan or mid-trimester scan
- Following results test which give an increased risk of Down Syndrome
- If previous pregnancy affected with a condition e.g. DS, CF
- If parent(s) carrier of chromosome rearrangement or genetic condition
Aims of Prenatal Testing
- To inform and prepare parents for the birth of an effected child.
- In utero treatment could be offered.
- Aids management of the rest of the pregnancy
- Allows parents to prepare for complications at or after birth.
- To allow TERMINATION of an affected foetus.
What are the three main types of prenatal testing?
1) Non-Invasive
Ultrasound/MRI
2) Minimally Invasive
Maternal Blood Test
Cell-Free Foetal DNA
3) Invasive
Chorionic Villus Sampling (CVS)
Amniocentesis
Ultrasound
- Early/dating scan (can be done from 5 weeks but best from 9 weeks)
- Nuchal Translucency and nasal bone
- High level/anomaly scan
- High level anomaly scan at 18-22 weeks so foetus is examined especially the brain, face, spin, heart, stomach, bowel, kidneys and limbs. This scan can be diagnostic in that it can detect many birth defects such as hydrocephalus, neural tube defects, limb/organ deformities, some heart defects.
- It is also a test where ‘soft’ markers are detected and can be suggestive of an underlying problem -if a marker is detected at ultrasound, a thorough check is made for other features of the chromosomal defect known to be associated with that marker
- U/S is also useful for determining the size and position of the foetus and the placenta, amount of amniotic fluid
MRI
20 weeks
High level and anomaly scan can be diagnostic - showing cleft lip, limb deformity or cardiac problem.
It can also show soft markers for other problems.
Nasal Bone - soft marker - presence or absence can indicate Down Syndrome.
When taken alongside NT and maternal age, looking at the nasal bone increases the sensitivity of DS screening.
Foetal Cardiac Scans can show cardiac problems - only done if the other scans indicate potential problem.
Maternal serum testing
- Tests maternal serum markers in the blood
- Detects increased risk of trisomy 21, trisomy 18 and/or neural tube defects.
Types of Maternal Serum Screening:
1st Trimester - 11-14 weeks - done alongside the NT measurement (looks for the presence of hCG (human chorionic gonadotrophin) and PAPP A (pregnancy associated plasma protein A)).
2nd Trimester - 16-20 weeks - done if they are booked in later in pregnancy - looks for hCG and PAPP A and AFP (alpha foetal protein) and uE3 (oestriol).
Private - combined 1st and 2nd Trimester Screening available
If these tests find that the woman is at high risk of some genetic diseases, she will be offered more invasive prenatal tests.
Cell free fetal DNA test
- Analysing the DNA fragments present in the maternal plasma during pregnancy (cell-free DNA).
- 10-20% of it comes from the placenta
- cffDNA is first detectable from about 4 -5 weeks gestation, but cannot accurately be detected until 9 weeks
- Offered when there is a X-linked condition in the family detects SRY gene on Y chromosome. If male then go on to prenatal test
- NIPD available for Achondroplasia, thanatophoric dysplasia, Apert syndrome - testing is free
- Also used for autosomal dominant single gene disorders from the father or arise de novo
- NIPD is also possible to alter management of pregnancies at risk of recessive conditions
- 12% chance that, if they do not detect a mutation, they are unable to confirm the presence of fetal DNA.
- CffDNA offered privately also
NIPD and NIPT
Non-invasive prenatal diagnosis (NIPD) or testing (NIPT) is based on a maternal blood test. In some cases, such as testing for the achondroplasia, it is diagnostic. But in other cases such as for Down’s syndrome an invasive test is required so the test is NIPT.
What are the benefits of NIPD and NIPT?
- Reduced number of invasive tests - due to identification of sex
- No risk of miscarriage
- Less expertise required to perform a blood test
- Invasive tests are more uncomfortable
- NIPD and NIPT can be offered earlier than invasive tests - results come earlier
What are the cons of NIPD and NIPT?
- Multiple pregnancies
- proportion of cell-free foetal DNA is reduced in women with a high BMI
- same implications as an invasive test- psychological
- invasive test may still be required to confirm an abnormal result
What are invasive prenatal tests?
Offered when there is a known risk shown by other prenatal testing and if there is a known genetic condition in the family that the baby is at risk of. Molecular, cytogenetic and biochemical tests are done and they are conducted under ultrasound guidance. They are diagnostic.
These tests are invasive and carry a miscarriage risk.
e.g. Chorionic Villus Sampling (CVS) and Amniocentesis
