Obstretrics Pt. 2 Flashcards
Why is prenatal diagnostic testing performed?
- Usually performed if there is suspicion of disease in the foetus, for example:
- Family history
- Past obstetric history (RhD alloimmunisation)
- Serum screening tests
- Ultrasound scanning (12 week dating or 20 week anomaly)
- Prenatal diagnosis often happens after some form of screening test
What are the attributes of a screening test?
- Relevance of the disease
- Effect of early diagnosis on management (e.g. offering termination)
- Sensitivity
- Specificity
- Predictive value
- Affordability
- Equity (available to all)
What are different types of prenatal diagnostic tests?
- Ultrasound diagnosis
- Invasive test- CVS or amniocentesis
- Invasive test- cordocentesis
- Ultrasound then invasive test
What conditions are diagnosed with ultrasound?
- Neural tube defect
- Gastroschisis
- Cystic adenomatoid malformation of lung
- Twin to twin transfusion syndrome
What conditions are diagnosed with CVS or amniocentesis?
- Downs syndrome
- Thalassaemia
- Cystic Fibrosis
What conditions are diagnosed with cordocentesis?
- Alloimmune thrombocytopaenia
What conditions are diagnosed with ultrasound first then invasive test?
- Congenital diaphragmatic hernia
- Exomphalos
- Ventriculomegaly
- Duodenal atresia
What does it mean when a woman initially has no IgM or IgG for a particular virus early in pregnancy but develops IgM or IgG later on?
- Had a clinical or subclinical infection with virus at some point during pregnancy
- This is usually only done if there are ultrasound results suggestive of infection or if there is a history of exposure to a particular virus
Describe cell-free foetal DNA (cffDNA)?
- Extracted from the maternal blood
- Uses
- Determination of foetal blood group in cases of RhD alloimmunisation
- Determine the sex of the foetus in X-linked disorders
- Diagnose skeletal dysplasias (e.g. achondroplasia)
- The levels of cffDNA increases with gestation, however, it decreases to undetectable levels by the first day after birth
What are the two most common invasive tests for prenatal diagnosis?
- Amnioncentesis and Chorion Villous Sampling are the two MOST COMMON invasive tests that are used to check the karyotype of the foetus or diagnose single gene disorders
- These tests carry a small but significant risk of miscarriage
- So, the seriousness of the condition should be enough to warrant the risk
What are three options usually available following results of a prenatal test?
- Continue
- May facilitate care around the time of delivery and may help the mother/partner prepare for the birth of a baby with a serious condition
- Termination of pregnancy
- Terminate, but provide information which may prove useful when counselling about recurrence risks in future pregnancies
What cells are sampled Chorion Villous Sampling? (Chorion Villous Biopsy)
How is CVS done?
- Foetal trophoblast cells in the mesenchyme of the villi divide rapidly in the first trimester
- CVS aims to take a sample of these rapidly dividing cells from the developing placenta
- This can either be done by:
- Passing a needle under ultrasound guidance through the abdominal wall and myometrium into the placenta
- Passing a fine catheter (or biopsy forceps) through the cervix into the placenta
- The laboratory can check the sample for aneuploidy
Why should the woman be ultrasound scanned initially before CVS?
- To confirm that the pregnancy is viable before the CVS
- To ensure that it is a singleton pregnancy (prenatal diagnosis in multiple pregnancy is more complicated)
- To confirm gestational age (CVS should NOT be performed < 10 weeks gestation)
- To localise the placenta and determine whether a transabdominal or transcervical approach is more appropriate
Which type of CVS is most commonly performed? (Transabdominal or transcervical)
- Transabdominal
What is the additional risk of miscarriage with CVS?
- 2%
What is placental mosaicism?
- Is sometimes found where two different cell types are found in the same sample (one cell line is normal and the other is abnormal)
- This pattern may be present in the placenta and NOT the foetus (confined placental mosaicism)
What is amniocentesis?
- Amniotic fluid contains amniocytes and fibroblasts shed from foetal membranes, skin and the foetal genitourinary tract
- Amniocentesis takes a sample of 15-20 mL of amniotic fluid containing these cells
- It is done by passing a needle under continuous direct ultrasound through the abdominal wall and myometrium into the amniotic cavity and aspirating fluid
- The total post-amniocentesis pregnancy loss risk = 1.9% (if performed > 15 weeks)
- Laboratories can provide a result for common aneuploidies (T21, 18, 13, X and Y) within 48 hours
- Full culture takes around 7-10 days
- Amniotic fluid can also be used to check for foetal viral infections
What is the advantage of CVS over amniocentesis?
- CVS can be performed earlier in the pregnancy
- This is at a stage where surgical termination of pregnancy is possible
- CVS also provides a larger sample of DNA
What is Cordocentesis?
What is the most common reason for it?
How does it work?
- This is performed when foetal blood is needed
- The MOST COMMON reason for cordocentesis is:
- Suspected severe foetal anaemia
- Thrombocytopaenia
- A needle is passed under ultrasound guidance through the abdominal wall and myometrium into the umbilical cord at the point where it insert into the placenta
- Can be performed from about 20 weeks
- Risk of miscarriage varies with indication and position of the placenta
Describe the prenatal diagnosis of Downs Syndrome?
- The screening test involves a combination of:
- Nuchal translucency (NT)
- Thicker in foetuses with Down syndrome
- Crown-rump length (CRL) from 11+2 to 14+1 weeks
- Maternal blood test showing hCG and PAPP-A levels from 10 - 14+1 weeks
- Nuchal translucency (NT)
- The mother’s age is also taken into account when determining risk
- A quadruple test (hCG, AFP, unconjugated oestriol and inhibin A) is the screening strategy of choice in the 2nd trimester
Describe Prenatal Diagnosis of Foetal Single Gene Disorders
When are prenatal tests offered?
- Certain autosomal dominant single gene disorders can be diagnosed antenatally
- Offered when:
- Father is known to be affected
- Risk of recurrence
- Condition is suspected on ultrasound:
- E.g. autosomal dominant severe skeletal dysplasia (achondroplasia, thanatophoric dysplasia)
- These are caused by a mutation in FGFR3
- PCR can be done to identify the FGFR3 mutation in cffDNA
- E.g. autosomal dominant severe skeletal dysplasia (achondroplasia, thanatophoric dysplasia)
Describe Foetal Blood Group test
- If RhD-positive DNA is amplified from the blood of an RhD-negative woman, it must have come from the foetus (thereby showing that the foetus is RhD-positive)
- This test is used in RhD-negative mothers with anti-D antibodies to determine the blood group of the foetus
- If the foetus is RhD-positive, it is at risk of alloimmune haemolytic disease
- Foetal blood group can be determined from samples taken in the first trimester
What are common musculoskeletal antenatal obstretric complications
- Backache
- Symphysis Pubic Dysfunction
- Carpal Tunnel Syndrome
As an antenatal obstetric complication, what is backache caused by and what are its complications?
How to manage backache?
- Very common
- Caused by:
- Hormone induced laxity of the spinal ligaments
- Shifting in the centre of gravity as the uterus grows
- Additional weight gain
- Causes exaggerated lumbar lordosis
- Advise correct posture, avoid lifting heavy objects, avoid high-heels, regular physiotherapy and simple analgesia (paracetamol or co-codamol)