In addition to these evidence-based recommendations, the guideline development group also identifies points of best clinical practice in the original guideline document.
Levels of evidence (Ia-IV) and grading of recommendations (A-C) are defined at the end of the "Major Recommendations" field.
Timing of Amniocentesis and Chorionic Villus Sampling (CVS)
A - Early amniocentesis performed before 14 completed weeks of gestation (14+0) is not a safe alternative to second-trimester amniocentesis or CVS.
It is recommended that an early amniocentesis is undertaken only in exceptional circumstances after the mother has been made fully aware of the potential complications. [Evidence level Ia/Ib]
B - It is recommended that CVS should not be performed before 10 completed weeks of gestation (10+0).
Consent
It is good clinical practice to obtain formal consent for amniocentesis or CVS before the procedure. Practice should conform to recommendations on consent from the General Medical Council and the Royal College of Obstetricians & Gynaecologists (RCOG). Use of the Department of Health consent form 3 is recommended.
Written or oral information should include what results are possible from the procedure; how, when, and by whom it is performed; and how their practice is monitored. Information should also be given on:
- National and local risks of the procedures
- Analysis (and subsequent storage) of the sample in the local cytogenetics laboratory
- Accuracy of the particular laboratory test being performed
- Culture failure rates
- Reporting time
- Method of communication of results
- Indications for seeking medical advice following the test
Where written material is not used, the counselling process, including verbal consent, needs to be clearly recorded in the patient's notes.
Method
B - Amniocentesis is associated with higher rates of successful taps and lower rates of "bloody" taps when performed under direct ultrasound control with continuous needle tip visualization.
The current recommendation for 'continuous ultrasound control' rests on the need to avoid 'bloody taps,' because the presence of blood interferes with amniocyte culture. [Evidence level III]
Best practice is that ultrasound scanning during the procedure should be performed by the person inserting the needle. An alternative technique involves ultrasound scanning being performed by a separate practitioner. Whatever the individual's views, there is no objective evidence favouring one technique over the other.
B - A transplacental approach may be appropriate if it provides easy access to a pool of amniotic fluid, but care should be taken to avoid the cord insertion.
In fact, if a clear pool of amniotic fluid can be reached only by passage through the placenta then this is the approach of choice [Evidence level IIb].
B - The outer needle diameter should not be wider than 20-gauge (0.9 mm).
B - CVS should always be performed under direct ultrasound control.
Skill of the Operator
B - Very experienced operators performing amniocentesis may have a higher success rate and a lower procedure-related loss rate. Occasional operators who perform amniocentesis less than ten times per annum may have increased rates of procedure-related loss.
B - Independent performance of amniocentesis and CVS should only occur following adequate training, which should include the use of a clinical skills model, assessment of interaction with patients, and supervised procedures.
Before undertaking procedures on women, consideration should be given to initial training using a clinical skills model. [Evidence level III]
As there are no data to guide practice, individual centres should agree a training and assessment process that is open and transparent, and with a clearly responsible trainer. Local deaneries and NHS trust clinical governance systems should have a role in ensuring quality training. It is suggested that trainers should be performing at least 50 ultrasound-guided invasive procedures per annum.
Third Trimester Amniocentesis
B - Third-trimester amniocentesis does not appear to be associated with significant risk of emergency delivery. Compared with mid-trimester procedures, complications including multiple attempts and bloodstained fluid are more common.
Control of Infection
B - Invasive prenatal testing in the first or second trimester can be carried out in women who carry hepatitis B or C. The limitations of the available data should be explained. Testing in women with human immunodeficiency virus (HIV) should be avoided, particularly in the third trimester.
Rhesus status should be available or obtained in every case. Rhesus prophylaxis with anti-D immunoglobulin must be offered following each procedure in line with national recommendations. [Evidence level Ia]
Organisation of Care
Trusts and organisations should ensure that the equipment, environment, staff training, arrangements for follow up, and links with related services carrying out pregnancy termination or support for women with diagnosed chromosomal or genetic disease are of sufficient standard and that these aspects of care are continuously reviewed.
Definitions:
Grading of Recommendations
Grade A - Requires at least one randomised controlled trial as part of a body of literature of overall good quality and consistency addressing the specific recommendation (evidence levels Ia, Ib)
Grade B - Requires the availability of well-conducted clinical studies but no randomised clinical trials on the topic of recommendations (evidence levels IIa, IIb, III)
Grade C - Requires evidence obtained from expert committee reports or opinions and/or clinical experience of respected authorities. Indicates an absence of directly applicable clinical studies of good quality (evidence level IV)
Levels of Evidence
Ia: Evidence obtained from meta-analysis of randomised controlled trials
Ib: Evidence obtained from at least one randomised controlled trial
IIa: Evidence obtained from at least one well-designed controlled study without randomisation
IIb: Evidence obtained from at least one other type of well-designed quasi-experimental study
III: Evidence obtained from well-designed non-experimental descriptive studies, such as comparative studies, correlation studies, and case studies
IV: Evidence obtained from expert committee reports or opinions and/or clinical experience of respected authorities
