Things to keep in mind:
- Informed choice – before ordering the test, discuss benefits, risks and limitations.
- Autonomy – the patient should choose whether to have prenatal screening.
- What prenatal screening options are available in your area?
- What option is most suitable for your patient?
- Which test will provide the optimal care for your patient?
- A screening test is not diagnostic.
The following recommendations are from:
SOGC Clinical Practice Guideline: Update on Prenatal Screening for Fetal Aneuploidy, Fetal Anomalies, and Adverse Pregnancy Outcomes. J Obstet Gynaecol Can 2017;39(9):805–817.
- All pregnant person’s in Canada, regardless of age, should be offered, through an informed counselling process, the option of a prenatal screening test for the most common fetal aneuploidies (II-A).
First trimester nuchal translucency should be interpreted for risk assessment only when measured by sonographers or sonologists trained and accredited for this fetal screening service and when there is ongoing quality assurance (II-2A). For aneuploidy, it should be offered as a screen with maternal serum biochemical markers in singleton pregnancies (II-2B).
Pregnant person’s age alone is a poor minimum standard for prenatal screening for aneuploidy, and it should not be used as a basis for recommending invasive fetal diagnostic testing when prenatal screening for aneuploidy is available (II-2D).
Health care providers should be aware of the prenatal screening modalities available in their province or territory (III-B). A reliable prenatal system needs to be in place ensuring timely reporting of results (III-C). Prenatal screening programs should be implemented with resources that support audited screening and diagnostic laboratory services, ultrasound, genetic counselling services, patient and health care provider education, and high-quality diagnostic testing, as well as resources for administration, annual clinical audit, and data management. In addition, there must be the flexibility and funding opportunities to adjust the program to new technology and protocols (II-3B).
A discussion of the risks, benefits and alternatives of the various prenatal diagnoses and screening options, including the option of no testing, should be undertaken with all patients prior to any prenatal screening. Following this counselling, patients should be offered (1) no aneuploidy screening, (2) standard prenatal screening based on locally offered paradigms, (3) ultrasound guided invasive testing when appropriate indications are present, or (4) maternal plasma cell-free DNA screening where available, with the understanding that it may not be provincially funded (II-2B).
Regardless of aneuploidy screening choice, all pregnant person’s should be offered a fetal ultrasound (optimally between 11 and 14 weeks) to confirm viability, gestational age, number of fetuses, chorionicity in multiples, early anatomic assessment, and nuchal translucency (NT) evaluation where available. The NT measurement for aneuploidy risk estimation (combined with maternal serum) should not be performed if cell-free DNA screening has been used. Every effort should be made to improve access to high quality first trimester ultrasound for all Canadian pregnant person’s. In areas where NT assessment is not available, a first trimester dating ultrasound improves the accuracy of maternal serum screening and the management of pregnancy (II-1A).
A large nuchal translucency (>3.5 mm) should be considered a major marker for fetal chromosomal and structural anomalies and requires genetic counselling, an offer of invasive testing with chromosomal microarray analysis, and detailed second trimester ultrasound follow-up (II-2A).
Pregnant person’s who are considering undergoing maternal plasma cell-free DNA (cfDNA) screening should be informed that:
It is a highly effective screening test for the common fetal trisomies (21, 18, 13), performed after 10 weeks” gestation (II-1A).
There is a possibility of a failed test (no result available), false negative or positive fetal result, and an unexpected fetal or maternal result (II-1A)
All positive cfDNA screening results should be confirmed with invasive fetal diagnostic testing prior to any irrevocable decision
- Management decisions, including termination of pregnancy, require diagnostic testing and should not be based on maternal plasma cfDNA results alone because it is not a diagnostic test (II-2B).
If a fetal structural abnormality is identified in a pregnant person regardless of previous screening test results, the pregnant person should undergo genetic counselling and be offered invasive diagnostic testing with rapid aneuploidy detection and reflex to microarray analysis if rapid aneuploidy detection is normal or inconclusive (II-2B).
Although cfDNA screening for aneuploidy in twin pregnancy is available, there is less validation data than for a singleton pregnancy and it should be undertaken with caution (II-2C).
Routine cfDNA screening for fetal microdeletions is not currently recommended (II-2B).
- If a fetal structural abnormality is identified, regardless of previous screening test results, genetic counselling and invasive fetal diagnostic testing should be offered, with rapid aneuploidy detection and reflex to microarray analysis if rapid aneuploidy detection is normal or inconclusive (II-2A).
The presence of an isolated fetal soft marker in the second trimester, with the exception of increased nuchal fold, should not be used to adjust the a priori risk for fetal aneuploidy (II-2D).
Universal screening for adverse pregnancy outcomes using maternal serum markers is currently not recommended outside of an investigational protocol with informed consent (II-2D).
There are a number of other guidelines which can be found on the SOGC website:http://www.sogc.org/index_e.asp. A number of these guidelines can be found as hyperlinks below:
a) SOGC Clinical Practice Guideline: Prenatal Screening for and Diagnosis of Aneuploidy in Twin Pregnancies. J Obstet Gynaecol Can 2011; 33(7):754–767.
b) SOGC Clinical Practice Guideline: Obstetrical Complications Associated with Abnormal Maternal Serum Markers Analytes. J Obstet Gynaecol Can 2008;30(10):918–932.
c) SOGC Clinical Practice Guideline: Pre-conceptional Vitamin/Folic Acid Supplementation 2007: The use of folic acid in combination with a multivitamin supplement for the prevention of neural tube defects and other congenital anomalies. J Obstet Gynaecol Can 2007;29(12):1003–1013.