Prenatal testing options for genetic abnormalities

Posted on 4 August 2017

Amniocentesis used to be the most common procedure to test for serious genetic birth defects, but nowadays there are other less invasive ways to test prenatally for genetic abnormalities.

Most babies are born without disabilities, but in some rare instances they may have inherited physical or mental problems. While no test or combination of tests can ever guarantee a baby will be born without disabilities, prenatal testing can flag possible issues to prepare the parents.

Amniocentesis is a procedure in which amniotic fluid is extracted from the sac around the unborn baby and tested to detect serious genetic birth defects. Although quick and almost painless, it can still be stressful for the mother and potentially harmful to the baby (as covered in our article on amniocentesis).

The following less-invasive blood tests are alternatives that may be requested by your doctor. However, in some instances you may still need an amnio even if you have the blood tests.

Maternal blood test for PAPP-A and free β-HCG

Done at 8-12 weeks (early is best).

Tests for levels of pregnancy-associated plasma protein A (PAPP-A) and free beta-human chorionic gonadotropin (free β-HCG), both produced by the placenta. Both of these at abnormal levels are considered markers for Down’s syndrome, the most common genetic condition with significant consequences.

The test has a 60% sensitivity and a 5% false positive rate for Down’s syndrome.

Maternal blood test for AFP, HCG and estriol (triple test)

Done at 15-20 weeks.

Tests for blood levels of alpha fetoprotein (AFP), a protein produced by the liver and yolk sac of a developing foetus; human chorionic gonadotropin (HCG), a hormone produced by the placenta; and estriol, an oestrogen produced by both the foetus and the placenta. At abnormal levels, these can signal foetuses at high risk of Down’s syndrome or a neural-tube defect such as spina bifida.

The test has a 60% sensitivity and a 5% false positive rate for Down’s syndrome.

Cell-free DNA testing or non-invasive prenatal testing (NIPT)

Done any time after 10 weeks.

Tests the mother’s blood for fragments of the foetus’s DNA, which is then screened for chromosomal abnormalities and a variety of genetic conditions. It can also provide information about foetal gender and rhesus (Rh) blood type.

The test has a 99+% sensitivity and less than 1% false positive rate. It is expensive, however, and not covered by most medical schemes.

Scans

Ultrasound scans are needed before certain blood tests to confirm exactly how far along the pregnancy is, and to exclude twins or miscarriage.

A combination of blood tests and scans can give more accurate results.

There is also a range of other scanning options to flag problems.

Understanding the test

Screening blood tests do not give a definitive answer about whether a condition such as Down’s syndrome is present or absent. They only give an indication of the likelihood, expressed either in numbers (risk of one in xxx) or as low, intermediate, or high risk.

The proportion of individuals with the abnormality who are identified by the test is called the sensitivity. The proportion of typical individuals who received a ‘high risk’ result is called the false positive rate. The a-typical foetuses who got a ‘low risk’ result are called false negative results. Ideally, a screening test needs a high sensitivity rate and low false positive rate.

References

Dr Lou Pistorius of Mediclinic Panorama (write up)

Prof Lut Geerts of Tygerberg Hospital (write up)

http://www.childmag.co.za/content/pregnancy-tests#.WTUlUFcwxeI

http://www.medicinenet.com/script/main/art.asp?articlekey=24602

https://patient.info/doctor/prenatal-screening-for-downs-syndrome

http://americanpregnancy.org/prenatal-testing/triple-screen-test/

http://www.mayoclinic.org/tests-procedures/noninvasive-prenatal-testing/home/ovc-20187358

Published in Babies

In the interest of our patients, in accordance with SA law and our commitment to expertise, Mediclinic cannot subscribe to the practice of online diagnosis. Please consult a medical professional for specific medical advice. If you have any major concerns, please see your doctor for an assessment. If you have any cause for concern, your GP will be able to direct you to the appropriate specialists.

Post a comment

Leave a reply