Testing for fetal abnormalities in pregnancy

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Antenatal screening screening tests are made up of tests that are low invasive giving you a risk (ie 1 in 500 or 1 in 6000 etc) or diagnostic tests which give a definitive answer as to whether or not your baby has an abnormality such as Downs Syndrome. Diagnostic tests are not carried out routinely and you will be offered one if a screening test determines that you are a higher risk for abnormalities such as Downs. Screening tests can also give you a risk factor for other rare  congenital abnormalities such as Edwards and Patau Syndrome.

Combined test and NIPT

For a long time now, doctors have been using the results of the Nuchal Translucency Scan which is done between 11-13 weeks to advise pregnant women and their partners on their risk of having a baby with Downs syndrome, or other chromosonal abnormalities. The Nuchal Translucency (NT) ultrasound scan looks at the fold of skin on the back of the baby’s neck at about 12 weeks. This is known as the combined test and has achieved a positive predictive value of about 84%. Women with a high risk have an option to proceed to chorionic villous sampling or amniocentesis. These are invasive tests that carry a miscarriage risk of about 1:100.

This has been developed further by looking at the nasal bone of the baby combined with a blood test (NIPT) to identify two pregnancy hormones.

It has been discovered that a blood sample taken from the mother can be analysed for cell free fetal DNA, which is essentially a marker in the mother’s blood of the DNA of the baby. During pregnancy, a minute number of fetal cells roam freely around the mother’s circulatory system!

The result of the simple blood test (NIPT), taken from 10 weeks onwards has a sensitivity for the detection of Downs syndrome greater than 99%, 97.4% of Edwards syndrome and 93.8% of Patau’s syndrome. This is commonly known as a NIPT test. It is available privately to everyone but only to a select number of women on the NHS. If the initial NT Scan and blood test results in a high risk for chromosonal abnormalities, a referral may be offered for a NIPT.

Why the NIPT test?

This is one of the most accurate non invasive prenatal tests to detect the chance of a chromosomal abnormality in a pregnancy.

Diagnostic prenatal screening tests

All diagnostic tests also carry a small risk of miscarriage (approx 1%) and you will need to weigh up the pros and cons before going ahead with one.

Chorionic Villus sampling (CVS)

This is a diagnostic test that involves taking a tissue sample from the placenta to identify for certain whether your baby has Down’s syndrome or a genetic abnormality. This can be done as the placenta contains the same genetic information as the baby. This test is carried out between 11-13 weeks gestation.

How is it done?

There are two procedures for CVS; one method extracts a sample of the placenta via the abdomen, and the other method carries out the procedure vaginally. With the abdominal method, a fine needle is inserted through your abdomen. Using an ultrasound scan as guidance,  the doctor removes a very small sample of placental tissue. To carry out CVS vaginally, the doctor inserts a small tube through the vagina and cervix  which then passes through the uterine wall to get to the placenta for tissue extraction. You have to wait about 10 days for the results for both procedures.

Amniocentesis

Amniocentesis is a diagnostic test used mainly to identify chromosomal abnormalities and is the most commonly used test for diagnosing Downs syndrome. During the test a sample of amniotic fluid containing cells from the baby’s system is taken from the uterus. It is a relatively quick and painless procedure that is offered around 16-19 weeks.

How is it done?

Using an ultrasound scan to guide the procedure, a long thin needle is inserted into the mothers abdomen into the amniotic sac and a small sample of amniotic fluid is extracted. This contains fetal cells, which are then grown in a culture in a laboratory to be analysed. as there is a small risk of miscarriage, you may be advised  to rest a day or two afterwards to minimise the risk. The majority of the results take 2-3 weeks although some laboratories offer provisional results after around a week.

Cordocentesis

This diagnostic test is also known as fetal blood sampling or umbilical vein sampling. During this test, blood is taken from the baby’s umbilical cord to diagnose Down’s syndrome when earlier screening tests have identified a potential problem. Since this is an extremely specialised procedure, it can only be carried out at a regional specialist fetal medicine centre in certain parts of the country.

It’s always worthwhile discussing antenatal screening tests in full with your doctor or midwife.

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