Antenatal Diagnostic tests – Identifying fetal abnormalities

Antenatal Diagnostic tests give a definitive answer as to whether or not your baby has an abnormality such as Downs Syndrome. These 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.

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 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.


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