SASUOG

South African Society for Ultrasound
in Obstetrics and Gynaecology

For Patients

Prenatal tests


Your Obstetrician/Gynaecologist will request some tests to check that you are healthy. These include the following:
 

Routine tests


  • Your ABO and rhesus blood group, to determine whether your baby is at risk of anaemia or jaundice (if there is an incompatibility between your blood group and that of the baby’s father);
  • Your haemoglobin levels, to determine whether you are anaemic;
  • Whether there are antibodies against rubella (German measles), HIV, hepatitis B and syphilis, to determine whether your baby is at risk of a congenital infection;
  • Your blood glucose levels ( in some cases), to determine whether you are at risk of diabetes (which might be underlying, or brought on by the pregnancy);
  • Your blood pressure, to determine whether you might be developing a condition called pre-eclampsia (high blood pressure brought on by the pregnancy);
  • Your urine, to determine whether you have a bladder infection, or whether there is protein in your urine, which might also indicate pre-eclampsia;
  • Whether you are a carrier of group B streptococcus, a bacterium which might be present in your vagina and could give rise to a pneumonia/meningitis in the baby (if attempting a normal vaginal delivery).
It is also possible to do some tests to determine whether your baby is at risk for certain physical or mental problems. These tests include prenatal screening tests (which give an assessment of risk) and prenatal diagnostic tests (which give a more conclusive answer on whether a baby, which is at risk of some problem, is definitely affected or not). The prenatal screening and diagnostic tests are optional, but provide valuable information for the management of the pregnancy.

It is important to realize that no test or combination of tests can ever guarantee a normal baby. On the other hand, the vast majority of babies are normal (even if no screening test had been done!). It is also important to make sure that your doctor is aware of any family or personal history of medical problems that might have an effect on the baby.
 

Screening and diagnostic tests


Screening tests are tests done on everyone. A screening test does not give a definitive answer about whether a condition is present or absent, but gives an indication of the likelihood (low, intermediate, or high risk). The majority of afflicted individuals (e.g. foetuses with Down syndrome) should be in the “high risk” group. The proportion of afflicted individuals that is detected by the test is called the sensitivity. The proportion of the normal individuals that is falsely labelled as high risk is called the false positive rate. Ideally, a screening test would have a high sensitivity (although it is never 100%), and low false positive rate (although it is never 0%).

A diagnostic test is a test done on someone at risk of a condition, and gives a definitive answer about whether the condition is present or absent. A diagnostic test has some features that makes it unsuitable for use as a screening test, such as risk or cost. The following screening tests are available in the three trimesters of pregnancy:
 

First trimester tests


The main aim is to detect foetuses at high risk of Down syndrome. Other benefits include detecting foetuses at risk of a chromosomal abnormality other than Down syndrome, dating the pregnancy accurately, detecting twins (or higher order multiple pregnancies) and diagnosing a miscarriage or some major foetal abnormalities.
 

Early combination test


The recommended first trimester screening test is the early combination test. This consists of a blood test (PAPP-A and free b-HCG) done at 8-12 weeks, followed by an ultrasound examination (“simple NT scan”) done at 12 – 13 weeks. The name “NT” refers to the measurement of the nuchal translucency thickness, a patch of fluid behind the foetus’ neck which is thickened in most foetuses with Down syndrome. The simple NT scan should be done by a practitioner (your obstetrician or someone else) who is accredited by the Fetal Medicine Foundation (the list of practitioners is available on https://fetalmedicine.org/lists/map/certified/NT) or an experienced/competent Obstetrician in NT scanning. The early combination test has a sensitivity of 85% for Down syndrome and 5% false positive rate.

If the early combination test demonstrates a low risk (?1:1000), no further testing is recommended. If it indicates a high risk (?1:100), a diagnostic test such as amniocentesis or chorionic villus sample (see below) is offered. If an intermediate risk (1:100 – 1:1000) is found, the possibilities would include a diagnostic test or further screening with an extended NT scan or cell-free DNA testing (see below).

Alternative screening tests


Alternatives to the early combination test in the first trimester include:
  • First trimester biochemical screening, which entails the blood test for PAPP-A and free b-HCG which is done as part of the early combination test. Ultrasound examination is still needed to determine the pregnancy duration accurately and exclude problems such as a major fetal abnormality, multiple pregnancy or miscarriage. This has a 60% sensitivity and a 5% false positive rate.
  • Simple NT scan only without biochemical screening. This has a 75% sensitivity and a 5% false positive rate.
  • Extended NT scan, which entails a similar procedure to the early combination screen, but with a more detailed ultrasound evaluation including evaluation of the nuchal translucency and nasal bone, as well as the foetal blood flow. There are fewer practitioners accredited for this (https://fetalmedicine.org/lists/map/certified/DV  ). The sensitivity for Down syndrome is 90% and the false positive rate 2.5%.
  • Cell free DNA testing, also called NIPT (non-invasive prenatal testing). This test is very accurate (?99% sensitivity and ?1% false positive rate), but currently very expensive. It can be performed any time after 10 weeks, but also requires an ultrasound examination beforehand to exclude problems such as a major fetal abnormality, multiple pregnancy or miscarriage.

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