Pregnancy Tests

Make informed decisions regarding tests during your pregnancy
By Glynis Horning

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Whatever your fitness level, family history and age, the excitement of pregnancy can come tinged with a quiet concern – will my baby be healthy?
 
“All prospective parents need to know what tests they are going for in pregnancy, be counselled before doing them and consider what they will do in the event of a positive screen,” says Durban-based foetal specialist Dr Ismail Bhorat. He advocates that two main risk assessments be routinely performed in pregnancy, both non-invasive.
 
First is an 11- to 14-week screen using a combination of nuchal translucency scan and nasal bone determination (both Down’s syndrome markers) with a first trimester maternal blood test for Beta Human Chorionic Gonadotropin (BHCG) and papp-a (placental hormones), indicators for Down’s, Edward’s and Patau syndromes. “This will give you an accuracy of 90 percent for a five percent false positive rate – an exceptionally high accuracy rate for a non-invasive test,” says Bhorat. Benoni-based gynaecologist Dr Mogi Lingham adds that as well as checking for chromosome abnormalities, the 11- to 14-week screen “also provides the most pertinent maternal and foetal risk assessment” and allows for the identification of pregnant women at high medical risk. “It provides the possibility to detect adverse pregnancy outcomes, including intrauterine growth retardation, pre-eclampsia, stillbirth and pre-term delivery.” Bhorat adds that in terms of screening for Down’s syndrome, the best test is the 11- to 14-week risk assessment, performed in an accredited centre.
 
The second main assessment is an 18- to 20-week congenital anomaly screen, a physical screen of various organ systems combined with a genetic sonogram, which is a soft marker screen for chromosomal anomalies. The scan and the first trimester screen can be combined for a Down’s syndrome detection rate of around 95 percent, says Bhorat.
 
If a patient misses the 11- to 14-week screen, the next best screening option is combining the 16- to 19-week triple test (biochemical test for Down’s syndrome), which itself has an accuracy of 60 percent for a five percent false positive rate, with a 20- to 21-week anomaly scan with a genetic sonogram (soft marker screen) achieving an accuracy of 85 percent for a five percent false positive rate at a risk cut-off of 1:300.
 
Only if results indicate a high risk of abnormality (greater than one in 300) are invasive procedures recommended – Chorionic Villus Sampling (CVS) or amniocentesis. According to Bhorat, recent studies have shown that the complication rate of these tests is lower than originally thought, generally quoted between 1:300-1:500. Some foetal units also offer a non-invasive option. A genetic sonogram can be performed at 18 to 20 weeks looking for markers for Down’s syndrome, says Bhorat. The main ones are nuchal fold, nasal bone, echogenic intracardiac focus (a bright spot seen in the baby’s heart indicating greater density), dilated kidneys, hyperechogenic bowel (a bright spot seen in the bowel), and short humerus and short femur (upper arm and leg bones).
 
 
 
Testing the tests
 
Cell-free DNA or Non-invasive Prenatal Testing (NIPT)
What is it? A maternal blood test that can detect fragments of the foetus’ DNA floating in the mother’s bloodstream just a few weeks into pregnancy. Scientists then look for abnormalities in the foetus’ chromosomes. Bhorat says that this is a relatively new test that has revolutionised the screening and testing procedures.
When is it done? The test can be done as early as 10 weeks and results are generally available between 7 and 14 days, depending on which test is used.
How safe is it? 100%
How accurate is it? The NIPT is an excellent test for detecting Down’s syndrome with an accuracy of 99 percent and false positive rate of 0.1 percent, but it may not detect other genetic or chromosomal anomalies. Bhorat cautions, though, that it is regarded as a screening test and does not replace existing screening procedures, but rather adds to accuracy and predictions. A positive predictive value will need to be confirmed either with CVS or an amniocentesis.

 

Alpha-fetoprotein (AFP) screening or triple blood screening
What is it? A sample of your blood is taken to test for levels of AFP, a protein produced and excreted by the foetus. A baby with an open spine (spina bifida) will excrete more, while one with Down’s syndrome will excrete less. The triple screen tests for AFP plus two hormones, unconjugated estriol (uE3) and BHCG, which are also markers for abnormalities.
When is it done? 16 to 19 weeks
How safe is it? 100%
How accurate is it? Sixty percent for a five percent false positive rate, if the high-risk cut-off is set at one in 300, says Bhorat. “That is, if our patient population is 100 000 and the incidence of Down’s syndrome across the board is one in 1 000 (actually the incidence in the general population is one in 800), then this test will pick up 60 babies with Down’s syndrome while some 5 000 women will test falsely positive – meaning healthy babies will be falsely determined to have an abnormality. The false positive readings can result from miscalculated birth dates, carrying multiple foetuses and other benign factors.”

 

Chorionic Villus Sampling (CVS)
What is it? A needle is inserted through the wall of your uterus or through your vagina and cervix, and a tissue sample is taken from the placenta to test for Down’s syndrome and other disorders (though not spina bifida).
When is it done? 11 to 14 weeks (first trimester)
How safe is it? There’s a less than two percent risk of miscarriage. Limb reduction defects have been reported, says Bhorat, but only if the test is done before 11 weeks. He puts risk of complications at under one percent if done in an experienced unit.
How accurate is it? 99.9%

 

Amniocentesis
What is it? An ultrasound scan that shows how your baby lies. A needle is inserted through your abdominal wall to draw fluid from the amniotic sac. It tests for Down’s syndrome, spina bifida and other foetal problems such as cystic fibrosis and muscular dystrophy. It can establish the sex of your baby; important if you carry a sex-linked genetic disorder.
When is it done? 15 to 20 weeks; results can take two to four weeks by which time you may be mid-pregnancy.
How safe is it? A recent study gives risk of miscarriage at less than two percent. Bhorat puts risk of complications at 0.5 percent if performed in an experienced unit.
How accurate is it? 99.9%

 

Cordocentesis
What is it? Foetal blood is taken by inserting a needle through your abdominal wall into the umbilical vein – a highly specialised procedure. It can detect Down’s syndrome, infection with diseases such as rubella and toxoplasmosis, and anaemia.
When is it done? After 18 weeks (second trimester)
How safe is it? Risk of miscarriage is less than two percent.
How accurate is it? 99.9% 

 

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