City switcher
City switcher
Search
Pregnancy Tests
Make informed decisions during your pregnancy
Main Image
Article
Whatever your fitness, family history and age, the excitement of pregnancy can come tinged with a quiet concern – will my baby be healthy? Having my first at 39 and second at 42, my concern was less quiet. Risk of a baby with Down’s syndrome (DS) soars from around 1 in 1 400 pregnancies before age 25 to 1 in 350 at 35, and 1 in 32 at 45, and other rarer chromosomal abnormalities also increase with age.
Today a slew of non-invasive tests can give early indications of your risk for problems such as these, but only amniocentesis and chorionic villus sampling (CVS) give a definite diagnosis. They also, however, carry a slight risk of miscarriage.
The choice can be challenging, especially when you are older: to risk having a special-needs child or no child at all (as fertility plummets with age)? For some, like my husband and me, it was relatively simple. After many contented years of just the two of us, we wanted a healthy child or no child. I had an amnio, and in due course, thankfully, two healthy boys.
But while I’m one of many women grateful for the tests available today, the feeling is not shared by all. Three years ago, Durban school shuttle operator Marcelle Riley was happily pregnant. Just 31, healthy and with no family history of problems, she was unconcerned, and only went for a routine blood test at 19 weeks (the triple blood screening). She was told it showed her chance of having a baby with DS was not high, and she proceeded with confidence.
“When she was born I wanted to hold and feed her immediately to bond, but doctors whisked her away,” says Marcelle. “They spoke to my partner, and an hour later he told me our daughter had DS. Then they sedated me – the hospital has had moms who ducked out and left their babies!”
In retrospect, Marcelle believes she should have been offered an amniocentesis after the blood test. “They told me my risk was about one in 2 224, which I learned subsequently was not that low.” Yet she’s not sure what she would have done if an amnio had confirmed a baby with DS.
“I like to think we wouldn’t have terminated. It’s been hard with our daughter, but I’m bonding more and more with her, and her dad and I can’t imagine life without her – he’s been amazing with her.”
When Marcelle became pregnant with their second child last year, people expected her to opt for all possible tests. Yet the only one she and her partner chose to have was a scan at 15 weeks. “After our first baby, we knew we wouldn’t terminate, whatever the result,” she explains. “We only had that scan because the GP suggested we see how the pregnancy was progressing. The measurements were all okay, but I was nervous the day I gave birth. Jessica was fine, and she and her sister are both doing well.”
“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 fetal specialist Dr Ismail Bhorat. “Every pregnant woman is at risk of genetic mutation irrespective of age – if age alone is used as a screening test, the detection rate for Down’s syndrome is only 30 percent.”
Although substantial research is being done in the area of DS, it is not yet treatable, he says, and a termination will be offered. For those patients who don’t want to terminate, “knowing the diagnosis will give them time to prepare for the outcome,” he says. “All children who have Down’s syndrome are mentally challenged, though severity varies, and all will require special-needs care.”
Caroline Willis, who heads Down Syndrome South Africa (DSSA) KZN branch, and has a daughter of 18 with DS, agrees. “Our organisation is neither for nor against termination,” she says. “Every parent needs to be equipped to make an informed decision for themselves, because it’s one they must live with for the rest of their lives.”
Better on the big screen?
Bhorat 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, see “testing the tests”), with a first trimester maternal blood test for beta human chorionic gonadotropin (BHCG) and papp-a (placental hormones), indicators for DS, 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,” he says. Benoni 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 fetal 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. These early diagnoses can help doctors to counsel patients and manage potential adverse outcomes.”
The second main assessment is an 18- to 20-week congenital anomaly screen, a physical screen of the 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, he says, the triple blood test for alpha-fetoprotein (AFP), unconjugated estriol (uE3) and BHCG is used with an accuracy rate of 60 to 65 percent (see “testing the tests”). “This is the most commonly used screening test, but the 11- to 14-week screen is far superior.”
Only if results indicate a high risk of abnormality (greater than one in 300), are invasive procedures recommended to confirm diagnosis – chorionic villus sampling (CVS) or amniocentesis.
Some fetal units also offer a non-invasive option. If a patient presents with a high-risk Down’s screen and doesn’t want invasive testing, a genetic sonogram is performed at 18 to 20 weeks looking for markers for DS, says Bhorat. The main ones are nuchal fold (see under “testing the test”), 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).
“As you can see, there are many options,” he concludes, “and the well-informed patient will choose what suits her.”
TESTING THE TESTS
Ultrasound
What is it?
Sound waves show an image that is used to establish the age and number of fetuses, and their size, position and possible abnormalities, such as spina bifida.
When’s it done?
All pregnant women are generally offered a dating scan in the first eight weeks, and a congenital anomaly scan around 18 to 20 weeks. Some are also offered a nuchal translucency (NT) scan around 11 to 14 weeks. This assesses the amount of fluid behind the neck of the fetus, an indicator of Down’s syndrome.
How safe is it?
There’s no hard evidence of scans affecting a baby.
How accurate is it?
About 90 percent for DS, says Bhorat.
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 fetus. A baby with an open spine (spina bifida) will excrete more, one with DS will excrete less. The newer triple screen tests for AFP plus two hormones, unconjugated estriol (uE3) and BHCG, are also markers for abnormalities.
When’s it done?
16 to 19 weeks
How safe is it?
Completely
How accurate is it?
60 percent for a five percent false positive rate, if the high-risk cutoff is set at one in 300, says Bhorat. “That is, if our patient population is 100 000 and the incidence of DS 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 it 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 fetuses 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 taken from the placenta to test for DS and other disorders (though not spina bifida).
When’s it done?
11 to 13 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 percent
Amniocentesis
What is it?
An ultrasound scan shows how your baby lies, and a needle is inserted through your abdominal wall and draws fluid from the amniotic sac. It tests for DS, spina bifida and other fetal 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’s 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 as less than two percent. Dr Bhorat puts risk of complications at 0,5 percent if performed in an experienced unit.
How accurate is it?
99,9 percent
Cordocentesis
What is it?
Fetal blood is taken by inserting a needle through your abdominal wall into the umbilical vein, a highly specialised procedure. It can detect DS, infection with diseases such as rubella and toxoplasmosis, and anaemia.
When’s it done?
After 18 weeks (second trimester)
How safe is it?
Risk of miscarriage is under two percent
How accurate is it?
99,9 percent
Questions to consider
Depending on where you go, some tests are offered routinely to all pregnant women or those at high risk of genetic problems such as Down’s syndrome and spina bifida, but all tests can be requested. Before deciding on any test, ask yourself and your doctor:
- How reliable is it?
- How risky is it?
- Why am I having it?
- If a problem is detected, would it change what I plan to do? (Else, why have it?)
- Would I terminate or prepare for raising a special-needs baby? Think through what both options would entail. Speak to your doctor and to support groups such as DSSA on 0861 369 672, dssaoffice@icon.co.za or visit downsyndrome.org.za
add your comments