Pregnancy is generally considered an exciting time for prospective parents and their families.
However, for many women and their partners, the happiness that comes with a positive pregnancy test is sometimes overshadowed by anxiety regarding the health of the baby, particularly if there is a history of miscarriages or health problems with previous babies.
Previously, the majority of expectant parents have relied solely on the traditional methods of ultrasound scans and blood tests carried out in the first trimester of pregnancy to help obtain more information about the health of their baby.
Screening Versus Diagnostic Tests – What’s The Difference?
It is important to note differences between a screening test and a diagnostic one. A screening test is one that detects whether the baby has a higher than normal risk of having a particular defect or condition, but cannot say definitively that this is the case. On the other hand, a diagnostic test is one that can tell whether or not the baby definitely has the defect or condition. Diagnostic tests tend to be invasive and carry a small risk of miscarriage.
The Combined Test
The most widely known screening method is often called the combined screening test, as it uses a combination of results from an ultrasound scan and blood test to calculate a risk of the baby having certain genetic conditions.
- This NT (nuchal translucency) ultrasound scan is carried out between 11 and 13-14 weeks of pregnancy. The ultrasound measures the amount of fluid lying under the skin at the back of the baby’s neck, whilst the blood test measures substances called B-HCG and PAPP-A. The NT measurement is often larger than normal if the baby has Down syndrome (extra chromosome 21) or certain other conditions.
- B-HCG is the chemical that is used in pregnancy tests to give a positive result and rises rapidly in the first 8-10 weeks of a pregnancy where it then tails off. Higher than normal levels of this hormone can indicate an increased risk for Down’s syndrome, whilst lower than normal levels are often associated with Edwards and Patau syndromes (extra chromosome 18) or Patau syndrome (an extra chromosome 13).
- PAPP-A is pregnancy-associated plasma protein – a protein produced by the placenta. Low levels of this hormone can be associated with increased risk of Down syndrome, Patau syndrome and Edwards syndrome. Low PAPP-A may also be associated with small babies, preterm deliveries and pre-eclampsia, and so extra scans may be advised at a later stage to check growth.
The results of the blood test, along with the mother’s age, weight, stage of pregnancy and any other information (smoker or non-smoker for example), are put into a computer to work out the chance of the baby having Down syndrome.
In the NHS in Scotland, a higher risk result is considered to be greater than a 1 in 150 chance of your baby having Down syndrome. If this is the case, then diagnostic tests such as amniocentesis or chorionic villus sampling (CVS) are offered.
Non-Invasive Prenatal Testing
An alternative option to proceeding to diagnostic testing at this point is to consider performing non-invasive prenatal testing (NIPT). This form of testing offers a great advantage over tests such as amniocentesis and CVS as they do not pose a risk to the baby or mother. The test involves taking a blood sample from the mother and analyses foetal DNA fragments in the maternal blood (this is called cell-free DNA).
There are several different prenatal cell-free DNA screening tests available including Harmony, Verifi, Panorama Test, and Maternity21P offered by private clinics.
In the case of the Harmony test, the blood test examines foetal DNA in the maternal bloodstream to screen for the increased chance for specific chromosome problems, such as the trisomies mentioned earlier – Down, Edwards And Patau syndromes. It can also determine some sex chromosome abnormalities (also called sex chromosome aneuploidy), where differences in the number of X and Y chromosomes can result in conditions with various health issues and/or behavioural problems. This test also offers an option for parents to learn the sex of their babies – possible even for mothers carrying twins.
New in Harmony Testing! More recently, Harmony has added a further option to test for a condition called DiGeorge syndrome (also known as 22q11.2 deletion syndrome). This is a condition caused by a missing piece (deletion) of chromosome 22, which can result in kidney, heart and growth problems.
Studies have shown that the Harmony test can detect more than 99% of babies with Down syndrome, whereas the traditional combined test detects around 79%.
It is important to note that NIPT is still considered a screening test, and prospective mothers with a high-risk result should proceed to amniocentesis or chorionic villus sampling for a definitive diagnosis.
The NHS does not routinely offer the DNA test at the moment, however this may change in the future.
These include the procedures amniocentesis and chorionic villus sampling (CVS).
Amniocentesis involves removing and testing a small sample of cells from the amniotic fluid – the fluid that surrounds the unborn baby in the womb (uterus). This is offered to women with a high-risk result from the combined testing, if there is a family history of a genetic condition, or if an abnormality is detected in the first trimester ultrasound scan. Amniocentesis is usually carried out between the 15th and 20th weeks of pregnancy, but may be later if necessary. It is generally avoided any earlier than this due to the increased risk of complications. Results can take a few days to a few weeks to come back depending on the condition being tested.
An alternative to amniocentesis is CVS. This is where a small sample of cells from the placenta is removed for testing and is carried out between the 11th and 14th weeks of pregnancy.
What are the risks?
One of the main risks associated with both amniocentesis and CVS is miscarriage. This is estimated to occur in 0.5-1% of women who have these tests. There are also some other risks, such as infection, or needing a repeat amniocentesis as not enough cells were sampled for accuracy.
Should I test at all?
It is important to remember that all prenatal screening tests are optional. There are several factors to consider when choosing which tests – if any.
What will you do with the test results?
Many people undergo prenatal screening in order to ease their anxiety about their baby’s health. However, if testing indicates that the baby has an abnormality, this can help some parents decide whether or not they wish to continue the pregnancy.
Other prospective parents may have no wish to end a pregnancy no matter what results indicate, but simply wish to be as informed as possible so they can plan for the baby’s care. In some cases, some treatment for problems can begin as early as during pregnancy, or immediately after the baby’s birth.
How accurate are the results?
It is essential to keep in mind that no test is 100% perfect and prospective parents need to be aware of the rates of false positives and negatives before deciding to proceed with a test.
What if I don’t want to test at all?
The choice to undergo any prenatal test – whether ultrasound or blood tests – is entirely up to the parents. There is no right answer and it is important to take the time to understand the risks and benefits of each test in order to help make the best decision for all concerned.
At ROC Clinic we are happy to discuss any aspects of prenatal testing with prospective parents. We can also offer the Harmony and Harmony Plus NIPT from 11 weeks gestation onward. Please get in touch if you wish more information.