Screening and Testing In Pregnancy
Prenatal Screening and Testing, what is it and when is it best done? Merlyn Glass, from Genetic Counselling Network, explains. There is clear difference between prenatal screening and prenatal invasive testing and it is important to understand this.
All of us usually have 46 chromosomes, 23 from each parent. The chromosomes are made up of DNA which contains all our genes, responsible for everything about us, not only our physical traits, but also how we function. Any numerical or structural abnormality of the chromosomes will cause the baby to have a problem.
- The easiest, and first screen, is to ask the mother’s age. Although any woman/couple is at risk for having a baby with a chromosomal abnormality, the chance increases exponentially as a woman’s age advances.
- The first sonar is usually done at eight weeks to confirm the gestation and viability of the pregnancy.
- Non-invasive prenatal test (NIPT) or cell- free fetal DNA test is the next screen which is done from nine weeks gestation. This test is expensive and therefore not accessible to all. An unborn baby’s DNA passes naturally into the mother’s blood stream. A sample from the mother is tested to see whether there is more DNA than expected. Although NIPT is usually offered to women who are at risk for having a baby with a chromosome abnormality, ideally any pregnant woman should have this option. The benefits of this early screen is that there is a less than 0,1% false positive rate and there is no risk to the pregnancy and results are available from 7-10 days after the sample is received in the laboratory. If the result shows that there is extra DNA, invasive testing is offered to confirm a diagnosis either by invasive testing.
- Whether or not a NIPT has been done, another sonar should be done at 12 weeks’ gestation and the nuchal translucency (NT) (the fluid filled space behind the fetus’ neck) should be measured. Any measurement up to 2.5mm is within normal limits. An increased NT could be indicative that the baby may have a chromosomal abnormality like Down syndrome or Turner syndrome, a cardiac abnormality, a skeletal abnormality, or that the mother may have had an early infection.
- At the same time, the nasal bone should be measured; a small or absent nasal bone may be familial, or show that the baby has a higher chance to have Down syndrome.
- A fetal anomaly scan should be done at 20 weeks gestation where the fetal heart and brain can be best visualised, as well as other fetal anatomy.
- Maternal serum screening
First trimester maternal serum screen (DS screen). At the time of the 12 weeks sonar, it is suggested that a first trimester maternal serum screen be done; this involves taking a blood sample from the mother and measuring two biochemical markers, PAPP A (pregnancy associated plasma protein A) and βhCG (beta human chorionic gonadotrophin). These levels, in conjunction with the NT and nasal bone 1 measurement, can detect 96% of babies with Down syndrome, as well as trisomies 13 and 18 (the latter two having poor outcomes). If the risk is shown to be increased, the patient/couple can opt for invasive testing. A positive test does not necessarily indicate that the baby has one of these conditions; there are many factors that influence the outcome:
+ Incorrect dating of the pregnancy
+ Bleeding in the pregnancy (which may not have been noticed by the mother)
+ Maternal weight
+ Beta-hCG levels are about 10% higher and PAPP-A levels about 10% lower in women who have had IVF
+ PAPP-A, free beta-hCG are higher in women who smoke
+ The serum marker levels are raised in twin pregnancies.
Second trimester maternal serum triple screen. This test is done ideally between 15-18 weeks gestation and measures Oestriol, bHCG and AFP. There is a 60% pick up rate, which means that there remains a 40% false positive rate. A positive test does not mean that the baby is affected, just that there is an increased risk.
Prenatal invasive testing
These include chorionic villus sampling (CVS), amniocentesis and cordocentesis.
When should invasive testing be done?
- If an abnormality has been detected on the NIPT
- If abnormalities have been seen on sonar:
- When ‘soft markers’ or ‘clues’ are seen, indicating an increased risk for having a baby with a genetic condition, for example an echogenic focus (a ‘bright’ area seen in the heart). This is mostly not a reason for concern, unless seen in conjunction with other factors, such as advanced maternal age or other markers.
- Skeletal abnormalities
- Skull or brain abnormalities (a strawberry shaped skull is often indicative of a baby with trisomy 18)
- When there has been a ‘positive’ maternal serum screening, either in the first or the second trimester of the pregnancy
- A couple has had a previous baby with a chromosome abnormality like Down syndrome; often this is done for reassurance rather than the risk
- One of the couple has a chromosome rearrangement in a ‘balanced’ form, and there is a chance to have a baby with an ‘unbalanced’ form, which can cause physical and intellectual abnormalities.
- If the couple are known to carriers for a ‘faulty’ gene for cystic fibrosis, thalassemia or other genetic conditions
- One of the couple has a specific condition for which there is a 50% chance of passing it on to children: for example, types of muscular dystrophies or dominant form of deafness.
- There is a known X-linked condition in the family which is passed on from mothers to sons, like haemophilia
- Chorionic villus sampling is done between 11 and 13.6 weeks gestation where a sample of the placenta, which is identical to that of the baby, is extracted via the abdominal wall and uterus and tested.
- An amniocentesis can be done between 16 and 22 weeks gestation where amniotic fluid, which contains fetal cells is extracted and tested. Both invasive procedures carry a risk of miscarriage of 2-3% and 0, 5-1% respectively. The couple will need to decide with careful counselling whether they want to proceed with one of these procedures. If a diagnosis of a chromosome abnormality is confirmed, they can opt to continue with the pregnancy, understanding the implications of the condition, or choose to terminate the pregnancy. Both choices come with enormous emotional issues, and it is important for the midwife, as well as the genetic counsellor, explain and support any decision made.
- Cordocentesis is offered very rarely where one needs to confirm a diagnosis when early testing has not been done; this involves extracting a blood sample from the cord, which also poses a risk for miscarriage.
Everyone wants to have a healthy ‘normal’ baby. It is the responsibility of the midwives to be fully informed of the testing that is available, so that these can be discussed with clients. If a client or a couple are concerned about having a baby with a genetic condition, it is helpful to refer them for genetic counselling. The counsellor will take a full history, assess the risks, discuss their options and explain all the relevant testing, screening and invasive diagnostic testing in detail so they can make an informed decision.
Contact numbers for Genetic Counselling
NHLS/WITS: 011 4899223 / 4
UCT: 021 406 6698 / 6995
STELLENBOSCH: 021-938-9807 / 9787
PRIVATE GENETIC COUNSELLORS: 010-5951005