What is antenatal screening?
Antenatal (or prenatal) screening is a process, which enables to identify unborn babies, who have higher risk of certain diseases. These are SCREENING tests, which tell you what is the CHANCE of having a baby with a specific condition (e.g. Down syndrome).
While it is never easy for a couple to decide to carry out prenatal screening because of the possibility of subsequently having to consider termination of pregnancy, prenatal screening currently is chosen by many couples. In the past, most pregnant women over the age of 35 or with family history of birth defects (or high risk women) were routinely offered prenatal screening for fetal chromosome abnormalities (see “trisomy”) by nonspecific biochemical maternal blood serum markers (MSS) and/or nuchal translucency ultrasound (NT/US) scans. Also, in most countries screening for Down syndrome and neural tube defects is offered for all pregnant women, by measuring biochemical markers in a blood sample obtained from a mother at 16 weeks gestation. However, the sensitivity of that approach is fairly low (~60%).
Biochemical maternal serum screening (MSS) involves looking for specific proteins and hormones in expectant mother's blood.
The extent of traditional biochemical and/or ultrasound screening typically depends on the timing when the mother contacts a health provider and is quite complicated:
|Screen type||Time||Markers measured in maternal blood||Sensitivity for Down syndrome||False positive rate||Diagnostic test if positive|
|First trimester combined (FTS) screening (is combined nuchal translucency scan)||11-14 weeks||PAPP-A (Pregnancy associated protein A) and hCG (Human chorionic gonadotropin) + combined with nuchal translucency (NT) ultrasound scan||80-85%||3-9%||Chorionic villus sampling (CVS) during 11-13 week or amniocentesis later|
|Second trimester biochemical screen||after 14 weeks||Triple screen: AFP (alfa fetoprotein), hCG, unconjugated estriol (uE3);
Quad screen: "Triple screen markers" + inhibin-A
|Triple screen: 70%;
Quad screen: 75-85%
|Triple screen: 7%;
Quad screen: 5-10%
Amniocentesis during 15-22 week
If subsequent 2nd trimester biochemical screen is undertaken together with FTS, it is called “integrated screen” - several visits for check-ups and considerable waiting time for each of the results involved. The maternal age is also considered in risk estimates.
FTS, second trimester screens (triple/quad) and ultrasound screen at 18 week of pregnancy also gives information about neural tube defects. Of every 100 pregnancies with an open neural tube defect (NTD), 80 (or 80%) will be detected with prenatal screening.
For other trisomies (Edwards and Patau syndrome), a mid-pregnancy scan at 18 week for nonspecific ultrasound markers can be offered.
However, these biochemical and US screening tests are not so accurate and reliable.
Detection rate of prenatal screening for Down syndrome has improved over time, with the most current NIPTs’ accuracy approaching 99.99%.
With the recent advancement of non-invasive molecular genomic technologies, biochemical screening approach can be regarded as outdated and maternal age 35 alone should no longer be used to determine who is offered prenatal screening. Importantly, you might be surprised to know, that age 35 as the screening cutoff was chosen back in 1979, because according to the estimates at that time, then the prevalence of trisomy 21 (1/270) approaches the estimated risk of fetal loss due to invasive procedures like amniocentesis (1/200). Currently, non-invasive prenatal screening (like NIPT) and invasive prenatal diagnostic testing choices should be driven primarily by prevention preferences of women and their partners.
In the majority of cases, prenatal screening will yield negative results, showing that your baby is almost certainly developing normally, and that certainty and peace of mind is priceless.
It is important to understand, that prenatal screening does not diagnose a condition – it can only show the risk of having a particular condition, therefore positive result of a prenatal screening does not mean that your baby definitely has a certain disorder. It also important to remember, that prenatal screening cannot find every birth defect – every pregnancy has a 2-3% risk (or 2-3 out of 100 instances) of having a condition which is not found by prenatal screening tests. If your test is positive, you should discuss it with trained genetic counsellor and choose to consider an invasive prenatal test (like amniocentesis or chorionic villus sampling) that can diagnose the suspected condition.
Read more about the benefits of prenatal screening here.