First trimester screening is a prenatal test that combines a maternal blood-screening test with an ultrasound evaluation of the fetus to identify the baby’s risk for specific chromosomal abnormalities, including Down syndrome (trisomy 21) and Trisomy-18.
What is the difference between a screening test and diagnostic test?
Tests, which tell if your baby has a particular disease or condition with certainty, are called diagnostic tests. On the other hand, screening tests only show if your baby’s chance is high or low for a specific disease or condition. These tests DO NOT diagnose a problem; they only signal further testing should be done.
What conditions are covered in first trimester screening?
The vast majority of babies are normal. However all women, whatever their age, have a small risk of delivering a baby with a physical or a mental abnormality. In some cases this may be due to chromosomal problems such as Down syndrome, Trisomy 13 or Trisomy 18. These conditions are mainly looked for in the first trimester screening.
These disorders are an important cause of congenital intellectual disability and may also have various medical problems involving the heart, digestive tract, and/or other organ systems.
Who should be screened?
Leading medical professional organisations recommend first trimester Down syndrome screening for all pregnant women, regardless of age.
How does first trimester screening assess your baby’s risk and what is its accuracy?
First trimester screening is not a diagnostic test, which means it cannot tell you whether your baby has Down syndrome, trisomy 13, or trisomy 18. Instead, the screening provides a probability that your baby might have Down syndrome, trisomy 13, or trisomy 18.
The screening results can either alert you and your doctor that your baby is at an increased risk for one of these chromosome disorders or be reassuring that your baby is at a lower risk for these conditions. Also, a negative or normal result (one that shows a decreased risk) does not mean that the baby will not have a chromosome abnormality. There is a 5% false positive rate for the test. Parents should be aware of the possibility of receiving abnormal results and then finding, after further testing, that the baby is normal. Finally, this screen is not designed to provide information about the possibility of other chromosome conditions, nor about many other genetic syndromes, genetic disorders, birth defects, or causes of intellectual disability.
When is the Nuchal Translucency scan performed?
We recommend the ultrasound is performed at week 12 of pregnancy. However, the ultrasound can be performed from 11 weeks through to 13 weeks and 6 days of pregnancy. It is a highly specialised ultrasound and takes about 20-30minutes.
The sonogram will confirm how far along your pregnancy is. In addition, a measurement of the fluid underneath the skin along the back of the baby’s neck, called the nuchal translucency (NT), will be taken. The sonogram will also determine if your baby’s nasal bone is present or absent. It will also check the blood flow across the valve of the right heart chamber and look for regurgitation (tricuspid regurgitation) and blood across the ductus venosus, the physiological valve regulating blood to the heart.
Combining your age-related risk with the NT measurement, nasal bone data, and blood work, provides you with one risk figure for Down syndrome and one risk figure for trisomy 13 or trisomy 18.