Women and Newborn Health Service
Health A – Z
King Edward Memorial Hospital
First Trimester Screening - Fetal Monitoring
First trimester screening (nuchal translucency) is a prenatal screening test performed at 11-14 weeks to identify pregnancies at increased risk of Down syndrome (Trisomy 21). Some other fetal abnormalities and multiple gestation can be detected on ultrasound at this early stage. Prenatal screening in the first trimester requires an ultrasound and blood test.
Further information and additional resources can be obtained from the Maternal Fetal Medicine Service.
Why is First Trimester Screening Performed?
How is First Trimester Screening Performed?
Why is First Trimester Screening Performed?
Physical and intellectual disability can result from Down syndrome which is caused by an extra number 21 chromosome. It is known to occur in 1 in every 650 pregnancies in Western Australia.
King Edward Memorial Ultrasound Department is a Fetal Medicine Foundation accredited centre, providing ultrasound and blood test results which are combined with pre-existing risk factors to achieve a detection rate of up to 90% of Down syndrome pregnancies. There is a false positive rate of 5% which means that 1 in 20 women will be shown as increased risk but they will not have a Down syndrome pregnancy.
How is First Trimester Screening Performed?
The nuchal translucency (NT) is the skin behind the neck which can be seen on an ultrasound sagittal section or profile view of the fetus. An NT measurement is taken in millimeters as seen on the image below ( the short vertical set of dotted lines). The crown rump length of the fetus is also measured to calculate pregnancy gestation ( the long horizontal set of dotted lines). A maternal blood test measuring two hormones (free beta hCG and PAPP-A) improves the accuracy of screening.
Whare are the Risks?
There are no known risks to the fetus in having ultrasound in pregnancy.
What do the Results Mean?
The results of First Trimester Screening will say either increased risk or not at increased risk of Down syndrome. Not at increased risk means the chance of having Down syndrome is less than 1 in 300. This does not guarantee the absence of Down syndrome. A First Trimester Screening result of increased risk does not mean the fetus definitely has Down syndrome. Most women in this group will NOT have a Down syndrome pregnancy but you will be offered the choice to have a diagnostic test to actually count the chromosomes in a tiny sample of cells.
This diagnostic test can be a Chorionic Villus Sampling (CVS) at about 12-14 weeks or Amniocentesis at 15 -18 weeks. Discussion with your doctor and your own preferences will help you decide which test is best for you. The results will exclude or confirm many (but not all) chromosomal abnormalities.
