First trimester screening for Down’s syndrome

The steady increase over the past 15-20 years in the efficiency of antenatal screening for Down’s syndrome has been achieved gradually through a series of technical discoveries.

At present there are four screening tests, and for a 5% false positive rate, the sensitivities are about 30% for maternal age alone, 60-70% for maternal age and second trimester biochemistry testing, 75% for maternal age and first trimester fetal nuchal translucency scanning, and 85% for maternal age with fetal nuchal translucency and maternal serum biochemistry at 11-14 weeks.

First trimester screening has obvious benefits over second trimester screening. These advantages include early diagnosis with subsequent safer and less traumatic early termination and of course for the majority of patients, early reassurance.

Cicero and colleagues (Lancet November 2001) examined 701 routine ultrasonographic scans done for women who were about to undergo prenatal diagnosis because of positive results on nuchal translucency screening. They checked whether the nasal bone was visible and found that in fetuses who were subsequently found to have Down’s syndrome, 73% had no nasal bone, compared with 0.5% in unaffected fetuses. Thus the absence of the nasal bone increases the risk of Down’s syndrome more than 140- fold and its presence reduces the risk three fold. If there is no correlation between the presence of the nasal bone and nuchal translucency or the serum markers (and this remains to be proven) the detection rate could be as high as 98% if using all three modalities.    

If the nuchal translucency and the nasal bone are assessed without any biochemical screening the detection rate is around 92%.

Furthermore by examining the  fetus  sonographically between 11 and 14 weeks together with the nuchal translucency measurement the majority of structural abnormalities (68%) can be diagnosed as well.

A 1st trimester complete anatomical survey by Ultrasound should look as follows:

Brain: Complete cranium, septum pellucidum, thalamus, choroid plexi, cerebellum, and ventricles.

Spine: Complete vertebrae seen in both transverse and coronal planes with normal overlying skin.

Face: Correct position of mandibles, maxillae and orbits.

Lungs: Normal shape, echogenicity and hypoechoic interface between abdomen and thorax.

Heart: Four-chamber view, symmetrical.

Abdomen: Normal cord insertion and abdominal wall.

GI tract: Single hypoechoic structure in left abdomen.

Kidneys: Visualisation of cortex and pelvis and bladder.

Extremities: Visualisation of long bones, correct posture of the hands and feet.

As is the case for every medical practice, sonographers undertaking risk-assessment by examination of the nuchal translucency, the fetal profile and anatomy in the first trimester must receive appropriate training and certification of their competence in doing these scans.

So where does this leave us and what does this mean to the various departments of health?

Despite a large body of published data on first trimester markers and accumulation of clinical experience with nuchal translucency scanning, screening for Trisomy 21 is still planned as a second trimester service.

The findings on the nasal bone published recently demand an urgent rethink of this policy.

Ermos Nicolaou

FCOG (SA), MD

Harris Birthright Research Centre for Fetal Medicine

King’s College Hospital , London

And Fetal Medicine Unit

Department of Obstetrics and Gynaecology

Chris Hani Baragwanath Hospital