Nuchal Translucency (NT)

Last updated 08.03.13

 

Index

 

Definition

-  First trimester ultrasound between 11/40 & 14/40 gestation of pregnancy to assess risk of Downs Syndrome

 

Features

-  First trimester ultrasound

-  Part of Maternal Serum Screening in First Trimester to asses for risk of Downs Syndrome

-  Beta hCG & PAPP-A

-  Non-invasive, no risk to pregnancy

-  Typically performed TA (Trans-abdominally)

-  TV (Trans-vaginal) if required ie: body habitus

- 75-80% sensitivity

- Measures accumulation of fluid at posterior of neck

- Sonographers require certification to assess NT

- Performed between 11-14/40

-  CRL 45-84mm (algorithim does not assess outside this range)

- Requires regular audit of performance

- Medicare Rebate if

-  Hyperemesis gravidaurm, DM in pregnancy, HTN in pregnancy, Pre-eclampsia

-  Liver or renal disease, AID, cardiac disease, thrombophillia

-  IBD, Coeliac disease, bowel stoma, abdominal wall scarring, abdo pain or mass

-  Previous spinal or pelvic trauma

-  Maternal infection, obesity, maternal age >35rs

-  Alloimmunisation

-  High risk pregnancy, drug dependency, poor obstetric history

-  Suspicion of ectopic pregnancy, risk of miscarriage, dimished symptoms of pregnancy

-  Suspected or known cervical incompetence or uterine abnormality, ART

-  Risk of fetal abnormality

-  Uncertain dates

-  Previous LUSCS, previous post dates delivery

 

Indication

-  Offered to all pregnant women in Australia

-  Provides risk estimate for Trisomy 21 (Downs Syndrome)

-  Not Dx

 

Procedure

-  Patient to have full bladder

-  First trimester ultrasound protocol plus assessment of nuchal

-  Margins of edges must be clear for proper placement of calipers

-  Fetus in midsagital plane

-  Nose tip visible

-  Image magnified to fill view w head, neck & upper thorax, 75% of image

-  Fetal neck in neutral position (not flexed or hyperextended)

-  Amnion must be seen as separate from NT line

-  Measurement taken w calipers

-  Placed on inner borders of nuchal space

-  None of horizontal cross bar crossing into nuchal space

-  Placed perpendicular to long axis of foetus

-  Taken at the widest space of NT

 

Nuchal 1

 

 

Results

-  Nuchal translucency measurement entered into algorithm to provide risk of Trisomy 21 (Downs Syndrome)

-  As CRL increases so does average NT

-  >2.5mm usually abnornmal but must be calculated

-  Increased nuchal thickness may also indicate

-  Trisomy 13, 18

-  Congenital heart defects

-  Skeletal dysplasia

-  Diaphragmatic hernia

-  DDx Cystic Hygroma which may be present in Downs Syndrome or Turners

 

 

 

 

References

 

AIUM Practice Guideliens: Obstetric US, October 1 2007 (incl caliper diagram)

US pics: www.fetal.com/NT%20Screening/02%20NT%20Imaging.html

Graph from RANZCOG 2007