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The Nuchal Scan Home > Services We Offer
The Nuchal Scan (11 to 13 weeks, 6 days gestation - CRL of 45 to 84 mm)
The crown-to-rump length (CRL) measurement provides an accurate assessment of the gestational age of the fetus and verify the estimated date of delivery (EDD). All the major organs of the body are developed by this stage and therefore it is a good time to exclude structural abnormalities of the fetus. The anatomy of the fetus is thereforte examined at this stage of pregnancy, to exclude any abnormalities of the skull and brain, arms, legs, stomach, spine, abdomen and bladder.
The majority of pregnancies are normal but in some cases, the baby is born with a chromosomal abnormality such as Down's syndrome. However, the only way to confirm this for sure is to perform an invasive test such as chorionic villous sampling (CVS) or amniocentesis. These procedures carry a risk of miscarriage between 0.5 to 1%.
Recent studies showed that ultrasound measurement of the nuchal translucency (an area beneath the skin) and presence of the nasal bone will detect almost 90% of Down's syndrome. Since it is non-invasive, there is no risk of miscarriage. Eleven to 13+6 weeks gestation is the ideal time to measure the nuchal translucency (NT) and look for the presence of the nasal bone.
The accuracy of this test is increased by measuring the level of two hormones in the mother's blood (Beta HCG & Papp-A) - See section on "Screening for Down syndrome"
An ultrasound image showing the NT measurement of a baby at 12 weeks
An ultrasound image showing the presence of the nasal bone
The nuchal scan between 11-13+6 weeks will provide the following information:
  • Accurate assessment of gestational age and hence verifying the expected date of delivery
  • Fetal heart rate measurement
  • Assessment of the risk of Downs syndrome by NT measurement and the presence or absence of the nasal bone
  • Determine the number of fetuses as multiple pregnancies needs closer monitoring.
  • Detection of certain major fetal abnormalities
  • Fetal viability. About 3% of pregnancies fail to carry on when scanned during this time. The diagnosis is made when no fetal heart movement is seen on ultrasound scan.