Ultrasound imaging of the fetus in the period from 11 weeks to 13 weeks 6 days gestational age is important to study fetal nuchal translucency or nuchal thickness. Nuchal translucency is the hypoechoic area in the subcutaneous region in the posterior aspect of the fetal neck. This hypoechoic area represents fluid collection in the nuchal region. The normal amount of fetal nuchal thickness can be upto 2 mm. (at 11 weeks). The normal nuchal translucency thickness can increase to about 2.8 mm. by 13 weeks 6 days. The above ultrasound image shows a sagittal section of the fetal neck with nuchal tranlucency or nuchal thickness of 1.8 mm. in a 11 week old fetus. This was a normal study. Note the separate band posterior to the neck- the amnion, a membrane present around the fetus. The amnion must be separately identified from the nuchal skin to prevent wrong measurement of the nuchal thickness.
A zoomed view of the nuchal thickness in the same fetus. Note the close proximity of the amniotic membrane.
The normal nasal bone is also visualized in this ultrasound study (see image above). All above images are courtesy of Dr. Prasenjeet Singh, India.
Reference: http://www.babycentre.co.uk/pregnancy/antenatalhealth/scans/nuchalscan/
This site gives more information on this topic:
http://www.fetalmedicine.com/fmf/FMF-English.pdf
http://www.fetalmedicine.com/fmf/training-certification/certificates-of-competence/11-13-week-scan/nuchal/
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