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9 weeks for blood sample - 12 weeks for scan!

The week chosen for NT

Nuchal translucency screening can only be conducted in a specific period during pregnancy, which coincides with the time when the embryo becomes a fetus. During this period (from 11 weeks until 13 weeks + 6 days) we can conduct a morphological study of the fetus’s primary structures (in most cases anticipating the second trimester’s  morphology scan results) as well as get information on the risk of chromosomal defects, of which half are trisomy 21 (Down) and the others trisomy 18 and 13.

The optimal gestational age for measuring the nuchal trasnlucency is 11 – 13+6 weeks and the size of the fetus has to be between 45mm and 84mm in length.

The reasons behind the choice of 11 weeks as minimum limit are:

  • Following a screening, the availability of a diagnostic test is essential. In this case the collection of villus sampling can only be conducted after 11 weeks.
  • Most structural anomalies can only be diagnosed after 11 weeks.

The reasons behind the choice of 13 weeks + 6 days as maximum limit are:

  • To be able to give the patient the option of terminating the pregnancy in the first rather than the second trimester.
  • After 13 weeks, the fluid in the back of the fetus’s neck tends to decrease
  • Due to unfavorable fetal position (e.g. vertical), the percentage of successful measurements tends to decrease after 14 weeks. It also becomes difficult to take an adequate ultrasound picture.

The study of nuchal translucency is  combined with an examination of maternal blood, which is best to do between 9 and 11 weeks.

The optimal solution is to arrange the first appointment earlier (between 9 and 11 weeks) for blood sampling. Another appointment can be fixed straight after at around 12 weeks, for the ultrasound scan. This way, should the patient decide to have an invasive procedure, can have a villocentesis.


Last Edit: 05/08/2013 8:17pm