11-13 week antenatal scan

11-13 week antenatal (nuchal translucency) scan is considered a routine investigation advised for fetal well-being as well as for early screening in pregnancy (see antenatal screening).

It includes multiple components and is highly dependent on the operator. Traditionally three factors are used to calculate the risk of trisomies:

• crown-rump length

  • must be 45 to 84 mm

  • gestation age 11 weeks 3 days to 13 weeks 6 days

• nuchal translucency

• fetal heart rate

Additional markers increase the detection rate and reduce false-positive rates:

• nasal bone

• ductus venosus flow

• tricuspid flow

Combining these factors with blood tests (i.e. dual marker) has been reported to achieve a nearly 95% detection rate for trisomies.

Apart from the screening protocol, early detection of major anomalies may be possible, and the checklist must include the following conditions:

• acrania

• encephalocele

• alobar holoprosencephaly

• iniencephaly

• body stalk deformity

• gastroschisis

• omphalocele

• limb reduction

• megacystis

• open spina bifida

See also

• intracranial translucency

• early structural scan