Second trimester ultrasound scan

The second trimester scan is a routine ultrasound examination in many countries that is primarily used to assess fetal anatomy and detect the presence of any fetal anomalies. 

The second trimester extends from 13 weeks and 0 days to 27 weeks and 6 days of gestation although the majority of these studies are performed between 18 and 23 weeks.

Terminology

Alternate names for the study include second trimester anatomy scan, fetal anomaly scan or TIFFA (targeted imaging for fetal anomalies).

Radiographic features

Biometry

The following measurements are routinely obtained to calculate estimated fetal weight (EFW) and estimated date of delivery (EDD):

• head circumference (HC)
• biparietal diameter (BPD)
• abdominal circumference (AC)
• femur length (FL)

Second trimester calculation of EDD based on these parameters has a generally accepted error margin of +/- 7 days.

Morphologic assessment

The following structures should be visualized at a routine second trimester morphology ultrasound.  

Fetal head

• fetal skull: integrity and shape
• fetal brain
  • ventricles and choroid plexus
  • cavum septum pellucidum
  • posterior fossa, including measurements of
    • transcerebellar diameter and
    • cisterna magna
  • nuchal fold thickness

Fetal face

• profile and nasal bone 
• orbits and lenses
• upper lip and palate
• mandible

Fetal heart and chest

• fetal heart rate and rhythm
• cardiac situs
• four chamber view
• outflow tract views
  • left ventricular outflow tract
  • right ventricular outflow tract
• aortic and ductal arches
• diaphragm and lungs

Fetal abdomen

• liver
• stomach (including situs)
• kidneys and renal arteries
• abdominal wall
• umbilical cord insertion
• bladder and umbilical arteries
• presacral space
• fetal gender

Fetal musculoskeletal system

• spine
  • transverse, longitudinal +/- coronal views and skin line
• upper limb
  • humera, including humeral length (HL)
  • radius/ulna: both sides
  • fingers and thumbs, including hand opening
• lower limb
  • both femora, including femoral length (FL) as part of biometric assessment
  • both tibia/fibula: sagittal views to demonstrate orientation of the ankles to screen for talipes.
  • both feet

Ancillary findings

• fetal lie
• cervical length
• placenta
  • site
  • distance to cervix 
• liquor volume
• umbilical cord including the number of cord vessels and evaluation of knots