Normal fetal biometry - second trimester

Case contributed by Alexandra Stanislavsky
Diagnosis almost certain


Routine second trimester ultrasound.

Patient Data

Gender: Female

Biparietal diameter (BPD) is measured from outer cortex of the near parietal bone to the inner cortex of the far parietal bone on a symmetric image, at the level of the thalami. Cerebellar hemispheres should not be seen in this plane. The biparietal plane is near-parallel to the ultrasound beam.

Head circumference (HC) is measured in this case by an elliptical tool on the same imaging plane.

Abdominal circumference (AC) is measured at the level of liver, preferentially at the confluence of the right and left portal veins and with stomach on view, from outer margin of skin.

Femur length (FL) measurement should be obtained with the femur perpendicular or near perpendicular to the ultrasound beam. Only the diaphysis is measured. 


Biometry table


The above measurements generate an ultrasound gestational age of 21w1d +/- 7d. This is compatible with the menstural gestational age of 20w2d (ie permissible 6 day discrepancy).

Case Discussion

These selected images from a second trimester morphology ultrasound demonstrate appropriate measurement of fetal biometry.  Note that the final gestational age (GA) calculation resulted in a 6 day discrepancy with the menstrual dates. This however is within the acceptable margin of error, variably taken as +/- 7 or 10 days.  The fetal size is therefore apropriate for dates, and the menstrual EDD should not be adjusted .

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