Gestational age

Dr Henry Knipe et al.

The gestational age (GA) of a fetus is most accurately determined by measuring the crown rump length (CRL) in the first trimester. The later the pregnancy, the larger the standard deviation of the estimated GA and thus EDD is.  Mean sac diameter (MSD) before a fetal pole is identified and biparietal diameter (BPD) after 11 weeks can also be used to support the calculation of the GA. 

The estimated date of delivery (EDD) should be calculated by last menstrual period (LMP) unless 1:

  • unknown LMP
  • GA by CRL is <10 weeks and differs from GA by LMP by ≥ 5 days
  • GA by CRL (+/- BPD) is 10-14 weeks and differs from GA by LMP by >7 days

Practical points

  • for twin pregnancies, gestational age is based on the largest twin's CRL 1

Related pathology

Ultrasound - obstetric
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Article information

rID: 63195
System: Obstetrics
Tag: cases, refs
Synonyms or Alternate Spellings:
  • Gestational age (GA)

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