Gestational age

Last revised by Jeremy Jones on 20 Sep 2021

The gestational age (GA) of an embryo/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 gestational age and thus estimated date of delivery (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 gestational age. 

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

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