Gestational age
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 :
- 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
Related pathology
See also
- automatic online gestational age calculator based on crown-rump length from www.perinatology.com
