Dating the Pregnancy Estimated Due Date Based on Ultrasound

    Ultrasound of the embryo and fetus before 14w0d of the pregnancy is the most accurate method to establish the pregnancy estimate due date (EDD). The crown-rump length (CRL) before 14w0d is accurate within 5-7 days of the actual due date. The earlier the CRL is done in pregnancy, the better the actual estimated due date can be calculated. After a CRL length measurement of 84 mm (14+ weeks of the pregnancy), the measurement of the CRL becomes less accurate and other measurements such as biparietal diameter (BPD), head circumference (HC),  abdominal circumference (AC), and femur length (FL) should be used.    

    Here are several guidelines to recalculate the estimated due date (EDD):

    1. If the LMP is unclear then the CRL before 14 weeks can be used to calculate the EDD
    2. If the CRL before 9 weeks is within 5 days of the EDD calculated by the first day of the last menstrual period (LMP) then LMP should be used to calculate the EDD. 
    3. If the CRL before 9 weeks is more or less than 5 days different from the first day of the last menstrual period (LMP) then the ultrasound date of the EDD should be used. 
    4. If the CRL between 9w0d and 13w6d is more or less than 7 days different from the date calculated from the first day of the last menstrual period (LMP) then the ultrasound date of the EDD should be used. 
    5. If the BPD/HC/AC/FL between 14w0d-15w6d are more or less than 7 days different from the date calculated from the first day of the last menstrual period (LMP) then the ultrasound date of the EDD should be used.
    6. If the BPD/HC/AC/FL between 16w0d-21w6d are more or less than 10 days different from the date calculated from the first day of the last menstrual period (LMP) then the ultrasound date of the EDD should be used. 
    7. If the BPD/HC/AC/FL between 22w0d-27w6d are more or less than 14 days different from the date calculated from the first day of the last menstrual period (LMP) then the ultrasound date of the EDD should be used. 
    8. If the BPD/HC/AC/FL between 28w0d and beyond are more or less than 21 days different from the date calculated from the first day of the last menstrual period (LMP) then the ultrasound date of the EDD should be used. 

     From: ACOG Committee Opinion October 2014

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