Get Started The Right Way – Establishing the EDD​

First trimester sonography has greatly improved our ability to calculate the estimated due date (EDD). However, there will be times that dating based on LMP does not match the ultrasound date. If the patient has an uncertain LMP or unreliable LMP, the earliest and most reliable sonogram should be used to establish the EDD. If the first day of her LMP is reliable, ACOG recommends redating as follows:

First trimester: based on CRL measurement 

8w6d or less: redate if discrepancy is > 5d
9w0d – 13w6d: redate if discrepancy is > 7d

Second trimester: based on BPD, HC, AC and FL
14w0d – 15w6d: redate if discrepancy is > 7d
16w0d – 21w6d: redate if discrepancy is > 10d
22w0d – 27w6d: redate if discrepancy is > 14d

Third trimester: based on BPD, HC, AC and FL
28w0d and beyond: redate if discrepancy is > 21d

First trimester ultrasound is the most accurate time frame for pregnancy dating and can increase the accuracy of the EDD even if LMP is known. If the CRL is greater than 84 mm, biometric (BPD, HC, AC, FL) parameters should be used to date the pregnancy. Mean sac diameter is not recommended for dating.

Consider a pregnancy without a dating ultrasound prior to 22 0/7 weeks “suboptimally dated.” Use caution when redating in the 3rd trimester as discrepancy may reflect growth restriction. Management should not be based on ultrasound alone but rather comprehensive clinical assessment.

In the setting of assisted reproductive technology (ART), the ART derived gestational age should be used for EDD using the age of the embryo and the transfer date. The age of the embryo is subtracted from the number of days between ovulation to delivery (280-14 = 266). For example, if the embryo is 3 days at transfer, the due date is 263 days from the date of transfer.
If you need assistance in establishing your patient’s EDD, feel free to give us a call.