Contributor: MFM Division (Dr. Dellinger)
Last Update: 1/29/2015
This document should be used to assign a final EDC. Please refer to document entitled Fetal Lung Maturity Testing, if interested in timing delivery for induction of labor or repeat cesarean.
Does the patient have a sure LMP?
No. Then the EDC will be established by a first trimester scan (CRL). If a first trimester scan is not available, then the earliest scan available will determine the EDC. In the event only a third trimester scan is available, the transcerebellar diameter (TCD) may be helpful in establishing and EDC, especially in the presence of fetal growth restriction.
Yes. Compare the LMP derived EDC to the ultrasound estimated due date (EDD). The standard deviations presented in table 1 can serve as guidelines for when the EDD should replace the EDC. Again, in cases where the first scan is performed in the third trimester, the TCD may be of use in establishing a more accurate EDD, especially with suspected IUGR.
Based on the data presented in the above chart, the following guidelines will be used for maintaining an LMP based EDC:
- At less than 9 weeks, the CRL should agree with LMP +/- 5 days.
- At 9 – 15 6/7 weeks, the CRL or composite measurements should agree +/- 7 days.
- At 16 – 21 6/7 weeks, the composite measurements should agree +/- 10 days.
- At 22 – 27 6/7 weeks, the composite measurements should agree +/- 14 days.
- At 28 weeks – term, the composite measurements should agree +/- 21 days.
- <span “font-size:11.0pt;line-height:115%;font-family:=”” calibri;mso-ascii-theme-font:minor-latin;mso-fareast-font-family:calibri;=”” mso-fareast-theme-font:minor-latin;mso-hansi-theme-font:minor-latin;mso-bidi-font-family:=”” “times=”” roman”;mso-bidi-theme-font:minor-bidi;mso-ansi-language:en-us;=”” mso-fareast-language:en-us;mso-bidi-language:ar-sa”=””>Evaluation of the TCD can be helpful late in pregnancy since it tends to be less affected by alterations in abnormal fetal growth. Work by Chavez et al on over 24,000 TCD data points has generated a nomogram listed below. Concordance between the actual and predicted gestational age was high (r = 0.92; P < .0001). This agreement was superior in the second trimester (r = 0.93; P < .0001) than in the third trimester (r = 0.81; P < .001). Between 17 and 21 weeks, and between 22 and 28 weeks of gestation, the predicted gestational age ranged between 0 and 4 days, and between 0 and 2 days, respectively, of actual gestational age. Between 29 and 36 weeks of gestation, predicted gestational age was within 5 days of actual gestational age; at 37 weeks of gestation, the predicted gestational age was discrepant by 9 days.
- Method for estimating due date. Committee Opinion no. 611. American College of Obstetricians and Gynecologists. Obstet Gynecol 2014; 124:863-6.
- American Journal of Obstetricis and Gynecology. Volume 189, Issue 4 , October 2003, Pages 1021-1025.
- American Journal of Obstetricis and Gynecology. Volume 191, Issue 3, September 2004, Pages 979-984.