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I have three different due dates. Which date is correct?

The earliest available dating ultrasound would be expected to give the most accurate estimate of your due date. Succeeding ultrasound examinations of the fetus would usually be used to decide whether or not the fetus is growing well according to the estimated due provided by the earliest ultrasound examination.

The estimated due date (EDD or EDC) is the date that spontaneous onset of labor is expected to occur. The due date may be estimated by adding 280 days ( 9 months and 7 days) to the first day of the last menstrual period (LMP). This is the method used by "pregnancy wheels". The accuracy of the EDD derived by this method depends on accurate recall by the mother, assumes regular 28 day cycles, and that conception occurs on day 14 of the cycle. Use of the LMP to establish the due date may overestimate the duration of the pregnancy, and can be subject to an error of more than 2 weeks [5-7].

In cases where the date of conception is known precisely, such as with in vitro fertilization, the EDD is calculated by adding 266 days to the date of conception.

There is good evidence that a dating ultrasound performed before 22 weeks, when available, should be used in preference to menstrual dates [1-3] to establish the estimated due date. Ultrasound uses the size of the fetus to determine the gestational age (the time elapsed since the the first day of the last menstrual period). The accuracy of the ultrasound estimate of the gestational age varies according to the gestational age.

Ultrasound Gestational Age Variation
Up to 20 weeks of gestation 7 days
Between 20 and 30 weeks of gestation 14 days
Beyond 30 weeks of gestation 21 days

The American College of Obstetricians and Gynecologists recommends that "if the estimated gestational age by a patient's last menstrual period differs from the ultrasound estimate by more than these accepted variations, the ultrasound estimate of gestational age should be used instead of the patient's menstrual cycle estimate."[4]

REFERENCES

1. Chervenak FA, Skupski DW, Romero R, et al: How accurate is fetal biometry in the assessment of fetal age?. Am J Obstet Gynecol 1998; 178:678.PMID:9579429
2. Mongelli M, Wilcox M, Gardosi J.: Estimating the date of confinement: ultrasonographic biometery versus certain menstrual dates. Am J Obstet Gynecol 1996; 174:278.PMID:8572021
3. Savitz DA, Terry Jr JW, Dole N, et al: Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. Am J Obstet Gynecol 2002; 187:1660.PMID:12501080
4. ACOG Practice Bulletin. Clinical management guidelines for obstetricians-gynecologists. Number 55, September 2004 (replaces practice pattern number 6, October 1997). Management of Postterm Pregnancy.Obstet Gynecol. 2004;104:639-46.PMID:15339790
5. Kramer MS, McLean FH, Boyd ME, Usher RH: The validity of gestational age estimation by menstrual dating in term, preterm and postterm gestations. JAMA 1988; 260:3306. PMID:3054193
6. Savitz DA, Terry JW Jr, Dole N, Thorp JM Jr, Siega-Riz AM, Herring AH.Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. Am J Obstet Gynecol. 2002 Dec;187(6):1660-6. PMID:12501080
7.Wilcox M, Gardosi J, Mongelli M, et al. Birth weight from pregnancies dated by ultrasonography in a multicultural British population. BMJ. Sep 4 1993;307(6904):588-91.PMID:8401014

Created 11/1/2007
Revised 3/5/2009


 

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