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
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.
The American College of Obstetricians and Gynecologists
recommends that "Ultrasonography may be considered to confirm menstrual dates if there is a
gestational age agreement within 1 week by crown rump measurements obtained in
the first trimester or within 10 days by an average of multiple fetal biometric
measurements obtained in the second trimester (up to 20 weeks of
gestation)". Ultrasound-established dates should take preference over menstrual
dates when the discrepancy is greater than 7 days in the first trimester and
greater than 10 days in the second trimester . Management decisions based on gestational age derived from a third trimester
ultrasound should be made with caution because of the normal variations in the
size of infants as well as growth disorders commonly seen in the third
Other Methods For Estimating the Gestational Age
A pelvic examination supported by good menstrual records in the first trimester
has been reported to be a reliable method for dating of pregnancy .
The fetal heart can be heard using Doppler ultrasound by 10 to 12 weeks in most
patients . The gestational age should ,therefore, be at least 10 to 12
weeks if fetal heart tone are heard.
Human Chorionic Gonadotropin Pregnancy Test
Human chorionic gonadotropin first becomes detectable in the mother's blood and
urine between 6 and 14 days after fertilization (3 to 4 weeks gestational age)
[11-13]. The gestational age would, therefore, be at least 3 to 4 weeks at the
time of a reliable hCG pregnancy test.
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in the assessment of fetal age?. Am J Obstet Gynecol 1998; 178:678.PMID:9579429
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2. Mongelli M, Wilcox M, Gardosi J.: Estimating the date of confinement:
ultrasonographic biometery versus certain menstrual dates. Am J Obstet Gynecol
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
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4. ACOG Practice Bulletin. Clinical management guidelines for
obstetricians-gynecologists. Number 55, September 2004 (replaces practice
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estimation by menstrual dating in term, preterm and postterm gestations. JAMA
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AH.Comparison of pregnancy dating by last menstrual period, ultrasound
scanning, and their combination. Am J Obstet Gynecol. 2002 Dec;187(6):1660-6.
7.Wilcox M, Gardosi J, Mongelli M, et al. Birth weight from pregnancies dated
by ultrasonography in a multicultural British population. BMJ. Sep 4
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age: a second-trimester comparison of pregnancies of known conception date with
pregnancies dated from the last menstrual period. Obstet Gynecol. 1989
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vitro. J Clin Endocrinol Metab. 1988 Dec;67(6):1322-4.PMID: 2461389
12. Wilcox AJ, et al. Time of implantation of the conceptus and loss of
pregnancy.N Engl J Med. 1999 Jun 10;340(23):1796-9.PMID: 10362823
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gonadotropin profiles in periimplantation urine samples. Biol Reprod. 2006
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Berghella V, et al., Does transvaginal sonographic measurement of cervical
length before 14 weeks predict preterm delivery in high-risk pregnancies?
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17. ACOG practice bulletin. Prevention of Rh D alloimmunization. Number 4, May 1999
(replaces educational bulletin Number 147, October 1990). Clinical management
guidelines for obstetrician-gynecologists. American College of Obstetrics and
Int J Gynaecol Obstet. 1999 Jul;66(1):63-70. PMID: 10458556
Fetal Lung Maturity. ACOG Practice Bulletin No. 97. American College of
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19. ACOG practice bulletin. Antepartum fetal surveillance. Number 9, October 1999
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