Pregnancy Due Date and Gestational Age Calculator

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Calculate estimated due date (EDD) and gestational age based on :

Conception date
      (date of ovulation, egg retrieval, or insemination)
Date of 3-day embryo transfer
Date of 5-day embryo transfer
Due date by sonogram (reverse calculation)
First day of last menstrual period

Calculate Due Date From Ultrasound Report

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 ovulation and 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.

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 measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate method to establish or confirm gestational age" [24].

The American College of Obstetricians and Gynecologists recommends that ultrasound-established dates should take preference over menstrual dates when the discrepancy between ultrasound dating and LMP is

  • Greater than 5 days before 9 0/7 weeks of gestation
  • Greater than 7 days from 9 0/7 weeks to 15 6/7 weeks
  • Greater than 10 days from 16 0/7 weeks to 21 6/7 weeks
  • Greater than 14 days from 22 0/7 weeks to 27 6/7 weeks
  • Greater than 21 days after 28 0/7 weeks. 

"Because of the risk of redating a small fetus that may be growth restricted, management decisions based on third-trimester ultrasonography alone are especially problematic; they need to be guided by careful consideration of the entire clinical picture and may require closer surveillance, including repeat ultrasonography to ensure appropriate interval growth." [24].

When twin pregnancy is the result of in vitro fertilization determination of gestational age should be made from the date of embryo transfer. Otherwise date the pregnancy using the larger fetus [22,23]

Other Methods For Estimating the Gestational Age

Clinical Examination

A pelvic examination supported by good menstrual records in the first trimester has been reported to be a reliable method for dating of pregnancy [9].

Doppler Ultrasonography

The fetal heart can be heard using Doppler ultrasound by 10 to 12 weeks in most patients [10]. 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.

Determining the Date of Conception

Because the human egg is capable of fertilization for only 12 to 24 hours after ovulation the date of ovulation may be taken as being the date of conception. However, ultrasound determination of the date of ovulation has the same imprecision as does the ultrasound estimate of the gestational age and, therefore, a precise date of conception cannot usually be determined as with in vitro fertilization.

In addition, although a woman is most likely to become pregnant if she has sex on the day of ovulation conception may also occur from live sperm still in her reproductive tract on the day of ovulation if she had sex for up to five days before ovulation [26,27].

Reviewed by Mark Curran, M.D. FACOG

All calculations must be confirmed before use. The suggested results are not a substitute for clinical judgment. Neither nor any other party involved in the preparation or publication of this site shall be liable for any special, consequential, or exemplary damages resulting in whole or part from any user's use of or reliance upon this material.

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