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Pregnancy Due Date & Gestational Age Calculator

Choose a method (LMP, Date from Ultrasound Report, Known EDD, IVF, Date of Conception) and enter the required dates/values. After the EDD is calculated, the gestational age on a given date may be calculated at the bottom of the “Important dates & windows”.

LMP will be cycle-corrected using: corrected LMP = LMP + (cycle length − 28 days).
Methods for Calculating the Estimated Due Date (EDD)

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 the date of conception. However, ultrasound determination of the date of ovulation has the same imprecision as the ultrasound estimate of gestational age and therefore a precise date of conception usually cannot be determined except with in vitro fertilization.

Although a woman is most likely to become pregnant if she has intercourse on the day of ovulation, conception may still occur from live sperm remaining in the reproductive tract if intercourse occurred up to five days earlier.

Determining the Estimated Due Date

The estimated due date (EDD) is the date spontaneous onset of labor is expected to occur. Using the LMP method, EDD is calculated by adding 280 days (40 weeks) to the first day of the last menstrual period. This assumes accurate recall, regular 28-day cycles, and ovulation on day 14. EDD by LMP can be off by more than two weeks.

When the exact date of conception is known (e.g., IVF), the EDD is calculated by adding 266 days to the conception date.

Ultrasound uses fetal size to determine gestational age. First-trimester ultrasound (up to 13 6/7 weeks) is the most accurate method. The earliest ultrasound with crown-rump length ≥ 7 weeks (≈10 mm) should be used to determine gestational age when available.

When to Prefer Ultrasound Dating Over LMP (ACOG/AIUM/SMFM Committee Opinion No. 700)

  • More than 5 days discrepancy before 9 0/7 weeks
  • More than 7 days discrepancy at 9 0/7–15 6/7 weeks
  • More than 10 days discrepancy at 16 0/7–21 6/7 weeks
  • More than 14 days discrepancy at 22 0/7–27 6/7 weeks
  • More than 21 days discrepancy after 28 0/7 weeks

Because redating a growth-restricted fetus in the third trimester may mask pathology, decisions should be based on the entire clinical picture.

Other Methods for Estimating Gestational Age

Clinical Examination

A pelvic examination in the first trimester supported by good menstrual history can be reliable for dating pregnancy.

Doppler Ultrasonography

The fetal heart is usually detectable by Doppler at 10–12 weeks.

Human Chorionic Gonadotropin (hCG)

hCG is detectable 6–14 days after fertilization (approx. 3–4 weeks gestational age).

Twins

With IVF twins, gestational age is assigned from the embryo transfer date. Otherwise, dating should generally be based on the larger twin to avoid misclassifying growth restriction.

Observed Gestational Age at Delivery by Plurality

Singleton Twins Triplets Quadruplets Quintuplets+
Average Gestational Age (weeks) 39 35 32 30 27

The earlier delivery of multiple gestations reflects increased preterm labor, hypertensive disorders, placental complications, fetal growth restriction, and stillbirth risk. Despite earlier average delivery, the definition of “full term” remains 39–40 6/7 weeks for all pregnancies.

References
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  4. ACOG Practice Bulletin No. 55 (Sep 2004). Management of Postterm Pregnancy. Obstet Gynecol. 2004;104:639–46. PMID: 15339790
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  10. Lohstroh P, et al. Daily immunoactive and bioactive hCG profiles in periimplantation urine samples. Biol Reprod. 2006;75(1):24–33. PMID: 16525035
  11. ACOG/AIUM/SMFM. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129:e150–4. PMID: 28426621  |  Full text
  12. ACOG Practice Bulletin No. 98. Ultrasonography in Pregnancy. Obstet Gynecol. 2008;112:1419–44.
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  17. Institute for Clinical Systems Improvement (ICSI). Routine Prenatal Care (13th ed, Aug 2009). Public copy of an adapted summary (PDF): Molina “Routine Prenatal Care”
  18. Martin JA, et al. Births: final data for 2009. Natl Vital Stat Rep. 2011;60(1):1–70. PMID: 22670489  |  CDC PDF
  19. Multifetal gestations: twin, triplet, and higher-order multifetal pregnancies. Practice Bulletin No. 144. Obstet Gynecol. 2014;123:1118–32. PMID: 24785876
  20. Page JM. Risk of stillbirth and infant death by each additional week of expectant management in twin pregnancies. Am J Obstet Gynecol. 2015;212(5):630.e1–7. PMID: 25797235
  21. Burgess JL, Unal ER, Nietert PJ, Newman RB. Risk of late-preterm stillbirth and neonatal morbidity for monochorionic and dichorionic twins. Am J Obstet Gynecol. 2014;210(6):578.e1–9. PMID: 24607757
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  23. Definition of term pregnancy. ACOG Committee Opinion No. 579. Obstet Gynecol. 2013;122:1139–40. PMID: 24150030
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SEE ALSO Due Date Calculator Using ACOG Criteria

SEE ALSO Fetal Development

Reviewed by: Mark Curran, M.D., FACOG — Updated: December 4, 2025