Calculate estimated due date (EDD) and gestational age from last menstrual period (LMP), ultrasound gestational age, known due date, IVF embryo transfer date, or a known conception date. The calculator also estimates calculated LMP, estimated conception date, and helpful pregnancy timing windows.

LMP method selected
Cycle-corrected
Uses your browser’s built-in date picker.
LMP will be cycle-corrected: corrected LMP = LMP + (cycle length − 28).
Defaults to today and is used for the displayed gestational age and fetal development link.
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After calculation, see Important dates & windows and use Fetal Development to jump to the matching week.
FAQ

How is an estimated due date (EDD) calculated from the last menstrual period (LMP)?

EDD is typically calculated as the first day of the LMP plus 280 days (40 weeks). If cycles differ from 28 days, adjusting by (cycle length − 28) days can improve the estimate.

How does ultrasound dating calculate a due date?

Ultrasound dating back-calculates from the gestational age (weeks + days) on the exam date to estimate the LMP and then derives EDD.

When should ultrasound dating be preferred over LMP?

When the discrepancy between LMP dating and ultrasound dating exceeds recommended thresholds, clinicians often use the ultrasound-based EDD.

How is EDD calculated for IVF/ART?

The calculator estimates conception from embryo transfer date and embryo age, then computes EDD as conception + 266 days.

What’s the difference between gestational age and fetal (conceptional) age?

Gestational age is counted from the LMP. Fetal or conceptional age is counted from conception and is usually about 2 weeks less than gestational age.

What if I don’t know my LMP?

You can use ultrasound gestational age, an established due date, IVF transfer date, or a known conception date to estimate EDD and gestational age.

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 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.

When the exact date of conception is known, such as with IVF, the EDD is calculated by adding 266 days to the conception date.

Ultrasound uses fetal size to determine gestational age. First-trimester ultrasound, using crown rump length (CRL) up to 13 weeks 6 days, is the most accurate sonographic method.

When to Prefer Ultrasound Dating Over LMP

  • 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

Observed Gestational Age at Delivery by Plurality

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

Multiple gestations often deliver earlier than singleton pregnancies, but this does not change how due date is initially assigned.

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 biometry versus certain menstrual dates. Am J Obstet Gynecol. 1996;174:278. PMID: 8572021
  3. Savitz DA, Terry JW Jr, 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/AIUM/SMFM. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129:e150–4. PMID: 28426621
  5. Butt K, Lim KI. Guideline No. 388-Determination of Gestational Age by Ultrasound. J Obstet Gynaecol Can. 2019 Oct;41(10):1497-1507. PMID: 31548039