How is an estimated due date calculated from the last menstrual period?

Estimated due date is typically calculated as 280 days or 40 weeks from the first day of the last menstrual period. If cycle length differs from 28 days, adjusting by cycle length may improve the estimate.

How does ultrasound dating estimate a due date?

Ultrasound dating uses gestational age measured on the ultrasound examination date and then back-calculates the estimated last menstrual period and estimated due date.

How is due date calculated for IVF pregnancy?

For IVF pregnancy, due date is calculated from embryo transfer date and embryo age at transfer. Conception is estimated from transfer date minus embryo age, and estimated due date is conception plus 266 days.

What if I do not know my last menstrual period?

If last menstrual period is unknown, due date can be estimated using ultrasound gestational age, an established due date, IVF transfer date, or a known conception date.

Pregnancy Due Date Calculator

Select Calculation Method

Choose a dating method. For LMP versus ultrasound discrepancies, use the separate ACOG criteria calculator linked under See also.

Enter First Day of Last Menstrual Period (LMP)

Uses your browser's built-in date picker.

Cycle length (days)

LMP will be cycle-corrected: corrected LMP = LMP + (cycle length - 28).

Enter Date for Gestational Age Calculation

Defaults to gestational age today, but you may also select a date to calculate the gestational age in the past or future.

After calculation, see Important dates & windows and use Fetal Development to jump to the matching week.

Results

Nothing to show yet.
Select a method, enter values, then press Calculate.

Results include EDD, gestational age for the selected date, calculated LMP, and an estimated conception date. The gestational age date field defaults to today but can be changed.

Estimated Due Date (EDD)

Gestational Age

Fetal development by gestational age

Fetal development -

Run a calculation to enable the week-by-week link.

View Fetal Development

- -

GA -w | completed weeks

Explore week (optional)

Week -

Tip: the fetal development page uses completed weeks (e.g., 36w 6d counts as 36 weeks).

Gestational age on selected date

Calculation method

Important dates, windows, and testing reminders

Includes pregnancy timing dates plus staff scheduling reminders for common antenatal surveillance start points. Use as a workflow aid with clinician judgment and current practice guidance.

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 LMP and ultrasound dating differ beyond guideline thresholds, the clinician may revise the EDD. This calculator does not automatically apply ACOG redating rules; use the linked ACOG calculator for that decision.

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) corresponds to 40 weeks of gestation; spontaneous labor may occur before or after this date. 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 Consider Revising the EDD Based on Ultrasound

These discrepancies are determined using gestational age by LMP on the ultrasound date. This calculator computes dates from the chosen method but does not compare LMP and ultrasound to make an ACOG redating recommendation.

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

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
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
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
ACOG/AIUM/SMFM. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129:e150-4. PMID: 28426621
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
ACOG. Prevention of Group B Streptococcal Early-Onset Disease in Newborns. Committee Opinion No. 797. ACOG guidance.
ACOG. Indications for Outpatient Antenatal Fetal Surveillance. Committee Opinion No. 828. ACOG guidance.