Estimated due date calculator

Name: Due date calculator
Author: Richard Injante

Estimated due date from your last period, conception, an ultrasound or IVF, with your weeks pregnant and milestones.

Content reviewed by Richard Injante Published by SolucionesAhora.com Updated on 2026-10-20 How we verify

Calculator inputs

Enter your figures, or press “See example” to watch it work.
If your case is unusual, open “Advanced options”.
Read the result and, below it, what it means and how it was worked out.

Which date are you working from?

The most reliable date comes from a first-trimester scan; then embryo transfer (IVF) and, if you know it, the conception date. The last period is a good first estimate if your cycles are regular.

First day of your last period

The first day of your last menstrual bleed, not the last.

Your cycle length (optional) días

Naegele’s rule assumes 28-day cycles. If yours is longer or shorter, the date shifts by that difference: with 30 days, two days later.

Days between the start of two periods

Conception date

The day of ovulation or of the intercourse that led to the pregnancy, if you know it for certain (for example, from ovulation tracking).

Scan date

The day you had the ultrasound.

Weeks of gestation at the scan (optional)

The complete weeks you were told at the scan (from the crown-rump length if it was in the first trimester).

Complete weeks

Extra days (optional)

The days added to the complete weeks (for example, 12+3: 3 days).

0 to 6

Transfer date

The day the embryo was transferred.

Age of the transferred embryo (optional)

The days of development the embryo had when it was transferred: a day-3 (cleavage stage) or day-5 (blastocyst) embryo are the most common.

Advanced options

Compare with a scan: date (optional)

Optional. If you have an ultrasound, enter its date and the measured age here to check whether it agrees with the last-period date, using ACOG’s thresholds.

Weeks at that scan (optional)

Complete weeks

Extra days at that scan (optional)

0 to 6

Calculate as of (optional)

The date on which you want to know how many weeks along you are. If you leave it empty, today is used.

Leave empty for today

Results

Enter your values and press “Calculate” to see the result.

Free, no sign-up! What if you could do this without a calculator? A five-minute game with challenges, lives and points. It clicks even when you cannot be bothered.

In short

What it calculates: Estimated due date from your last period, conception, an ultrasound or IVF, with your weeks pregnant and milestones.
Formula used: Last period (Naegele): EDD = last period + 280 days + (cycle − 28)
Example: Example 1. From the last period.

Due date calculator

The estimated due date (EDD) falls 40 weeks after the first day of your last period. But that rule, Naegele’s, assumes a 28-day cycle with ovulation on day 14, and not every pregnancy fits: so this calculator also accepts a conception date, an ultrasound or an embryo transfer from IVF.

Pick the date you have and you get the due date, how far along you are today (weeks + days), the trimester, when the milestones fall (full term, post-term…) and, if you have an ultrasound, whether its date differs enough from your period date for ACOG to suggest redating. Everything runs in your browser.

How it works

Every method looks for the same starting point: “day zero” of the pregnancy, equivalent to the first day of a period in a 28-day cycle. From there 280 days are added. With the last period it adjusts for the difference between your cycle and 28 days; with conception it subtracts 266 days; with an ultrasound it subtracts the measured age; with IVF it subtracts 14 days plus the embryo’s age.

To compare the period date with an ultrasound, ACOG sets how big a difference it accepts depending on the age at which the scan was done: the earlier, the smaller the margin, because the measurement is more precise.

Formula

Last period (Naegele): EDD = last period + 280 days + (cycle − 28)
Conception: EDD = conception + 266 days
Ultrasound: EDD = scan date + 280 − measured age (in days)
IVF: EDD = transfer + 266 − embryo age (261 days with a day-5 embryo; 263 with day 3)
Weeks pregnant = (date − day zero) ÷ 7
ACOG redating threshold, by age at the scan:
up to 8+6: 5 days · 9+0 to 15+6: 7 days · 16+0 to 21+6: 10 days · 22+0 to 27+6: 14 days · from 28+0: 21 days

Worked example

Example 1. From the last period.
Last period on 2026-02-01 and a 28-day cycle: EDD 2026-11-08. On 2026-08-21 you are at week 28+5, already in the third trimester (which began 2026-08-16), with 79 days to go.

Example 2. With a 30-day cycle.
With the same period but a 30-day cycle, the EDD moves two days later: 2026-11-10, and estimated conception is 2026-02-17.

Example 3. IVF.
Transfer of a blastocyst (day-5 embryo) on 2026-03-01: EDD 2026-11-17. With a day-3 embryo it would be 2026-11-19.

Example 4. The scan contradicts the period.
By the last period (2026-02-01) the EDD is 2026-11-08. A scan on 2026-05-04 measures 12+0, which gives 2026-11-16: eight days apart, more than the 7 ACOG accepts at that age. The criteria suggest redating from the scan; the decision is your obstetrician’s.

Explanation

Why we count from the last period

Gestational age is measured from the first day of the last period, not from conception, because that date is almost always known and conception is not. That is why a “4-week” pregnancy has really only been going a few days.

Which method is most reliable

**First-trimester ultrasound** (up to week 13+6): the most precise; ACOG treats it as the reference for dating a pregnancy.
**IVF**: the transfer date and the embryo’s age are known exactly.
**Known conception**: good, if ovulation was tracked.
**Last period**: fine with regular cycles; less precise if they are irregular or you are unsure of the date.

When to redate from the scan

If the scan differs from the last period by more than ACOG allows (5 days up to week 8+6, 7 up to 15+6, 10 up to 21+6, 14 up to 27+6 and 21 after that), redating from the scan is suggested. Otherwise the last period stands. The decision always belongs to the professional looking after you.

Terms of pregnancy

ACOG distinguishes early term (37+0 to 38+6), full term (39+0 to 40+6), late term (41+0 to 41+6) and post-term (from 42+0). Before 37 weeks it is preterm. The due date sits in the middle of full term, but most babies are born across a range of weeks, not on the day given.

Common mistakes

Counting from conception instead of the last period.
Using the last day of the period instead of the first.
Applying Naegele’s rule with very irregular cycles.
Treating a calculator’s date as final when there is an early scan.
Planning for the exact due date.

Limits of this calculator

Naegele’s rule assumes a 280-day gestation with ovulation on day 14; it takes no individual factors into account.
ACOG’s redating thresholds apply to the age measured at the scan (crown-rump length in the first trimester) and are given as a guide; check them against the original text (Committee Opinion 700).
For IVF it uses the convention of subtracting 14 days plus the embryo’s age (261 days after a day-5 transfer).
It does not work for multiple pregnancies and does not replace medical follow-up.
It accepts scan weeks from 4 to 41.

Notice

This calculator is for information. The due date and any decision about your pregnancy should be confirmed by your obstetrician or midwife.

Frequently asked questions

How many babies arrive on the due date?

Very few. The due date is a central point; most births happen between weeks 37 and 42.

Which date counts, the scan or the last period?

If the scan is from the first trimester and differs by more than ACOG allows, the scan date is used. If they agree, the period date. Your obstetrician decides.

How is it worked out with IVF?

The embryo’s age plus 14 days is subtracted from the transfer date and 280 days are added: 261 days after the transfer for a day-5 embryo, 263 for a day-3 one.

What if my cycles are irregular?

The last period loses accuracy: an early scan is much better for dating the pregnancy.

Why are weeks counted from the last period?

Because it is the date known for certain. That is why a confirmed pregnancy already counts about four weeks.

When does the third trimester start?

At week 28+0, 196 days after day zero. The second trimester starts at 14+0 (day 98).

What does “full term” mean?

From 37 weeks it is no longer premature; ACOG distinguishes early term (37 to 38+6), full term (39 to 40+6), late term (41) and post-term (from 42).

Is my data sent to a server?

No. The calculation runs in your browser.

Sources and references

The formula behind this calculator is not ours. These are the sources it comes from, in APA 7 format, in case you want to go to the original or cite them in your own work.

American College of Obstetricians and Gynecologists. (2017). Committee Opinion No. 700: Methods for estimating the due date. Obstetrics & Gynecology, 129(5), e150–e154. https://doi.org/10.1097/AOG.0000000000002046 https://doi.org/10.1097/AOG.0000000000002046 The current standard: a first-trimester scan overrides Naegele’s rule; dating of IVF pregnancies.
American College of Obstetricians and Gynecologists. (2013). Committee Opinion No. 579: Definition of term pregnancy. Obstetrics & Gynecology, 122(5), 1139–1140. https://doi.org/10.1097/01.AOG.0000437385.88715.4a https://doi.org/10.1097/01.AOG.0000437385.88715.4a The definition of early, full, late and post-term.
Mittendorf, R., Williams, M. A., Berkey, C. S., & Cotter, P. F. (1990). The length of uncomplicated human gestation. Obstetrics & Gynecology, 75(6), 929–932. https://pubmed.ncbi.nlm.nih.gov/2342739/ Measured that real gestation runs a little longer than the 280 days the classic rule assumes.

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