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Pregnancy Due Date Calculator

This pregnancy due date calculator estimates your due date from the first day of your last period, a known conception date or an ultrasound's gestational age, then see how far along you are on any date you choose.

Dating methodLMP / conception / ultrasound dateReference date

Estimates only. A clinician establishes the obstetric due date. LMP assumes a 28-day cycle; conception adds 266 days, not 280. Dates remain in this tab and are not saved.

Choose a date to see the estimate.

How the due date is estimated

From your last period (LMP)due date = first day of LMP + 280 days
From conceptiondue date = conception date + 266 days
From an ultrasounddue date = scan date + 280 − (weeks × 7 + days at the scan)
Gestational age on a date280 − days from that date to the due date
Trimesterfirst < 14 weeks · second 14–27 weeks 6 days · third 28 weeks +

Pregnancy is counted from the first day of the last period, not from conception. In a textbook 28-day cycle, ovulation and conception happen about two weeks later. That is why the conception method adds 266 days instead of 280. Enter whole weeks and days for the scan. Reference date is the day you want the gestational age for, usually today.

Estimates only. A clinician establishes the obstetric due date. LMP assumes a 28-day cycle; conception adds 266 days, not 280. Dates remain in this tab and are not saved.

Worked example

The same pregnancy dated three ways, with gestational age checked on October 15, 2026:

Method	Due date	Age on October 15, 2026
Last period (LMP)LMP March 2, 2026	December 7, 2026	32 weeks 3 days
ConceptionConceived March 16, 2026	December 7, 2026	32 weeks 3 days
UltrasoundScan May 4, 2026 at 9 weeks 2 days	December 5, 2026	32 weeks 5 days

LMP and conception agree because conception is two weeks after the LMP. The scan puts the due date 2 days earlier, at December 5, 2026. At 9 weeks 2 days, a difference that small is within ACOG's 7-day threshold, so a clinician would normally keep the LMP date.

On October 15, 2026 the LMP dating gives 32 weeks 3 days, the third trimester, with 53 days to the estimated date.

When an ultrasound changes the due date

An LMP date depends on remembering the right day and on a regular cycle. ACOG says that ultrasound measurement of the embryo or fetus in the first trimester, up to 13 weeks 6 days, is the most accurate way to set or confirm gestational age. Once set, the due date should be changed only in rare cases. If the scan disagrees with the LMP date by more than the amounts below, the scan date is used instead.

Redating thresholds (ACOG)

Age at the scan	Redate if LMP and scan differ by
Up to 8 weeks 6 days	More than 5 days
9 weeks 0 days to 13 weeks 6 days	More than 7 days
14 weeks 0 days to 15 weeks 6 days	More than 7 days
16 weeks 0 days to 21 weeks 6 days	More than 10 days
22 weeks 0 days to 27 weeks 6 days	More than 14 days
28 weeks 0 days and later	More than 21 days

Later scans are less precise for dating, so the tolerated gap grows. A pregnancy with no ultrasound before 22 weeks 0 days is considered suboptimally dated.

What "term" means

Label	Gestational age
Early term	37 weeks 0 days to 38 weeks 6 days
Full term	39 weeks 0 days to 40 weeks 6 days
Late term	41 weeks 0 days to 41 weeks 6 days
Postterm	42 weeks 0 days and later

The due date is 40 weeks 0 days, the middle of full term. Births before 37 weeks are preterm.

IVF and other assisted reproduction

Date these pregnancies from the treatment. In Ultrasound mode, enter the transfer date with 2 weeks 5 days for a day-5 embryo (transfer + 261 days) or 2 weeks 3 days for a day-3 embryo (transfer + 263 days).

This is an estimate, not medical advice. Only your midwife, obstetrician or doctor can set your due date. Speak to them about any bleeding, pain or other concerns. Don't wait for a date from a calculator.

Sources: ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (2017); ACOG Committee Opinion No. 579, Definition of Term Pregnancy (2013). Checked 15 October 2026.

Frequently asked questions

How is a pregnancy due date calculated?+

The standard method adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is Naegele's rule. An LMP of March 2, 2026 gives an estimated due date of December 7, 2026. Your clinician may confirm or change it after an early ultrasound.

How do I calculate my due date from the conception date?+

Add 266 days (38 weeks) to the conception date, not 280. Pregnancy is counted from the LMP, which is about two weeks before conception in a 28-day cycle. Conception on March 16, 2026 gives December 7, 2026 — the same date as the LMP example two weeks earlier.

How many weeks pregnant am I?+

Gestational age is counted from the first day of your LMP, so you are already about two weeks "pregnant" at conception. Choose Reference date to see your age on any day. With an LMP of March 2, 2026, you are 7 weeks 0 days on April 20, 2026 and 32 weeks 3 days on October 15, 2026.

How many days are left until my due date?+

The calculator shows "Days to estimated date" counted from the reference date. With an LMP of March 2, 2026, there are 53 days to go on October 15, 2026. Change the reference date to count from another day.

What is the difference between gestational age and fetal age?+

Gestational age is counted from the first day of the last period and is the age doctors and scans use. Fetal (or conceptional) age is counted from conception, so it is about two weeks less. A pregnancy at 12 weeks gestational age has a fetus about 10 weeks old. This calculator always reports gestational age.

How do I enter a result from a dating scan?+

Choose Ultrasound, enter the scan date and the gestational age the report gives in weeks and days. The due date is the scan date plus 280 days minus that age. A scan on April 27, 2026 showing 8 weeks 0 days gives December 7, 2026 — the same as the LMP example, so the dates agree.

What if my cycle is longer or shorter than 28 days?+

The LMP method assumes a 28-day cycle with ovulation around day 14. With longer cycles you probably ovulated later, so the LMP-based due date may be too early; with shorter cycles, too late. This calculator doesn't adjust for cycle length. If you know when you conceived, use Conception; otherwise a first-trimester ultrasound is the standard way to confirm dating.

My ultrasound due date is different from my LMP due date. Which one counts?+

ACOG says a first-trimester ultrasound (up to 13 weeks 6 days) is the most accurate way to set or confirm gestational age. Your clinician changes the due date to the scan date when the two differ by more than a set number of days — for example more than 5 days before 9 weeks, or more than 7 days from 9 weeks to 13 weeks 6 days. The table on this page lists all the thresholds.

How do I calculate an IVF due date?+

ACOG says pregnancies from assisted reproduction should be dated from the treatment. Choose Ultrasound, enter the embryo transfer date, and set the age to 2 weeks 5 days for a day-5 embryo or 2 weeks 3 days for a day-3 embryo. A day-5 transfer on March 20, 2026 gives December 6, 2026 (transfer + 261 days); a day-3 transfer gives December 8, 2026 (transfer + 263 days).

When do the trimesters start?+

The calculator uses the usual boundaries: the first trimester runs to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days until birth.

Is a baby likely to be born on the due date?+

Not necessarily. The due date marks 40 weeks 0 days, but ACOG counts any birth from 37 weeks 0 days to 41 weeks 6 days as term. Treat the date as the middle of a window, not a deadline.

Are the dates I enter saved or sent anywhere?+

No. The dates are calculated in your browser, kept only in this tab and cleared when you close or reload it. Nothing is uploaded or stored. Download summary saves a small text file to your own device.