TRENES

Due Date Calculator

Estimate your baby’s due date (EDD) and trimester start dates from the first day of your last period.

Written by Kübra Yıldız · Reviewed by Furkan Tur · Updated

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Your due date — the estimated date of delivery, or EDD — is the anchor every prenatal appointment, scan and milestone is measured against. It’s calculated from the first day of your last menstrual period (LMP), not from conception, because the LMP is the one date most people can pin down reliably.

Enter the first day of your last period and your usual cycle length, and this tool gives your estimated due date plus the dates each trimester begins. It’s an estimate: only about 1 in 20 babies actually arrives on the due date itself.

Due Date Calculator: step-by-step worked example

Example inputs
First day of last period
August 3, 2026
Cycle length (days)
30
  1. 1

    Naegele's rule: base length

    Pregnancy = 280 days (40 weeks) from the last period

    = 280 days

  2. 2

    Cycle-length correction

    Correction = Cycle − 28 days

    30 − 28 = 2 days

  3. 3

    Total days

    Total = 280 + correction

    280 + (2) = 282 days

  4. 4

    Estimated due date

    Due date = Last period + total days

    August 3, 2026 + 282 = May 12, 2027

  5. 5

    Start of 2nd trimester

    Last period + correction + 14 weeks (98 days)

    August 3, 2026 + 100 = November 11, 2026

  6. 6

    Start of 3rd trimester

    Last period + correction + 28 weeks (196 days)

    August 3, 2026 + 198 = February 17, 2027

Result · Estimated due dateMay 12, 2027
Pregnancy timeline
  1. Last period
    August 3, 2026
  2. 2nd trimester
    November 11, 2026
  3. 3rd trimester
    February 17, 2027
  4. Due date
    May 12, 2027

Good to know: Based on Naegele's rule (last period + 280 days, with the due date and trimester dates shifted by the difference if your cycle isn't 28 days). Only 4–5% of babies arrive on the exact date; birth between weeks 37 and 42 is normal. A first-trimester ultrasound date is more reliable — follow your obstetrician.

Due Date Calculator — how it works

Naegele’s rule

The standard method adds 280 days (40 weeks) to the first day of your last period:

StepCalculation
StartFirst day of last menstrual period (LMP)
Add280 days (40 weeks)
ResultEstimated due date (EDD)

A common shortcut is the same thing stated differently: take the LMP, subtract 3 months, add 7 days and a year.

The cycle-length adjustment

The 280-day rule assumes a textbook 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, ovulation shifts and so does your true due date. This tool adjusts for the cycle length you enter: a 32-day cycle pushes the estimate roughly 4 days later, a 24-day cycle about 4 days earlier.

Trimesters

Pregnancy is split into three trimesters, dated from the LMP:

TrimesterWeeks
First0 – 13
Second14 – 27
Third28 – birth

Why it’s only an estimate

An LMP-based date assumes regular cycles and a known period date. A first-trimester ultrasound measures the baby directly and is more accurate — if your scan gives a different date, clinicians use that. A normal, full-term birth can happen any time from 37 to 42 weeks.

Worked example

Suppose your last period began on 1 March 2026 and you have a regular 28-day cycle. Adding 280 days gives an estimated due date of 6 December 2026. The second trimester starts 14 weeks after the LMP (around 7 June), and the third trimester at 28 weeks (around 13 September). If your cycles instead run 32 days, ovulation is later, so the estimate moves about four days out to roughly 10 December.

Estimated due date and pregnancy week as of 12 October 2026, by last period date

Assumes a regular 28-day cycle (Naegele’s rule: first day of last period + 280 days). An ultrasound may adjust the date.

Last periodDue dateWeek todayTrimester
1 January 20268 October 202640 weeks 4 days3
15 January 202622 October 202638 weeks 4 days3
1 February 20268 November 202636 weeks 1 day3
15 February 202622 November 202634 weeks 1 day3
1 March 20266 December 202632 weeks 1 day3
15 March 202620 December 202630 weeks 1 day3
1 April 20266 January 202727 weeks 5 days2
15 April 202620 January 202725 weeks 5 days2
1 May 20265 February 202723 weeks 3 days2
15 May 202619 February 202721 weeks 3 days2
1 June 20268 March 202719 weeks 0 days2
15 June 202622 March 202717 weeks 0 days2
1 July 20267 April 202714 weeks 5 days2
15 July 202621 April 202712 weeks 5 days1
1 August 20268 May 202710 weeks 2 days1
15 August 202622 May 20278 weeks 2 days1
1 September 20268 June 20275 weeks 6 days1
15 September 202622 June 20273 weeks 6 days1

Frequently asked questions

Is the due date counted from my last period or from conception?+

From the first day of your last menstrual period (LMP). Because conception typically happens about two weeks after the LMP, “40 weeks pregnant” actually means about 38 weeks since conception. The LMP is used because it’s the date most people can identify precisely.

How accurate is a due date?+

It’s an estimate. Only around 4–5% of babies are born on their exact due date; most arrive within the two weeks either side. A first-trimester ultrasound is more accurate than an LMP calculation and takes precedence if the two disagree.

What if my cycle isn’t 28 days?+

Enter your real average cycle length. The classic rule assumes a 28-day cycle with ovulation on day 14; longer or shorter cycles shift ovulation and the due date accordingly, and this tool adjusts for it.

What counts as full term?+

A birth between 37 and 42 completed weeks is considered term. Before 37 weeks is preterm; after 42 is post-term. The due date marks the middle of the normal range, not a deadline.

Can I rely on this instead of seeing a doctor?+

No. This is an informational estimate, not medical advice. Your due date should be confirmed by a healthcare provider, usually with an early ultrasound, and prenatal care should follow their guidance.

Sources

About this page

Publisher:
Kalkoro — Teknolojig Bilişim Teknolojileri A.Ş., İstanbul
Written by:
Kübra Yıldız — Editor (Health, nutrition, pregnancy, education, exam scores)
Reviewed by:
Furkan Tur — Editor

Parameters come from official sources; the last update date is shown at the bottom of the page.

Spotted a wrong result? Use the "Report a bug" button at the top of the page; every report is reviewed.

How we calculate and our accuracy policy →

This tool is for general information only and is not medical diagnosis, treatment or advice. Consult a physician for decisions about your health.

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