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Reverse Due Date Calculator

HealthPregnancy and Fertility

The estimated due date from your scan or clinic letter.

Optional: shows the gestational age on this date.

Estimated first day of last period (LMP) —
Estimated conception date
—
Likely fertile window
—
Weeks pregnant on chosen date
—
Second trimester starts
—
Third trimester starts
—
Days from LMP to due date
—

A reverse due date calculator counts backwards from an estimated due date. The last menstrual period (LMP) is 280 days earlier and conception is about 266 days earlier. A due date of 7 October 2026 points to an LMP of 31 December 2025 and conception around 14 January 2026, assuming a 28-day cycle.

About this tool

Clinic letters and scan reports usually give a due date, not the dates behind it. People want those dates back for many reasons: to see when conception probably happened, to fill in a form that asks for the LMP, or to know how many weeks along they are on a given day. This calculator takes the due date, applies Naegele's rule in reverse, and corrects the period start for cycles longer or shorter than 28 days. It also lists the likely fertile window, the dates each trimester begins and, if you pick a date, the gestational age on that day. The limit is biological: ovulation varies by several days even in regular cycles, so the conception date is an estimate with a margin of about a week, and it cannot establish paternity.

How to use it

  1. Enter the due date

    Use the date from your dating scan or your clinician, since an ultrasound-adjusted date is the most reliable starting point.

  2. Set your cycle length

    Use the minus and plus buttons to set your usual cycle from 21 to 45 days; 28 matches the standard rule.

  3. Optionally pick a date to check

    Choose any day to see how many weeks and days pregnant that date corresponds to.

  4. Read the results

    The LMP appears first, followed by the conception estimate, fertile window, trimester start dates and the working.

Examples

Due 7 October 2026, 28-day cycle

Due date
2026-10-07
Usual cycle length
28

Result Estimated first day of last period (LMP): 2025-12-31
Estimated conception date: 2026-01-14
Likely fertile window: 2026-01-09 to 2026-01-15
Weeks pregnant on chosen date: —
Second trimester starts: 2026-04-08
Third trimester starts: 2026-07-15
Days from LMP to due date: 280

  1. Days from LMP to due date = 280 + (28 − 28) = 280
  2. LMP = 2026-10-07 − 280 days = 2025-12-31
  3. Conception (ovulation) ≈ 2026-10-07 − 266 days = 2026-01-14
  4. Fertile window = 2026-01-09 to 2026-01-15 (5 days before ovulation to 1 day after)

Counting 280 days back lands on 31 December 2025 as the period start, and conception falls about two weeks later, on 14 January 2026.

Same due date, 32-day cycle

Due date
2026-10-07
Usual cycle length
32

Result Estimated first day of last period (LMP): 2025-12-27
Estimated conception date: 2026-01-14
Likely fertile window: 2026-01-09 to 2026-01-15
Weeks pregnant on chosen date: —
Second trimester starts: 2026-04-04
Third trimester starts: 2026-07-11
Days from LMP to due date: 284

  1. Days from LMP to due date = 280 + (32 − 28) = 284
  2. LMP = 2026-10-07 − 284 days = 2025-12-27
  3. Conception (ovulation) ≈ 2026-10-07 − 266 days = 2026-01-14
  4. Fertile window = 2026-01-09 to 2026-01-15 (5 days before ovulation to 1 day after)

Ovulation in a 32-day cycle comes about four days later, so the period start moves back to 27 December 2025 while conception stays on 14 January.

How far along on 1 March 2026

Due date
2026-10-07
Usual cycle length
28
Check weeks pregnant on
2026-03-01

Result Estimated first day of last period (LMP): 2025-12-31
Estimated conception date: 2026-01-14
Likely fertile window: 2026-01-09 to 2026-01-15
Weeks pregnant on chosen date: 8 weeks 4 days
Second trimester starts: 2026-04-08
Third trimester starts: 2026-07-15
Days from LMP to due date: 280

  1. Days from LMP to due date = 280 + (28 − 28) = 280
  2. LMP = 2026-10-07 − 280 days = 2025-12-31
  3. Conception (ovulation) ≈ 2026-10-07 − 266 days = 2026-01-14
  4. Fertile window = 2026-01-09 to 2026-01-15 (5 days before ovulation to 1 day after)
  5. Gestational age on 2026-03-01 = 60 days = 8 weeks 4 days

Sixty days have passed since the 31 December LMP, which is 8 weeks 4 days of pregnancy.

How it is calculated

LMP = due date − (280 + (cycle length − 28)) days; conception ≈ due date − 266 days

280
Days from the first day of the last period to the due date in a 28-day cycle (40 weeks)
266
Days from ovulation and conception to the due date (38 weeks)
cycle length
Days from the start of one period to the start of the next

Naegele's rule sets the due date 280 days after the LMP, assuming ovulation on day 14. Ovulation is usually about 14 days before the next period, so in a longer cycle it comes later and the LMP sits further back from the due date, by the difference from 28 days. Conception is tied to ovulation, so it stays 266 days before the due date whatever the cycle. Trimesters are counted from the LMP: the second begins at 14 weeks and the third at 28 weeks. The fertile window spans the five days before ovulation and the day after.

Sources

When not to use it

  • Do not rely on it to date conception for paternity questions; the margin is several days either way.
  • For IVF pregnancies the transfer date gives the conception date directly and more precisely.
  • Irregular cycles make the LMP adjustment unreliable, so a scan date is the better reference.

Common mistakes

  • Entering the date of a positive test instead of the due date.
  • Treating the conception estimate as the exact day of intercourse, when sperm can survive up to five days.
  • Leaving the cycle at 28 days when your cycles are regularly much longer or shorter.

Frequently asked questions

How is the conception date worked out from a due date?

Pregnancy is about 266 days from ovulation to birth, so the calculator subtracts 266 days from the due date. That gives the likely ovulation day, which is when fertilisation usually happens. Intercourse could have been up to five days earlier, because sperm survive that long.

Why does my cycle length change the LMP but not the conception date?

The second half of a cycle, from ovulation to the next period, is fairly constant at about 14 days. A longer cycle means ovulation came later after the period started. The due date is anchored to ovulation, so only the period start moves.

My scan changed my due date. Which one do I enter?

Enter the scan-adjusted date. ACOG regards first-trimester ultrasound as the most accurate way to date a pregnancy, and when it moves the due date the dates derived from it shift by the same number of days.

Why is the pregnancy counted from before conception?

Dating starts at the LMP because that date is usually known, while ovulation is not. As a result someone is already about two weeks pregnant by the standard count on the day of conception, and 40 weeks in total sits between the LMP and the due date.

How reliable is the estimated fertile window?

It is a six-day span ending the day after the estimated ovulation. Ovulation timing varies from cycle to cycle, so the true window can fall a few days either side, especially when the due date came from the LMP rather than from a scan.

Can I find out how many weeks pregnant I was on a past date?

Yes. Pick that date in the optional field and the tool counts the days since the estimated LMP and shows them as weeks and days. It accepts dates from the LMP up to 310 days after it.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 19). Reverse Due Date Calculator. Tools Directory. Retrieved October 25, 2026, from https://tools.milunir.com/health/pregnancy-and-fertility/reverse-due-date-calculator/

MLA

Tools Directory Editorial Team. “Reverse Due Date Calculator.” Tools Directory, 19 Oct. 2026, https://tools.milunir.com/health/pregnancy-and-fertility/reverse-due-date-calculator/. Accessed 25 Oct. 2026.

Chicago

Tools Directory Editorial Team. “Reverse Due Date Calculator.” Tools Directory. Last modified October 19, 2026. Accessed October 25, 2026. https://tools.milunir.com/health/pregnancy-and-fertility/reverse-due-date-calculator/.

Link<a href="https://tools.milunir.com/health/pregnancy-and-fertility/reverse-due-date-calculator/">Reverse Due Date Calculator – Tools Directory</a>