Plain Figures

Pregnancy Due Date Calculator

Estimate your due date from the first day of your last period, from conception, or from an IVF embryo transfer, and see how far along you are today.

Work it out from
Your dates

Days from one period’s first day to the next’s. 20 to 45.

Estimated due date

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Enter your numbers to see the result.

How the due date is estimated

ACOG gives the average length of pregnancy as 280 days, or 40 weeks, counted from the first day of the last menstrual period[1]. It also counts pregnancy as starting two weeks after that day [2], so from conception it is 266 days. A cycle longer or shorter than 28 days shifts ovulation, and the due date with it [3].

Last period: LMP + 280 + (cycle − 28) days Conception: + 266 days IVF day 5: + 261, day 3: + 263

The LMP and cycle rule is ACOG’s 280 days plus a day-for-day cycle adjustment; the conception and IVF offsets are arithmetic from the same facts.

Example: last period, 28-day cycle

First day of last period Saturday, January 10, 2026: + 280 days = Saturday, October 17, 2026.

Example: a 35-day cycle

Same last period with a 35-day cycle: 280 + (35 − 28) = 287 days, so Saturday, October 24, 2026, a week later.

Example: conception and IVF

Conception on Sat, Mar 14, 2026: + 266 days = Sat, Dec 5, 2026. A day-5 embryo transferred on Sun, Mar 1, 2026: + 261 days = Tue, Nov 17, 2026; a day-3 embryo: + 263 = Thu, Nov 19, 2026.

Example: how far along on a given day

With a due date of Sat, Oct 17, 2026, the count starts 280 days earlier. On Sat, Jun 20, 2026 that is 23 weeks 0 days: the second trimester.

The week you are in today is worked out in your browser from your device’s date. Dates here are estimates; your doctor or midwife sets the date that counts.

Questions

How accurate is a due date?

It is an estimate. MedlinePlus says a due date “is not exact, but an estimate” [4]. Many women also cannot remember the exact day of their last period, and cycles differ in length. Most babies are born before or after it.

Can an ultrasound change my due date?

Your ob-gyn may confirm or adjust the date with an early ultrasound, especially when the last period date is uncertain [2]. ACOG adds that once a due date has been selected, it does not change however many later ultrasounds you have [1]. Use the date your care provider gives you.

Why does cycle length matter?

ACOG says ovulation typically happens about 14 days before the next period starts, however long the cycle is [3]. A 35-day cycle means ovulation about a week later than in a 28-day cycle, so the due date moves a week later. The calculator allows 20 to 45 days.

Where do the IVF dates come from?

They are arithmetic, not a quoted rule. A day-5 embryo is counted as five days past fertilization, so its transfer date is five days after the equivalent conception date: due date = transfer + 261 days (+ 263 for a day-3 embryo). Your clinic’s own dating is the one to use.

What do the trimester and term labels mean?

ACOG defines the trimesters as the first day of the last period to 13 weeks 6 days, 14w0d to 27w6d, and 28w0d to 40w6d [2]. The NICHD calls birth before 37 weeks preterm [4]; the AAP says full term is 39 weeks [5]; ACOG calls 41 to 42 weeks late term and over 42 weeks postterm [1].

Sources

  1. American College of Obstetricians and Gynecologists. When Pregnancy Goes Past Your Due Date (FAQ069; last reviewed March 2026). Accessed .
  2. American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy (FAQ156; last reviewed March 2026). Accessed .
  3. American College of Obstetricians and Gynecologists. Trying to Get Pregnant? Here’s When to Have Sex (last reviewed August 2025). Accessed .
  4. Eunice Kennedy Shriver National Institute of Child Health and Human Development (NIH). Pregnancy (fact sheet). Accessed .
  5. American Academy of Pediatrics, HealthyChildren.org. Corrected Age For Preemies (last updated December 10, 2018). Accessed .

Last updated .