Due Date Calculator

How this due date calculator applies Naegele's rule, adjusted for cycle length

The most common way to estimate a due date starts from the first day of the last menstrual period (LMP), not from conception itself, because most people can date their last period far more confidently than they can date conception. The textbook version of this method, known as Naegele's rule, simply adds 280 days (40 weeks) to that date. Those 280 days are not arbitrary: a full-term pregnancy is usually counted as 40 weeks from the LMP, even though actual conception typically happens around two weeks into that window, near ovulation. This page is, in effect, a naegele's rule calculator adjusted for your own average cycle length, rather than assuming everyone ovulates on day 14, and it works the same relationship a physical pregnancy wheel calculator has used for decades, edd and lmp on opposite ends of the same 280-day count. The due date itself is also sometimes called the EDC, estimated date of confinement, an older clinical term for the same edc dating math this calculator runs, and if you have seen the abbreviation LMP and searched l.m.p meaning specifically, it is simply short for last menstrual period.

If what you actually want is “what is 40 weeks from today” rather than from a specific LMP date, entering today's date as the last period start answers that directly, landing exactly 280 days out.

The plain 280-day version of the rule assumes a textbook 28-day cycle with ovulation on day 14. Real cycles vary, and a longer or shorter average cycle shifts ovulation, and so the likely conception date, later or earlier relative to the LMP. This calculator adjusts for that: the 280-day estimate is shifted by the same number of days the entered average cycle length differs from 28. A 35-day average cycle, seven days longer than the textbook cycle, pushes the estimated due date seven days later; a 21-day average cycle pulls it seven days earlier.

In symbols, the estimate is the last period plus 280 days, shifted by how far the cycle strays from 28, and the conception-date method swaps in 266 days.

Worked example. Take a last period starting 1 January 2026, with a standard 28-day cycle. Adding 280 days lands on 8 October 2026, so that becomes the estimated due date, and the likely conception date, roughly 14 days after the LMP, falls around 15 January 2026. Now compare that against a 35-day average cycle starting from the same 1 January 2026 last period: the cycle-length adjustment adds a further seven days on top of the 280, moving the estimated due date to 15 October 2026, exactly a week later than the 28-day estimate, purely because ovulation is assumed to happen later in a longer cycle.

Run through this calculator, the same 1 January 2026 last period gives the due dates below for eight average cycle lengths. Each extra day of cycle moves both the due date and the likely conception date one day later.

The conception-date method, explained with its own worked example

When a conception date is actually known, for example from fertility tracking or a single-cycle timeline, or from a known IVF transfer date, a more direct estimate skips the cycle-length assumption entirely: a full-term pregnancy is counted as 38 weeks, or 266 days, from conception itself, rather than 40 weeks from the last period. The 40-weeks-from-LMP and 38-weeks-from-conception figures describe the same underlying length of pregnancy; they just start the clock from two different, roughly two-week-apart reference points. An IVF transfer date is a common source of a known conception date input: entering it here answers an ivf transfer due date or embryo transfer due date question the same 266-day way, once translated to an equivalent conception date from the embryo's age at transfer, and a fresh or a frozen embryo transfer due date calculator differs only in that translation step, which is exactly the kind of input an ivf pregnancy calculator needs to be more precise than a plain LMP-based one.

Worked example. Take a conception date of 1 March 2026. Adding 266 days lands on 22 November 2026, which becomes the estimated due date under this method. Because 2026 is not a leap year, that 266-day count runs through a plain 28-day February with no adjustment needed; a conception date whose 266-day window crosses a leap year's 29 February would run through one extra day in that stretch, which the calculator's date arithmetic accounts for automatically rather than needing a manual correction.

For an IVF transfer, the embryo's age stands in for the conception date: subtract the age at transfer from 266 and add the result to the transfer date. The table uses a 1 March 2026 transfer.

Current week, average baby size, and the 42-week milestone table

Whichever method produces the due date, the result panel also shows where the pregnancy stands today: the current week and day, the trimester, percent complete with a progress bar, and, for weeks 8 through 42, the commonly published average baby length and weight for that specific week, effectively answering “how old is my baby” in the same breath as the due date itself. The milestone table starts at week 8, so if you already know you are 8 weeks pregnant, or eight weeks pregnant spelled out, the current-week row already shows this week's baby size average, and later on the same table a search for 16 week fetus size is answered directly by that week's row. All of it is derived from the same due date, working back through the standard 280-day count to an equivalent last period date, then measuring today against that, the same way you would calculate gestational age or calculate pregnancy progress by hand, the exact method this site's pregnancy calculator already uses, reused here rather than recalculated a second way, so the two tools never disagree about the same underlying facts.

Below that sits a full week-by-week table running through week 42, not just the standard 40, with the current week highlighted and auto-scrolled into view. The extra two rows matter because reaching the due date is not the end of the normal range, as the term-window section below explains; the table stays useful for the small share of pregnancies that run past 40 weeks, not just the majority that do not. Average baby length is measured crown-to-rump (head to bottom) through week 19 and head-to-heel from week 20 onward, which is why the published figure jumps at that point rather than climbing smoothly, a real feature of how these reference charts are built rather than an error. Every figure is a population average, not a prediction: individual measurements vary meaningfully, especially in the third trimester, and an ultrasound is the only way to know an individual baby's actual size.

Due date as a reference point: early, full, late and postterm

A due date reads like a single day, but the American College of Obstetricians and Gynecologists (ACOG), together with the Society for Maternal-Fetal Medicine (SMFM), defines a full six-week window around it, which is also the standard answer to how many weeks pregnancy is full term: early term runs from 37 weeks 0 days through 38 weeks 6 days, full term runs from 39 weeks 0 days through 40 weeks 6 days, late term runs from 41 weeks 0 days through 41 weeks 6 days, and postterm is 42 weeks 0 days and beyond. If a search for “wog meaning” or “meaning wog” brought you here, WOG is shorthand for weeks of gestation, the same running count of weeks and days shown at the top of this page; w.o.g meaning is identical, just written with periods. Outcomes measured across large populations are, on average, best within the full-term window specifically, which is part of why a doctor may frame 39 weeks, not 37, as the real target for a scheduled delivery that has no medical reason to happen earlier, and why a pregnancy that continues into postterm is typically monitored more closely and often discussed for induction.

Commonly cited estimates put only around 4 to 5 percent of babies as arriving on their exact calculated due date, with many sources suggesting a large majority deliver within about a week either side of it and a substantial majority within about two weeks, though the precise percentages quoted vary somewhat by source. Whatever the exact figures, the practical takeaway is the same: a full-term delivery anywhere from about three weeks before the due date to two weeks after it is entirely normal, and the number this calculator returns is best treated as a useful planning midpoint, not a deadline.

The gestational age shown alongside an LMP-based result works the same way: it is a running count of weeks and days since the entered last period, calculated from today's date, and it is left blank whenever that count would not make sense, for example when the entered last period date is still in the future.

An early ultrasound, typically performed in the first trimester, is considerably more accurate than either method here, because it measures the size of the fetus directly rather than working backward from a remembered date or an assumed cycle length. Whenever an ultrasound-based due date is available, it should be treated as the more reliable figure, and this calculator's estimate as the earlier, rougher starting point it was always meant to be. For pregnancy detection (how and when hCG becomes detectable) and pregnancy management (medication categories, weight gain, exercise and nutrition guidance by trimester), see the fuller coverage on the pregnancy calculator page rather than repeating the same ground here.