The moment a pregnancy is confirmed, one question crowds out every other: when is the baby due? That date anchors everything that follows — every antenatal appointment, every screening test, every scan. Here’s the good news: learning how to calculate pregnancy due date estimates is really just one addition problem, once you know which date you’re supposed to start from.
Most people reach for the wrong starting point. The standard clinical method doesn’t begin with the day you conceived. It begins about two weeks earlier, with the first day of your last menstrual period. That sounds backwards — and it is, a little — but there’s a good reason for it, which we’ll get into below.
In this guide you’ll learn how to calculate your pregnancy due date five different ways: the classic 280-day rule, the quick shortcut doctors scribble on notes, the correction for longer or shorter cycles, the method for when you know the exact conception date, and the transfer-date maths used for IVF pregnancies. You’ll also see five fully worked examples, the common mistakes that throw the answer off, and an honest look at how accurate these dates really are.
Table of Contents
Why Pregnancy Is Dated From Your Last Period, Not Conception
To understand how to calculate pregnancy due date figures properly, you first need to accept the quirk at the heart of obstetrics: pregnancy is dated from the first day of your last menstrual period (LMP), not from conception.
Why? Because conception is invisible. You can’t feel it happen, and unless you were tracking ovulation with tests, you can’t pin down the exact day. But the first day of your last period is a date most people can remember or find in a diary or an app. So medicine built the entire system around it.
The result is a built-in offset of roughly two weeks. Ovulation typically happens about 14 days after the start of your period, and conception follows within a day or two. So when a pregnancy is described as “6 weeks along” by LMP, the embryo is really only about 4 weeks old. A pregnancy that lasts “40 weeks” from the last period is really about “38 weeks” from conception.
Two terms help keep this straight:
- Gestational age — time since the first day of the LMP. This is what doctors, midwives and due-date calculators use.
- Fetal age — time since conception. Two weeks shorter, and rarely used in practice.
Every formula in this guide works in gestational age unless it says otherwise.
How to Calculate Pregnancy Due Date with Naegele’s 280-Day Rule
The workhorse of pregnancy dating is Naegele’s rule, named after the German obstetrician Franz Naegele, who formalised it in 1812. It assumes a regular 28-day cycle with ovulation on day 14, and it says:
Estimated due date (EDD) = first day of LMP + 280 days (40 weeks)
That’s it. Add 280 days to the first day of the last period, and you’ve got the date printed on every antenatal record. More than two centuries after Naegele proposed it, this is still the default calculation used by clinicians and pregnancy due date calculators worldwide — including the free pregnancy due date calculator on this site.
The quick shortcut doctors use
Counting 280 days across nine months of varying lengths is tedious, so there’s a handy shortcut:
EDD = LMP minus 3 months, plus 7 days, plus 1 year
Take the first day of the last period, go back three calendar months, add a week, and roll the year forward. For a last period starting on 1 January 2026: go back to 1 October 2025, add 7 days (8 October), add a year — 8 October 2026. As we’ll see in the worked example below, that lands on exactly the same date as the 280-day count here.
A word of honesty, though: the shortcut doesn’t always agree with the 280-day count. Months are different lengths, and leap years interfere, so the shortcut can land a day or two off the true 280-day total. When they disagree, the 280-day count is the authoritative one. You’ll see a real example of the discrepancy below.
Worked Example 1: Standard 28-Day Cycle
Let’s walk through the full 280-day count, because once you’ve seen it done properly, you’ll never doubt the shortcut again.
Given: first day of LMP = 1 January 2026; cycle length 28 days (the classic assumption).
We count forward 280 days, month by month:
| Month | Days counted | Running total |
|---|---|---|
| January (2nd–31st) | 30 | 30 |
| February 2026 | 28 | 58 |
| March | 31 | 89 |
| April | 30 | 119 |
| May | 31 | 150 |
| June | 30 | 180 |
| July | 31 | 211 |
| August | 31 | 242 |
| September | 30 | 272 |
| October (1st–8th) | 8 | 280 |
We start counting from 2 January because 1 January is day zero — the starting line, not the first day of the journey. A common beginner error is counting the LMP day itself as day 1, which shifts everything forward by one day.
Estimated due date: 8 October 2026.
Quick sanity check: forty weeks is 280 days, and we’ve counted exactly that. And the shortcut agrees: 1 Jan − 3 months = 1 Oct 2025; + 7 days = 8 Oct; + 1 year = 8 Oct 2026. Both methods converge.
How to Calculate Pregnancy Due Date When Your Cycle Isn’t 28 Days
Naegele’s rule assumes ovulation on day 14 of a 28-day cycle. But ovulation doesn’t care about the calendar — it happens roughly 14 days before the next period, not 14 days after the last one. So in a 35-day cycle, ovulation typically falls around day 21, a full week later than the textbook assumption. That pushes conception — and the due date — a week later too.
The correction is beautifully simple:
EDD = LMP + 280 days + (your cycle length − 28)
A 35-day cycle adds 7 days. A 24-day cycle subtracts 4 days. And yes, the calcnit pregnancy due date calculator asks for your average cycle length precisely so it can apply this correction.
Worked Example 2: 35-Day Cycle
Given: LMP = 15 March 2026; average cycle length 35 days.
First, the unadjusted 280-day count: March (16th–31st) 16 days, April 30 (46), May 31 (77), June 30 (107), July 31 (138), August 31 (169), September 30 (199), October 31 (230), November 30 (260), then 20 days into December to reach 280. Unadjusted EDD: 20 December 2026.
Now the correction: 35 − 28 = +7 days.
Estimated due date: 27 December 2026.
Notice something instructive here: the quick shortcut (−3 months + 7 days + 1 year) applied to 15 March gives 22 December — two days off the true 280-day count of 20 December. Months differ in length, and that’s exactly why the day count is the authority. A two-day error matters when screening tests are scheduled to the week.
When You Know the Conception Date
If you were tracking ovulation — with LH strips, temperature charting or a known date — you can skip the LMP entirely and date from conception:
EDD = conception date + 266 days (38 weeks)
The maths is identical to the LMP method: conception happens roughly 14 days after the LMP, and 280 − 14 = 266. The free due-date tool on this site uses this exact method — it adds 266 days to a known conception date.
Worked Example 3: Known Conception Date
Given: conception = 15 February 2026.
Counting forward: February (16th–28th) 13 days, March 31 (44), April 30 (74), May 31 (105), June 30 (135), July 31 (166), August 31 (197), September 30 (227), October 31 (258), then 8 days into November to reach 266.
Estimated due date: 8 November 2026.
One caution: a “conception date” from a fertility tracker is really the ovulation date (or the day after), which is the standard proxy. Fertilisation follows within about 24 hours, so the error introduced is negligible — far smaller than the natural two-week window around any due date.
IVF Pregnancies: Transfer-Date Dating
IVF is the one situation where the conception date is known to the hour — and yet LMP-based dating is still used as the common language. The trick is a fixed offset that converts the embryo’s age into gestational age.
A day-5 blastocyst is 5 days old, and fertilisation happened roughly 14 days into the standard cycle, so its gestational age at transfer is 5 + 14 = 19 days. The due date is therefore 280 − 19 = 261 days after transfer. For a day-3 (cleavage-stage) embryo, the gestational age at transfer is 3 + 14 = 17 days, so the offset is 280 − 17 = 263 days:
| Embryo stage at transfer | Gestational age at transfer | Days to add |
|---|---|---|
| Day-5 blastocyst | 19 days | 261 days |
| Day-3 embryo | 17 days | 263 days |
Worked Example 4: Day-5 Blastocyst Transfer
Given: transfer = 10 March 2026, day-5 blastocyst.
Counting 261 days: March (11th–31st) 21, April 30 (51), May 31 (82), June 30 (112), July 31 (143), August 31 (174), September 30 (204), October 31 (235), then 26 days into November to reach 261.
Estimated due date: 26 November 2026.
A quick cross-check you can do in your head: 261 days after transfer is the same as treating the transfer date as “19 days pregnant” and adding 261. Some clinics quote it as “transfer date + 38 weeks − 5 days” — identical arithmetic.
Ultrasound Dating: The Most Accurate Method
Here’s the hierarchy of accuracy, and it’s worth knowing because it determines whose number wins when methods disagree:
- First-trimester ultrasound (8–13 weeks) — the gold standard. Measuring the crown–rump length (CRL) of the embryo dates the pregnancy to within ±5 to 7 days. This is the method ACOG (the American College of Obstetricians and Gynecologists) treats as definitive — if the scan date differs from the LMP date by more than the margin of error, the scan date is used.
- Conception-date calculation — excellent, if the date is reliable.
- LMP + Naegele’s rule — good, but dependent on a regular cycle and an accurately remembered date.
Why does ultrasound win? Because it measures the pregnancy directly rather than inferring it from cycle averages. Later scans are progressively less useful for dating — by the third trimester, babies vary enough in size that a scan’s margin of error stretches to around ±3 weeks, which is why clinics pin the official due date to the earliest reliable scan and resist moving it.
What if you don’t remember your LMP at all? Then the ultrasound is your dating method — a first-trimester scan can establish the gestational age on its own, and from there the due date is simple addition.
Reading Your Result: Gestational Age and Trimesters
Once you have a due date, every conversation with your care provider happens in weeks and days. The notation “20w0d” means 20 weeks and 0 days of gestational age — days elapsed since the LMP, not a calendar fraction. (140 days = exactly 20 weeks 0 days.)
Worked Example 5: Gestational Age “Today”
Given: LMP = 1 June 2026. Today is 19 October 2026.
Days elapsed: June (2nd–30th) 29, July 31 (60), August 31 (91), September 30 (121), October (1st–19th) 19 → 140 days.
Gestational age today: 20 weeks 0 days. (And for reference, the due date for this LMP is 1 June + 280 days = 8 March 2027.)
Want to do this for any two dates instantly? The date difference calculator on this site counts the exact days between your LMP and today, which you then divide by 7 to get weeks and days.
The pregnancy is conventionally split into three trimesters:
| Trimester | Span | Ends (for our Example 1, LMP 1 Jan 2026) |
|---|---|---|
| First | 0w0d – 13w6d | 8 April 2026 (day 97) |
| Second | 14w0d – 27w6d | 15 July 2026 (day 195) |
| Third | 28w0d – birth | begins 16 July 2026 (day 196) |
The calcnit tool reports these milestones automatically alongside the due date — “First Trimester Ends”, “Second Trimester Ends” and “Third Trimester Starts” — so you don’t have to count them by hand.
How Accurate Is a Due Date, Really?
Time for the honest answer: a due date is the centre of a window, not a deadline. Only about 5% of babies arrive on their exact due date — the site’s own calculator flags this too. The majority arrive within about two weeks on either side; one commonly cited figure is that roughly 70% deliver within 10 days of the EDD.
That’s why clinicians classify the whole neighbourhood of the due date rather than treating 40w0d as a cliff edge:
| Term | Gestational age |
|---|---|
| Preterm | before 37w0d |
| Early term | 37w0d – 38w6d |
| Full term | 39w0d – 40w6d |
| Late term | 41w0d – 41w6d |
| Postterm | 42w0d and beyond |
So for our Example 1 (EDD 8 October 2026), “full term” runs from 1 October to 14 October. Arriving anywhere in that fortnight is the textbook ideal outcome — which is exactly why fretting over a due date that shifts by a few days after a scan is wasted energy. If your scan moves the date within the margin of error, the original LMP date usually stands; the scan only takes over when the gap exceeds it, per ACOG’s guidance on estimating the due date.
Common Mistakes
1. Dating from conception instead of the LMP. The most frequent error. If you know you conceived on, say, 1 February and you add 280 days, you’ll overshoot by two weeks. From conception, add 266.
2. Forgetting the cycle-length correction. A 35-day cycler who uses unadjusted Naegele’s rule gets a date a full week too early — and then spends a week being told the baby is “overdue” before it even reaches term.
3. Trusting the shortcut blindly. The −3 months + 7 days trick is a convenience, not a formula. Around long and short months it drifts by a day or two (we saw 22 vs 20 December above). For anything that matters — screening windows, induction discussions — use the 280-day count.
4. Mixing up IVF transfer offsets. Day-5 transfer means +261 days; day-3 means +263. Swapping them introduces a two-day error — small, but pointless.
5. Panicking when the dating scan moves the date. A shift of a few days is normal and usually means the original date stands. It only changes the plan when the scan and the LMP disagree by more than the scan’s margin of error.
The Bottom Line
Calculating a pregnancy due date is five minutes of arithmetic: add 280 days to the first day of your last period, adjust for your cycle length, and cross-check with a first-trimester scan if you can. These methods are for estimation and education — your midwife or doctor’s official dating always takes precedence. But don’t stop at the hand calculation — run the numbers through the pregnancy due date calculator and it’ll hand you the estimated date, your current gestational age and all three trimester milestones in one go.
And if you’re already thinking ahead to the nutrition side of pregnancy — how much you should actually be eating as the trimesters progress — our guide to daily calorie needs walks through the formulas for that too.
Frequently Asked Questions
Can my due date change after the dating scan?
Yes, but only when it should. If the first-trimester scan’s estimate differs from your LMP-based date by more than about 5–7 days, your provider will revise the due date to match the scan — ultrasound is more accurate than calendar arithmetic. Smaller differences are ignored, and the original date stands.
What if I don’t remember the date of my last period?
You’re not stuck. A first-trimester ultrasound can date the pregnancy from the embryo’s measurements alone — the crown–rump length at 8–13 weeks is accurate to about a week. Many pregnancies are dated this way entirely.
Does a longer cycle mean a later due date?
Yes — roughly. Because ovulation happens about 14 days before your next period, a 35-day cycle means you ovulated around day 21 instead of day 14, pushing conception (and the due date) about a week later. The rule of thumb: add your cycle length minus 28 to the standard 280-day date.
How is the due date calculated for IVF?
From the transfer date, not the LMP. Add 261 days for a day-5 blastocyst transfer, or 263 days for a day-3 embryo transfer. These offsets account for the embryo’s age plus the standard two-week gestational-age offset.
What does “40 weeks pregnant” actually mean?
It means 40 weeks have passed since the first day of your last menstrual period — not since conception. The embryo itself is only about 38 weeks old. This LMP-based “gestational age” is the universal language of antenatal care, so a “40-week pregnancy” is a 38-week-old baby, by design.
Is it true that only about 5% of babies arrive on their due date?
Yes. The exact figure varies by study, but roughly one in twenty babies is born on the estimated date. The rest arrive in a window around it — most within about two weeks either side. Think “due week” (or “due fortnight”) rather than due date, and you’ll have a calmer pregnancy.