Naegele's rule estimates your due date by adding exactly 280 days — 40 weeks — to the first day of your last menstrual period, then shifting that result by (your cycle length − 28) days to match when you actually ovulated. The math is straightforward, and the Pregnancy Due Date Calculator applies the same rule obstetricians and midwives use worldwide, so the figure you get online lines up with what the clinic hands you at a first visit. The 280-day convention deliberately dates pregnancy from your period rather than from conception, which is why the standard "40 weeks" of human gestation technically includes the two weeks before the egg and sperm ever met. Working the formula through by hand only takes a moment once you know your LMP and your typical cycle length, and the same arithmetic is what gets written on the chart — meaning a quick calculation on your phone is genuinely useful for tracking prenatal appointments, screening windows, and trimester milestones.
What Naegele's Rule Actually Says
The rule dates back to the early 19th century and remains the default pregnancy-dating method in most clinics because it needs only one input: a calendar date. It assumes a regular 28-day menstrual cycle with ovulation on day 14 and fertilization shortly after. Since first-day-of-last-period plus 280 days lands you squarely on the estimated date of delivery (EDD), the formula reduces to one line:
EDD = first day of your last menstrual period + 280 days
Three things follow from this. First, every calculation is anchored to a single calendar date you already know, which is why the rule has stayed in textbooks for two centuries. Second, the 280-day window assumes nothing unusual about your cycle — that assumption has to be corrected when it isn't true. Third, because the rule dates pregnancy from the LMP, gestational age always runs about two weeks "ahead" of the embryo's actual age since fertilization. Per the Wikipedia entry on Naegele's rule, this dating convention is still the foundation of modern obstetric dating, even though ultrasound has become the standard for refinement. If you want to audit the day-counting on paper, an online add/subtract days guide walks through the arithmetic in calendar-accurate form.
How to Calculate Your Due Date Step by Step
The fastest way to get a clinically meaningful estimate is to use the Pregnancy Due Date Calculator, which does the date arithmetic for you and recalculates the moment you type:
- Click the date field and select the first day of your last menstrual period (LMP). This is day one of your cycle and the anchor for the entire calculation.
- If your cycle is not 28 days, change the cycle-length field to your real average, anywhere from 20 to 45 days. The default is 28.
- Read your estimated due date and current gestational age on the right side of the tool. Both update as you type — nothing needs to be submitted.
- Note the result somewhere durable (a notes app, your patient portal, or a printed prenatal summary) so you can bring it to your first appointment.
If you want to check the math by hand, the calculation runs as follows. Take the first day of your last period — say, January 1, 2025 — then count forward exactly 280 days. January has 31 days (30 left after day 1), February contributes 28, March 31, April 30, May 31, June 30, July 31, August 31, and September 30, which together add up to 272 days. Eight more days into October gives you October 8, 2025 as the due date for a 28-day cycle. For a 30-day cycle, add two more days and you land on October 10, exactly as the "cycle length minus 28" adjustment prescribes.
How Cycle Length Shifts Your Date
The 28-day assumption is the single biggest source of error in LMP-based dating. Ovulation does not actually happen on day 14 in most women — it happens roughly two weeks before the next period, so a longer cycle pushes ovulation later and conception later, and the due date moves later by exactly the same number of days. The table below shows how much the estimate shifts for the cycle lengths the tool accepts.
| Average cycle length | Days added vs. 28-day baseline | Effect on due date |
|---|---|---|
| 21 days | −7 days | One full week earlier |
| 25 days | −3 days | A few days earlier |
| 28 days | 0 days | Baseline estimate |
| 30 days | +2 days | Two days later |
| 35 days | +7 days | One full week later |
| 40 days | +12 days | Nearly two weeks later |
| 45 days | +17 days | Just over two weeks later |
The tool rejects cycle-length values outside the 20–45-day window because anything below 20 or above 45 days usually reflects an irregular or anovulatory pattern that LMP dating cannot safely handle — confirming those pregnancies with a first-trimester ultrasound is the smarter path. Cycles as short as 21 days already shift the due date a full week earlier than the textbook estimate, so entering an accurate number matters more than people often realize.
Reading Gestational Age Correctly
Gestational age is the same thing as "weeks pregnant" on your chart, and it is measured as the number of completed weeks and days since the first day of your LMP. So when your provider says you are "six weeks and two days" at a first appointment, the embryo itself is closer to four weeks old — the two-week offset exists because pregnancy is conventionally dated from the period, not from the moment of conception. The calculator displays gestational age in exactly this format and updates it live against today's UTC date, so the figure you see in the morning is the same one you would see again that evening regardless of your time zone, daylight-saving changes, or recent travel. UTC-based arithmetic is what keeps the gestational-age counter stable across regions, a subtle bug that trips up many simpler calculators that rely on the device's local clock.
This is also why hCG thresholds, ultrasound windows, and NIPT timing are all quoted in gestational weeks rather than weeks-since-conception. Understanding the offset helps you read pregnancy apps and tracker charts correctly — a "week 8 milestone" target is not the same as eight weeks after sex.
Why a Due Date Isn't a Deadline
An estimated due date is the statistical midpoint of a normal term pregnancy, not a delivery appointment. Large cohort studies consistently report that only about 4% of babies are born on their exact estimated date, and a healthy delivery can happen any time between 37 completed weeks and 42 completed weeks. Treating the EDD as a hard deadline sets up unnecessary anxiety in the final weeks and feeds the common "overdue at 40 weeks" misunderstanding — in reality, 40 weeks is the midpoint, not the cliff edge.
For practical planning, it is far more useful to map out a window: full-term begins at 37 weeks, early term at 39 weeks, late term at 41 weeks, and post-term crosses 42 weeks. Knowing your window is more useful than fixing on a single date, especially when deciding when to install the car seat, when to schedule a last work trip, or when labor signs warrant a call to the clinic. The Pregnancy Due Date Calculator gives you the midpoint; the clinical window is what you and your provider actually plan around.
When an Ultrasound Changes the Date
A first-trimester ultrasound (typically done between 7 and 12 weeks) measures the crown–rump length of the embryo and is generally more accurate than LMP dating within a margin of about five to seven days. Many clinics will revise the official due date on the chart if the ultrasound estimate differs from the LMP estimate by more than roughly a week in the first trimester, because the embryo is growing fast enough at that stage that small differences in size map onto measurable day-count differences. Ultrasound becomes the primary dating tool whenever cycles are irregular, the LMP is uncertain, conception happened while breastfeeding, or hormonal contraception was recently stopped.
The calculator remains the right starting point for planning milestones and tracking gestational age, but for the date your provider actually puts on the chart, the ultrasound is what wins once it has been done — and the calculator is a useful cross-check against that ultrasound-based number rather than a permanent assignment.
Cases That Warrant a Provider's Eyes
There are several scenarios where LMP-based dating is unreliable enough that you should discuss the result with an obstetrician or midwife rather than relying on the calculator alone:
- Irregular cycles, or cycles that swing significantly in length month to month.
- Recent discontinuation of hormonal contraception, including the pill, patch, ring, or hormonal IUD.
- Conception while still breastfeeding, which can delay ovulation past the LMP-implied day.
- Uncertainty about when the last period actually started, or implantation bleeding mistaken for a period.
- Any bleeding, pain, or other symptom that prompts earlier-than-routine prenatal contact.
The tool is a planning aid — useful for trimesters, prenatal appointment windows, screening-test timing, and baby-planning logistics. It is not a diagnosis and never replaces prenatal care. Confirming the dates with a qualified provider keeps the estimate aligned with the rest of your medical record and catches the edge cases the formula assumes away.