The exact math behind your estimated delivery date, why it lands 280 days out, and which dating method your doctor actually trusts most.
Published August 23, 2026 · Reviewed by the Ihsabha Editorial Team
The moment a pregnancy test turns positive, the very next question is almost always "so when's the baby due?" That single date gets written on hospital forms, shared with family, and used to plan everything from maternity leave to the nursery — yet very few people know how it's actually worked out, or that doctors sometimes use two or three different methods that don't always agree with each other. This guide breaks down exactly how a due date is calculated, why the standard count is 280 days rather than a round nine months, and when your provider will trust an ultrasound measurement over the calendar math.
A due date — formally called the Estimated Date of Delivery, or EDD — is not a prediction of the exact day labor will start. It's the midpoint of a normal-length pregnancy: the date at which a full-term pregnancy reaches 40 weeks of gestational age. Because gestational age is measured from the first day of your last menstrual period (LMP) rather than from the moment of conception, the "40 weeks" clock actually starts running about two weeks before you were biologically pregnant at all. That quirk trips a lot of people up, and it's the key to understanding every calculation method below. It's also worth reading Due Date by Conception Date: How to Calculate It From When You Conceived if this situation applies to you. Rather than working this out by hand every time, the Due Date Calculator can handle the whole calculation for you.
The formula almost every calculator and clinician starts with is called Naegele's Rule, named after the 19th-century German obstetrician who formalized it:
In practice, nobody counts 280 days on their fingers. The shortcut version — mathematically identical — is to take the first day of your LMP, subtract three calendar months, then add seven days and one year. If your last period started on March 10, 2026, subtracting three months gives December 10, 2025; adding seven days gives December 17, 2025; adding one year gives your estimated due date of December 17, 2026.
A full-term pregnancy, measured from actual conception, lasts about 266 days — roughly 38 weeks. So why does the standard due-date formula use 280 days instead? Because doctors can't reliably know the exact moment conception happened, but most people can identify the first day of their last period with reasonable confidence. Since ovulation and fertilization typically occur around day 14 of a textbook 28-day cycle, adding those 14 "extra" days onto the 266-day conception-based count gives the familiar 280-day, 40-week standard. It's a deliberate trade-off: a start point everyone can actually pinpoint, at the cost of building in a two-week assumption about when ovulation happened.
Once you're pregnant, a first-trimester ultrasound offers a second, independent way to estimate your due date. Between roughly 8 and 13 weeks, a sonographer measures the crown-rump length (CRL) — the length of the fetus from head to bottom — and compares it against standardized growth charts built from thousands of dated pregnancies. Because early fetal growth is remarkably consistent across pregnancies regardless of the mother's cycle length, this measurement is considered the single most accurate dating tool in modern obstetrics, generally accurate to within three to five days.
For most of pregnancy, both methods should land within about a week of each other, and your provider will typically keep whichever due date was set first. But when they disagree by more than roughly 5 to 7 days in the first trimester (or more than 10 days in the early second trimester), medical guidelines recommend switching to the ultrasound-based due date. The reasoning is simple: LMP dating assumes a textbook 28-day cycle with ovulation on day 14, and a huge number of people simply don't have that exact cycle — irregular periods, longer or shorter cycles, and even mis-remembered dates all introduce error that an ultrasound measurement sidesteps entirely.
If you were tracking ovulation with an ovulation predictor kit, basal body temperature charting, or fertility app, you may know your actual conception window rather than just your LMP. In that case, the calculation shifts to the shorter, biologically accurate count:
This method removes the guesswork about when ovulation happened during your cycle, which is exactly why it tends to produce a more personalized estimate for people with cycles that are longer, shorter, or less predictable than 28 days. Ihsabha's Conception Date Calculator can work this out for you directly from either your LMP or your due date. For an instant, practical check, the Due Date Calculator is ready to go.
For pregnancies conceived through IVF, the due date calculation is actually the most precise of all, because the exact age of the embryo at transfer is known down to the day. A Day 3 embryo transfer counts as 3 days of gestational age on the transfer date, and a Day 5 (blastocyst) transfer counts as 5 days. From there, the formula is simply 280 days minus the embryo's age at transfer, counted forward from the transfer date. Because there's no ambiguity about ovulation timing at all, IVF-based due dates are generally treated as more reliable than either LMP or standard ultrasound dating.
Naegele's Rule assumes a 28-day cycle with ovulation on day 14 — an assumption that simply doesn't hold for a large share of people. A common adjustment is to add or subtract one day from the standard due date for every day your average cycle differs from 28. For a consistent 32-day cycle, for example, that means adding roughly four days to the standard LMP-based estimate, since ovulation likely happened later than day 14. This is one of the clearest cases where an ultrasound measurement in the first trimester is worth prioritizing over the calendar formula, since it sidesteps the cycle-length assumption entirely.
One detail that confuses almost everyone at some point is the gap between gestational age and fetal age. Gestational age — the number used on every due-date calculator, ultrasound report, and prenatal chart — is counted from the first day of your last menstrual period. Fetal age (sometimes called embryonic or conceptional age) is counted from the actual moment of fertilization, roughly two weeks later. So when a chart says you're "10 weeks pregnant," the embryo itself is closer to 8 weeks old developmentally. Clinicians universally use gestational age for consistency, since it's tied to a date almost every patient can report, but it's worth knowing the distinction exists so a fetal development app that uses fetal age doesn't leave you thinking your dates don't match. For a deeper look, our guide on How Accurate Is a Pregnancy Due Date? Why Only 5% of Babies Arrive On Theirs covers this in more detail.
Because Naegele's Rule is just "subtract three months, add seven days, add one year," you can sketch a rough due date in your head for any LMP month without a calculator:
| Last Period Started In | Rough Due Date Falls In |
|---|---|
| January | Early-to-mid October |
| February | Early-to-mid November |
| March | Early-to-mid December |
| April | Early-to-mid January |
| May | Early-to-mid February |
| June | Early-to-mid March |
| July | Early-to-mid April |
| August | Early-to-mid May |
| September | Early-to-mid June |
| October | Early-to-mid July |
| November | Early-to-mid August |
| December | Early-to-mid September |
This table is a rough mental shortcut only — the exact day still depends on which day of the month your period started, so treat it as a ballpark for planning conversations, not a substitute for the precise calculation above.
Every formula in this guide is built around a singleton pregnancy, but twin and higher-order pregnancies statistically run shorter than single-baby pregnancies — on average, a twin pregnancy delivers a few weeks earlier than the standard 40-week mark, and triplets earlier still. Naegele's Rule and conception-based dating still give you a starting reference point for a multiples pregnancy, but your care provider will typically treat that 40-week figure as an upper bound rather than a realistic target, and will schedule monitoring accordingly from the second trimester onward. Use the Due Date Calculator below to check your own numbers quickly and accurately.
Once you have a due date, it's useful to see how the standard 40-week count is divided into the three trimesters used throughout prenatal care:
| Trimester | Gestational Weeks | Approximate Duration |
|---|---|---|
| First Trimester | Weeks 1 – 13 | ~13 weeks |
| Second Trimester | Weeks 14 – 27 | ~14 weeks |
| Third Trimester | Weeks 28 – 40 | ~13 weeks |
Note that weeks 1 and 2 of "pregnancy" fall before conception actually occurred — another reminder that gestational age is a dating convention, not a literal count of how long the baby has existed.
Depending on which country or which chart you're reading, the same calculated date can appear under several different labels: Estimated Date of Delivery (EDD), Estimated Date of Confinement (EDC, an older term still used on some forms), or simply "due date." All three refer to the identical 40-week calculation — there's no difference in the math behind them, only in terminology that's shifted over time and across regions. If a form or app asks for your EDC, you can treat it exactly the same as EDD or due date.
If you've ever entered the same last period date into two different pregnancy apps and gotten due dates a day or two apart, you haven't made a mistake — the apps are simply making slightly different rounding or leap-year handling choices under the hood. Naegele's Rule technically assumes exactly 280 days, but the "subtract three months, add seven days, add a year" shortcut can land a day off depending on how a given month's length is handled in the underlying code, particularly around February in a leap year. These differences are trivial in practice — a day either way falls well within the broader multi-week uncertainty already built into any due-date estimate — but it's worth knowing so a one-day mismatch between two tools doesn't cause unnecessary worry.
Skip the manual date math. Ihsabha's Due Date Calculator takes the first day of your last period (or your conception date) and instantly returns your estimated delivery date, your current gestational week, and which trimester you're in — with no counting required.
Once you have an estimated delivery date, it becomes the reference point for nearly every milestone that follows — when each prenatal screening is scheduled, when your third trimester begins, and when your leave paperwork needs to be filed. If you know your conception date rather than your LMP, Ihsabha's Conception Date Calculator can convert between the two, and the Pregnancy Calculator can tell you exactly how many weeks and days pregnant you are today based on either starting point.
The standard formula, known as Naegele's Rule, adds 280 days (40 weeks) to the first day of your last menstrual period. In practice, most clinicians simply subtract three months from that date and add seven days and one year, which lands on the same result.
In the first trimester, yes. A crown-rump length measurement taken between 8 and 13 weeks is considered the most accurate dating method available and is used to adjust the due date if it differs from the LMP-based estimate by more than about five to seven days.
Because ovulation and conception typically occur around day 14 of a 28-day cycle, not on day one. Counting from the last menstrual period adds roughly two extra weeks compared to counting from actual conception, which is why the two methods differ by about 14 days.
Yes. If your first-trimester ultrasound measurement falls outside the expected range for your LMP-based due date, your provider will typically update your due date to match the ultrasound estimate, since it's considered more reliable at that stage.
Every method in this guide produces an estimate, not a guarantee — even the most precise dating tool available, first-trimester ultrasound, still carries a margin of a few days. A due date is best treated as the center of a normal delivery window rather than a fixed appointment, which is why full-term birth is defined as anywhere from three weeks before to two weeks after it. If your calculated date and your provider's clinical estimate disagree, always defer to the figure set during your prenatal care, since it accounts for measurements and context a calendar formula simply can't capture.
Plan ahead with Ihsabha's Ovulation Calculator and IVF Due Date Calculator. For related reading on Ihsabha's blog, see IVF Due Date Calculator: How Your Due Date Is Calculated From Embryo Transfer Day, Twin Pregnancy Due Date: How It's Calculated and Why Twins Usually Arrive Early and How to Calculate Your Due Date Without Knowing Your Last Period.