Pregnancy Due Date Calculator

An estimated due date (EDD) is exactly that — an estimate, built from a well-established formula that's been in clinical use for roughly 200 years. This calculator estimates a due date from three possible starting points — last menstrual period, a known or estimated conception date, or an IVF transfer date — and lays out the resulting trimester timeline.

How Naegele's rule works

This formula, attributed to German obstetrician Franz Naegele in the early 1800s, rests on two assumptions: a 28-day menstrual cycle, and ovulation occurring on day 14 of that cycle. Since human gestation from conception averages about 266 days, and conception typically follows LMP by about 14 days under those standard assumptions, the total works out to 280 days from LMP — the “40 weeks” figure used throughout pregnancy dating, even though true gestation from conception is closer to 38 weeks.

What’s notable about Naegele’s rule is how long it’s remained the clinical standard despite being over two centuries old and built on averages rather than individual measurement. Its durability comes down to a combination of simplicity (it requires only a single, generally memorable date) and reasonable accuracy at the population level — while any individual due date carries real uncertainty, the rule performs well enough across large populations that more complex alternatives haven’t displaced it as the default starting point, even as ultrasound dating has become the more precise method once pregnancy is confirmed.

How this calculator works

Each of the three methods ultimately resolves to an equivalent “LMP” reference point, then applies the same 280-day rule from there:

The conception and IVF methods are more direct, since they start from a known or closely-estimated fertilization date rather than needing to infer one — which is part of why ultrasound dating (which can estimate fetal age directly from measurements) and IVF dating are generally considered more precise than LMP-based dating for someone who has that information available.

Worked example

Using this calculator’s own default example — a last period roughly 8 weeks before today, standard 28-day cycle:

The results panel breaks this down explicitly — LMP, estimated conception, EDD, current gestational age, and days remaining — so each number in the calculation is visible rather than just the final due date on its own. The timeline view below the headline result plots the same underlying dates against key milestones (end of first trimester, viability, full term), giving a fuller picture of where a given date falls across the whole pregnancy rather than showing the due date in isolation.

Why cycle length matters

Naegele’s rule assumes a 28-day cycle with ovulation on day 14 — but many people have cycles that run longer or shorter than that, and cycle length differences show up almost entirely in the first half of the cycle (the follicular phase, from LMP to ovulation), while the second half (the luteal phase, from ovulation to the next period) stays close to 14 days for most people regardless of total cycle length.

This means a longer cycle pushes ovulation — and therefore conception and the due date — later than the standard calculation assumes, while a shorter cycle pulls it earlier. For a 35-day cycle (7 days longer than standard), the adjusted due date runs about a week later than the flat 280-day calculation; for a 24-day cycle (4 days shorter), it runs about 4 days earlier. This adjustment only applies to the “Last Period” method, since the conception and IVF methods already start from a specific date rather than needing to infer ovulation timing from cycle length.

It’s worth being clear about what this adjustment can and can’t correct for. It accounts for a consistently longer or shorter average cycle — someone who reliably cycles every 32 days rather than every 28. It can’t account for cycle-to-cycle variability within an individual (a cycle that runs anywhere from 26 to 34 days from month to month, for instance), since the calculator only has a single average-length input to work with. For anyone with meaningfully irregular cycles, this average-based adjustment is still a reasonable starting estimate, but it carries more inherent uncertainty than it would for someone with consistently regular cycles — which is exactly the kind of situation where an early dating ultrasound adds the most value over calendar-based estimation alone.

The three trimesters

The terminology around “term” has become more specific in recent years: full-term is 39–40 weeks, early-term is 37–38 weeks, late-term is 41 weeks, and post-term is 42 weeks and beyond — a more precise breakdown than the older, single “term” label (37–42 weeks) once implied, reflecting research showing meaningfully different average outcomes across that range rather than treating the whole 37–42 week window as equivalent.

This shift in terminology, adopted by ACOG and the Society for Maternal-Fetal Medicine in 2013, reflected accumulating research showing that outcomes for babies born early in the 37–38 week range differ somewhat, on average, from those born at 39–40 weeks — enough of a difference that clinical guidance now generally favors scheduling non-medically-indicated deliveries (elective inductions or cesareans without a medical reason) no earlier than 39 weeks when possible, rather than treating any point in the 37–42 week range as equally optimal. This is general population-level guidance rather than a rule that applies identically to every pregnancy — individual circumstances can and do change what timing is appropriate for a specific patient, which is a decision made with a healthcare provider rather than from a due date calculation alone.

How accurate is a due date, really?

Only about 5% of babies are actually born on their calculated due date — the large majority arrive somewhere within the broader window of 37 to 42 weeks, with the highest concentration in the days surrounding, but not exactly on, the calculated date. This isn’t a flaw in the calculation; a due date was never intended as a precise prediction, but as the statistical center of a genuinely wide normal range, useful for scheduling prenatal care and monitoring rather than for predicting an exact birth day.

First-time pregnancies statistically tend to run slightly longer on average than subsequent pregnancies for the same person, and there’s a modest tendency for delivery timing to run somewhat similar across a person’s own pregnancies and even show some family-level patterns — though these are population-level tendencies with wide individual variation, not something to plan around as a reliable predictor for any specific pregnancy. The due date remains the most useful single planning anchor precisely because it’s calculated the same standardized way every time, making it comparable across patients and providers even though no individual date carries strong predictive precision on its own.

This is worth keeping in perspective for anyone who finds themselves anxiously watching a due date approach: the wide normal range around it (statistically, most deliveries cluster within about two weeks either side) is the expected pattern, not a sign anything is off track. Providers generally become more actively concerned about timing only well outside that range — significantly preterm, or meaningfully post-term — which is exactly why prenatal monitoring intensifies as a pregnancy approaches and passes the due date, rather than treating the date itself as a hard deadline.

An early ultrasound (ideally before 13 weeks) is generally considered the most accurate way to confirm gestational age, since fetal measurements at that stage correlate very closely with actual gestational age regardless of cycle irregularity or uncertain LMP recall — which is why a due date established early in pregnancy by ultrasound often takes precedence over an LMP-based calculation if the two disagree by more than about a week.

Conception and IVF dating methods

For anyone tracking ovulation directly (through basal body temperature, ovulation predictor kits, or a fertility app) or who otherwise knows their approximate conception date with more confidence than a standard cycle assumption would provide, the conception-date method skips the cycle-length inference step entirely and calculates directly from that known date — generally a more precise starting point when it’s available.

IVF dating is the most precise of the three methods this calculator offers, since the embryo’s exact age at transfer is a known, controlled clinical fact rather than an estimate. A day-5 (blastocyst) transfer is dated differently from a day-3 transfer specifically because the embryo is already 5 days past fertilization at the moment of transfer, versus 3 days — the calculation accounts for that known age difference directly rather than estimating it.

This precision advantage is part of why fertility clinics typically provide their own calculated due date at the time of a successful transfer, rather than leaving patients to work it out independently — the clinic has the exact transfer date and embryo age on record, removing the two biggest sources of uncertainty (recalled LMP date and assumed ovulation timing) that affect the other two methods. Patients using this calculator’s IVF option are essentially reproducing that same clinic-provided calculation.

What this calculator can't tell you

This tool works entirely from calendar dates and standard population averages — it has no way to account for individual variation in actual cycle patterns beyond the average length entered, doesn’t know about any irregular ovulation timing in a specific cycle, and can’t incorporate ultrasound measurements, which become the more authoritative dating method once they’re available. Anyone with irregular cycles, uncertain LMP dates, or any pregnancy-related medical concern is better served by a healthcare provider’s direct assessment — including an early dating ultrasound — than by any calendar-based calculation alone, this one included.

It’s also worth being clear that this calculator provides general informational estimates rather than personalized medical guidance, and doesn’t account for individual health history, prior pregnancy complications, or other factors a healthcare provider would consider when establishing and monitoring a pregnancy timeline.

For context, a “confirmed” pregnancy timeline in clinical practice typically gets locked in early — once an ultrasound-based gestational age is established, usually in the first trimester, that date generally becomes the reference point used for the remainder of prenatal care, rather than being continually recalculated from LMP as the pregnancy progresses. This calculator, and calendar-based due date estimation generally, is most useful in the earliest stages before that clinical confirmation happens — as a starting orientation rather than a running clock meant to be checked repeatedly against a provider’s established dating.