The moment a pregnancy test turns positive, one question tends to arrive before all the others: when is this baby actually due? A due date calculator answers it in seconds — and below you'll find a real, working one. Enter the first day of your last period, tell us your cycle length, and we'll do the math the way your doctor would.
But a date on a calendar is only the beginning of the story. So after you've got your number, we'll walk through how the calculation actually works (it's called Naegele's rule, and it's simpler than it sounds), why your cycle length nudges the result, how to think about your conception date and gestational age, what each trimester holds, and why "due date" is one of the most misunderstood numbers in all of pregnancy. Let's start with the tool.
Pregnancy Due Date Calculator
Based on the first day of your last menstrual period (LMP), adjusted for your cycle length.
Your estimated due date
—
How far along
—
Trimester
—
Est. conception date
—
Got your date? Lovely. Now let's make sense of it — because understanding why that day landed where it did will help you read everything from your ultrasound reports to your maternity-leave paperwork with a lot more confidence.
Quick answer
To calculate your due date, add 280 days (40 weeks) to the first day of your last menstrual period — known as Naegele's rule — and adjust if your cycle isn't 28 days. The free calculator above does it instantly; only about 4% of babies arrive on the exact date.
What this guide covers
How is a due date actually calculated?
Here's the quotable version: your estimated due date is the first day of your last menstrual period plus 280 days — about 40 weeks. That's it. Everything else is refinement on top of that one core idea.
It can feel strange that pregnancy is measured from your last period, since you obviously weren't pregnant then. But this is the convention the entire medical world uses, for a very practical reason: almost everyone knows when their last period started, while very few people know the exact day they conceived. The first day of bleeding is a clear, observable event. Ovulation and conception are invisible. So clinicians anchor the whole 40-week count to the one date you can actually report — your last menstrual period, usually abbreviated LMP.
That's why the calculator above asks for your LMP rather than a conception date. You feed it the start of your last period, it adds 280 days, and it adjusts for how your personal cycle differs from the textbook 28-day average. The output is your expected date of delivery — the single most-referenced number of your whole pregnancy, even though, as we'll see, it's better understood as the middle of a likely window than a fixed appointment.
Key takeaway
A due date is your last period's first day plus 280 days (40 weeks). Pregnancy is counted from your last period, not from conception, because the start of your period is the one date almost everyone can pinpoint.
Naegele's rule: the 200-year-old formula behind the number
The method our calculator uses has a name: Naegele's rule, after Franz Karl Naegele, the German obstetrician who popularized it in the early 1800s. Despite its age, it remains the backbone of due-date estimation in clinics around the world, and it's elegantly simple. The classic version goes like this: take the first day of your last menstrual period, subtract three months, then add seven days and one year. Land on that date and you've got your estimated due date.
If that mental arithmetic makes your head spin, you're not alone — and it's exactly why the day-counting version is easier to trust. Adding 280 days to your LMP produces the identical answer with none of the calendar gymnastics. Forty weeks. Two hundred and eighty days. That's the rule in its purest form, and it's what the tool above runs the instant you hit calculate.
Naegele's rule is over two centuries old, yet it still underpins the due date on virtually every modern ultrasound report. Simple, observable, and good enough — that's why it has lasted.
There's one important assumption baked into Naegele's rule, and it's worth naming because it's where the calculation can drift from your reality: it assumes a 28-day cycle with ovulation falling neatly on day 14. For a great many people, that assumption holds well enough. But cycles are gloriously varied, and if yours runs consistently longer or shorter than 28 days, the standard formula can be off by several days. That's the gap our calculator closes — and the reason it asks for your cycle length.
Why your cycle length changes the math
Picture two people who both started their last period on exactly the same day. One has a tidy 28-day cycle; the other has a 35-day cycle. The textbook formula would hand them an identical due date — but their bodies are on different schedules. The person with the longer cycle almost certainly ovulated later, which means conception happened later, which means the pregnancy will, on average, reach full term a little later too.
This is the insight our tool builds in. Rather than blindly assuming day-14 ovulation for everyone, it adjusts your estimate by the difference between your average cycle length and 28. The formula it uses is straightforward:
- Estimated due date = LMP + 280 days + (cycle length − 28) days
- Estimated conception date = LMP + (cycle length − 14) days
So a 35-day cycle pushes your due date about a week later than the naive calculation, while a 24-day cycle pulls it about four days earlier. These shifts are not trivial. A week's difference can change which side of a month your due date falls on, when your maternity leave begins, and how your early ultrasounds get interpreted. It's a small adjustment with an outsized effect on how well your estimate fits you rather than a statistical average.
It helps to understand the logic underneath the adjustment. Ovulation tends to happen a fairly fixed number of days — about fourteen — before your next period, regardless of how long your cycle runs. What varies between a short cycle and a long one is mostly the first half, the follicular phase, where the egg matures. A 35-day cycle usually means a longer follicular phase and therefore later ovulation, around day 21 rather than day 14. Since conception can only follow ovulation, a later ovulation means a later conception and, on average, a later birth. By subtracting 28 from your cycle length and shifting the due date by that difference, the calculator approximates where your personal ovulation fell — a far better assumption than pretending every body ovulates on the same day.
If you've spent any time learning the rhythm of your body — and a good cycle tracker makes that effortless — you'll already have a reliable sense of your average cycle length. If you're not sure, 28 is a reasonable default, and you can always refine the estimate once a first-trimester ultrasound gives you a more precise reading. Understanding your cycle in the first place is exactly the kind of self-knowledge our guide to the menstrual cycle phases is designed to give you, and it pays off the moment you start trying to conceive.
Conception date vs. gestational age: clearing up the confusion
This is the part that trips up almost everyone, so let's slow down. There are two different clocks running during pregnancy, and mixing them up is the source of endless confusion.
The first clock is gestational age, counted from the first day of your last period. This is the number your doctor, your app, and your ultrasound all use. When someone says "I'm 12 weeks pregnant," they almost always mean 12 weeks of gestational age — 12 weeks since their LMP.
The second clock is fetal age (sometimes called conceptional age), counted from the actual moment of conception. Because conception typically happens around two weeks after your last period began, fetal age always runs about two weeks behind gestational age. So at "8 weeks pregnant" by the standard count, the embryo has only been developing for roughly 6 weeks.
This two-week offset explains a famous source of bewilderment: the day you get a positive test, you're often already counted as about four weeks pregnant, even though conception may have been only two weeks earlier. Nothing is wrong — it's just the gestational clock, which started ticking at your last period. Our calculator estimates your conception date as your LMP plus your cycle length minus 14, which lines up with when ovulation most likely occurred, while still dating the pregnancy itself from your LMP so it matches what your provider will use.
The two-clock rule
Gestational age counts from your last period; fetal age counts from conception, about two weeks later. Your due date, your app, and your ultrasound all speak in gestational age — so that's the number to anchor to.
From your last period to your due date — all in one private app
Vyve tracks your cycle, helps you spot ovulation, and flips into pregnancy mode the moment you're expecting — with your data encrypted on your phone, never sold.
Try Vyve today"How many weeks pregnant am I?"
This is one of the most-searched pregnancy questions, and the calculator above answers it precisely for the day you're reading this. But here's how to reason about it yourself, because you'll be asked the question a lot — by apps, by forms, and by every relative who finds out.
Your weeks pregnant — your gestational age — is simply the time elapsed since the first day of your last period, expressed in weeks and days. If your last period started exactly 70 days ago, you're 10 weeks and 0 days pregnant (70 divided by 7). If it started 75 days ago, you're 10 weeks and 5 days. Clinicians write this as "10+5," and you'll see that shorthand all over your maternity notes. Each week of pregnancy runs from the completed week to the next, so being "10 weeks" really means you're in your eleventh week of pregnancy — another small but persistent source of confusion.
Knowing your exact gestational age matters for more than curiosity. It determines the timing of important screenings and tests, it tells you which developmental milestones your baby is reaching, and it's the reference point for whether things are progressing on schedule. A pregnancy is considered full term from 39 weeks 0 days through 40 weeks 6 days, with 37 to 39 weeks classed as "early term" and anything past 42 weeks as "post-term." That's the window your 40-week due date sits inside — the peak of a curve, not a hard line.
If you're at the very start of this journey and still reading the early signals — the tender breasts, the sudden exhaustion, the metallic taste, the late period — our companion guide to the early signs of pregnancy walks through what those first weeks can feel like and when a test is most reliable. Pair it with the calculator above and you'll have both the "am I?" and the "how far along?" covered.
The three trimesters: a map of your 40 weeks
Pregnancy is conventionally split into three trimesters, each roughly thirteen weeks long. They aren't arbitrary — each maps onto a genuinely different phase of how you'll feel and what your baby is doing. Knowing which trimester your due date math places you in helps you anticipate what's ahead instead of being surprised by it.
The first trimester spans weeks 1 through 13. It's the most transformative and, for many, the most physically demanding stretch. From a cluster of cells, all of your baby's major organs begin to form. Meanwhile you may be wrestling with the greatest hits of early pregnancy: nausea, crushing fatigue, sore breasts, and a nose that can suddenly detect a sandwich from two rooms away. It's also, statistically, the period of highest risk, which is why many people wait until it's behind them to share their news.
The second trimester runs from weeks 14 through 27 and is often called the "honeymoon" of pregnancy, and not without reason. For a lot of people the nausea lifts, energy returns, and the pregnancy starts to feel real in the best way — this is when you'll typically feel those first fluttering movements and, if you choose, learn the baby's sex at the anatomy scan. The baby grows dramatically, and the bump that announces your pregnancy to the world usually arrives here.
The third trimester covers weeks 28 through 40 (and occasionally beyond). The finish line. Your baby puts on weight and matures the systems needed for life outside the womb, while you contend with the practical realities of carrying a near-newborn: backache, breathlessness, swollen ankles, and sleep that gets more elusive by the week. It's a stretch of nesting, hospital bags, and a deepening anticipation as your due date approaches.
| Trimester | Weeks | What's happening with baby | Common for you |
|---|---|---|---|
| First | 1–13 | Major organs form; heartbeat begins | Nausea, fatigue, tender breasts |
| Second | 14–27 | Rapid growth; movement felt; anatomy scan | More energy, visible bump |
| Third | 28–40 | Weight gain; lungs mature; positions for birth | Backache, nesting, broken sleep |
Your due date marks the close of week 40 — the conventional endpoint of the third trimester. But as the next section makes clear, the body rarely reads the calendar quite so literally.
How accurate is a due date, really?
Time for the single most important fact in this entire guide, because it will save you a great deal of unnecessary stress in your final weeks: only about 4 percent of babies are born on their exact due date. Read that again. Ninety-six out of a hundred babies arrive on a different day.
This isn't a flaw in the calculation — it's the nature of biology. Your due date is the statistical midpoint of when babies tend to arrive, not a scheduled event. The overwhelming majority of births happen in the two weeks on either side of the estimated date. A pregnancy that reaches anywhere from 37 to 42 weeks is considered entirely normal, which is a window of more than a month. So the right mental model isn't "the baby is due on March 14th." It's "the baby is most likely to arrive sometime in the couple of weeks around March 14th."
| Timing | Gestational age | What it means |
|---|---|---|
| Preterm | Before 37 weeks | Early arrival; may need extra monitoring |
| Early term | 37–38 weeks | Common and usually healthy |
| Full term | 39–40 weeks | The ideal window; due date sits here |
| Late term | 41 weeks | Past the date but still normal |
| Post-term | 42+ weeks | Provider may discuss induction |
Holding this loosely is genuinely good for you. People who treat their due date as a hard deadline often spend the final fortnight in a state of mounting frustration when nothing happens "on time" — when in fact a first baby arriving a few days to a week past the estimate is completely ordinary. Think of the due date as a helpful signpost for planning, not a promise your body made.
The 4 percent rule
Only around 4 percent of babies are born on their due date. The vast majority arrive within two weeks either side, and 37 to 42 weeks is all considered normal. Treat your date as the middle of a window, not a deadline.
When and why your due date might get revised
Your LMP-based estimate is the starting point, but it isn't always the final word. The most accurate way to date a pregnancy is an early ultrasound, ideally in the first trimester. Between roughly 8 and 13 weeks, the baby's size — specifically a measurement called the crown-rump length — is so consistent from pregnancy to pregnancy that it can pin gestational age to within a few days. Early in pregnancy, every baby grows at almost exactly the same rate, which makes that measurement a powerful clock.
If the ultrasound's estimate differs significantly from your LMP date — typically by more than about five to seven days in the first trimester — your provider will usually adjust your official due date to match the scan. This commonly happens for people with irregular cycles, those who aren't certain of their last period date, or anyone who ovulated unusually early or late. It's not a sign that anything is wrong; it's simply the more precise measurement taking precedence over the estimate.
This is also why the calculator above is best understood as a wonderful first answer rather than a clinical verdict. It gives you an excellent, personalized estimate from information you can supply today, no appointment required — perfect for that exciting early stretch when you just want to know roughly when. Once you've had your dating scan, you'll have an official date, and the two will usually be close. If they differ, trust the scan and let your app follow suit.
From due date to delivery: tracking pregnancy with Vyve
Calculating your due date is a single moment. Living the 40 weeks that follow is a journey — and that's where having the right companion app changes the experience. Vyve was built first as a privacy-first cycle and ovulation tracker, but it doesn't abandon you the moment you conceive. It has a dedicated pregnancy mode designed to carry you smoothly from cycle tracking, through trying to conceive, into pregnancy itself.
The transition is seamless by design. The same app that learned your cycle rhythm and helped you understand your fertile window flips into a week-by-week pregnancy companion, tracking your gestational age, mapping your trimesters, and letting you log the symptoms — the nausea, the cravings, the kicks — that make up the real texture of being pregnant. Instead of starting over in a brand-new app and re-entering your history, your story continues in one place.
And here is the part we care about most, because pregnancy data is some of the most sensitive information you will ever generate. The fact that you're pregnant, how far along you are, any complications, any losses — that is precisely the data the advertising economy most wants to get its hands on, and precisely the data you should be most careful about handing over. Vyve's answer is structural: your information is encrypted and stored on your own device, not pooled on a server building a profile of your pregnancy. We don't sell your pregnancy data to advertisers or data brokers, because we never hold it in a form we could sell. For a life event this intimate, that distinction isn't a marketing line — it's the whole point.
The moment an app knows you're pregnant, you become extraordinarily valuable to advertisers. Vyve's stance is simple: that knowledge should stay on your phone, and stay yours.
We want to be honest about what an app can and can't do. Vyve doesn't replace your midwife, your OB-GYN, or your dating scan — nothing on a phone should. What it offers is a calm, private, intelligent place to understand your cycle, estimate your due date, and then track the weeks as they unfold, with your data treated as the deeply personal thing it is. From your last period to the day you finally meet your baby, the goal is the same one that runs through everything we build: powerful insight, complete privacy, and a tool you'd be happy to hand to someone you love.
Your pregnancy, week by week — and entirely your own.
Join the Vyve early-access list and track your cycle, conception, and pregnancy in one private app that keeps your most sensitive data off the cloud.
Try Vyve todayFrequently asked questions
How do I calculate my due date?
The most common method is Naegele's rule: take the first day of your last menstrual period (LMP), add 280 days (40 weeks), and that's your estimated due date. If your cycle is longer or shorter than 28 days, adjust by adding or subtracting the difference. The calculator near the top of this page does all of this automatically, including a conception-date estimate and how many weeks pregnant you are today.
How accurate is a due date calculator?
A due date calculator gives a solid estimate, but it is only an estimate. Only about 4 percent of babies are born on their exact due date, and most arrive within the two weeks before or after. A first-trimester ultrasound is the most accurate way to date a pregnancy, and your provider may adjust your due date based on it.
Can I calculate my due date from my conception date?
Yes. If you know your conception or ovulation date, add 266 days (38 weeks) to estimate your due date, since pregnancy is counted from the last period but conception happens about two weeks later. Our calculator estimates conception as your LMP plus your cycle length minus 14 days, then dates the pregnancy from your LMP so it matches what your provider will use.
How many weeks pregnant am I?
Your gestational age is counted in weeks and days from the first day of your last menstrual period, not from conception. So at the moment of a positive test you are often already around 4 weeks pregnant. Enter your last period date in the calculator above and it will tell you exactly how many weeks and days along you are today.
Does cycle length change my due date?
Yes. Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is regularly longer or shorter, you likely ovulate later or earlier, which shifts your due date. Our calculator adjusts the estimate by the difference between your average cycle length and 28 days for a more personal result.