Pregnancy Due Date Explained

How Your Due Date Is Calculated

Nearly every due date starts with the same basic idea: count forward 40 weeks from the first day of your last period. That simple rule has a name, a bit of history, and a couple of quirks worth understanding.

What Is Naegele's Rule?

Naegele's Rule is the standard formula used to estimate a due date. It's named after Franz Karl Naegele, a German obstetrician who published it in the early 1800s after observing that pregnancies tend to last about 10 lunar months from the last period. The calculation is simple:

1. Start with the first day of your last menstrual period.
2. Add 7 days.
3. Subtract 3 months.
4. Add 1 year if the result lands in the past.

For example, if the first day of your last period was January 1st: add 7 days to get January 8th, subtract 3 months to get October 8th, and add a year if needed. The result, October 8th, is exactly 280 days — 40 weeks — after January 1st. That's your estimated due date.

The rule assumes a fairly typical 28-day cycle with ovulation around day 14. If your cycles run longer or shorter than that, the math can be adjusted, which we'll cover in the calculation section below.

Why Do We Add 7 Days?

This is one of the most common questions about due date math, and the answer comes down to calendar arithmetic. Pregnancy is measured as 280 days (40 weeks) from your last period. Calendar months don't all have the same number of days, so simply subtracting 3 months from a date doesn't land you exactly 280 days later. Adding 7 days closes that gap, bringing the total to the full 280 days that defines a standard, full-length pregnancy counted from the last period.

Why Pregnancy Is "Dated" Before You're Actually Pregnant

Here's the part that trips up a lot of people: your last period happens before you ovulate, and you can only conceive after ovulation. Since ovulation typically occurs around two weeks after your period starts, the standard "40 weeks" of pregnancy actually includes about two weeks during which you weren't pregnant yet.

In other words, a pregnancy dated at 40 weeks from your last period is closer to 38 weeks from the actual date of conception. Doctors use LMP dating anyway because most people have a reasonable idea of when their last period started, even if they don't know the exact day they conceived.

Due Date by Ultrasound vs. Last Menstrual Period

LMP-based dating is a useful starting point, but it isn't the only method, and it isn't always the most precise one. Ultrasound dating measures the baby directly, which sidesteps a lot of the guesswork built into Naegele's Rule.

First-Trimester Ultrasound Dating

A first-trimester ultrasound, ideally performed before about 14 weeks, measures the crown-rump length (CRL) — the distance from the top of the head to the bottom of the fetus. At this early stage, growth is remarkably consistent from one pregnancy to another, so CRL measurements can estimate gestational age to within about 5 to 7 days. (ACOG) That's why the American College of Obstetricians and Gynecologists (ACOG) recommends first-trimester ultrasound as the most accurate way to date a pregnancy, even when the last period is known.

This matters because studies suggest only about half of women recall their last period date with real precision. An early ultrasound removes that guesswork almost entirely.

When Your Doctor Might Change Your Due Date

If your LMP-based due date and your ultrasound-based due date disagree by more than a certain number of days, ACOG guidelines call for using the ultrasound date instead. How much disagreement it takes depends on how far along you are when the scan happens — the earlier the scan, the smaller the discrepancy needed to trigger a change, because early measurements are so much more precise.

*Third-trimester redating is used cautiously, since an apparent discrepancy this late may reflect the baby's own growth pattern rather than an error in earlier dating.

Once a due date has been set from an accurate first-trimester ultrasound, it's generally not revised again based on later scans. As pregnancies progress, babies grow at increasingly individual rates, so a third-trimester measurement is a poor tool for "correcting" a date that was already well established early on. If your due date shifted after a scan, our guide to what happens when a due date changes walks through why that happens and what it does — and doesn't — mean.

How Accurate Is a Due Date, Really?

This is worth saying plainly: a due date is the statistical middle of a range, not a deadline. Research consistently finds that only around 4 to 5% of babies are born on their exact estimated due date. About 90% arrive within roughly two weeks before or after it, and around 60% are born somewhere between 39 and 41 weeks of pregnancy.

That spread is normal, not a sign that anything went wrong with the calculation. Spontaneous labor is triggered by a complex set of signals between a baby and the parent's body, and the exact timing of that trigger varies naturally from one pregnancy to the next. It's more useful to think of your due date as the center of a several-week window than as a single day to count down to.

That reframe also matters for planning: a due date gives your care team a benchmark for timing screening tests, tracking growth, and deciding when extra monitoring makes sense — not a fixed appointment on the calendar.

Is a Due Date Based on 37, 39, or 40 Weeks? Understanding "Term"

Your actual due date is always calculated at 40 weeks. The confusion usually comes from a separate idea: "term," the range of weeks during which a birth is considered medically normal. In 2013, ACOG and the Society for Maternal-Fetal Medicine replaced the single broad label "term" with four more specific categories, because research showed that outcomes differ depending on exactly when in that range a baby is born. (ACOG)

So a due date is technically "40 weeks," while "term" — the safe range to be born within — starts at 37 weeks and extends through 42 weeks. Both numbers are correct; they're just answering different questions.

Your Due Date Week by Week

A due date isn't just a single number at the end — it's the finish line for a series of stages, each with its own developmental milestones. Here's a brief overview of how the countdown breaks down.

First Trimester (Weeks 1–13)

This stage covers the two weeks before conception through the point where major organs have formed. It's also the window for the most accurate dating ultrasound, so if you haven't had a first-trimester scan yet, this is the time to schedule one.

Second Trimester (Weeks 14–27)

Often the most comfortable stretch of pregnancy, this trimester includes the anatomy scan (usually around 18–22 weeks), when the baby's organs and growth are checked in detail and many parents learn the baby's sex, if they choose to.

Third Trimester (Weeks 28–Birth)

Growth accelerates, appointments become more frequent, and your body starts preparing for labor — sometimes with the false-alarm contractions we cover later in this guide. This is also when your due date moves from an abstract number to something you're actively watching.

For a deeper, week-by-week breakdown of what's happening for you and your baby at each stage, see our Pregnancy Week-by-Week Guide.

Are First Babies Usually Late or Early?

There's a widely held belief that first pregnancies run long. The evidence leans in that direction but isn't completely settled. Several studies and clinical observations suggest first-time mothers are somewhat more likely to deliver at or slightly after their due date, with spontaneous labor sometimes starting a few days later on average compared to subsequent pregnancies. Some data also suggests labor tends to progress faster once it starts in later pregnancies, simply because the body has done it before.

That said, other clinicians point out that birth order alone doesn't reliably predict timing, and that plenty of first babies arrive early or right on schedule. Factors like cycle length, family history, body mass index, and the baby's sex have all been studied as possible influences on how long a pregnancy runs — and no single factor tells the whole story. Treat "first babies come late" as a loose trend worth being aware of, not a rule your own pregnancy has to follow.

How to Calculate Your Own Due Date

You don't need to memorize Naegele's Rule to work out your own estimate. Here's the practical version:

1. Find the first day of your last period. This is day one of your dating window, not the last day of bleeding. If you've been tracking your period already, you likely have this recorded.
2. Count forward 280 days (40 weeks). That lands you on your estimated due date if your cycle is close to the typical 28 days.
3. Adjust for cycle length if yours isn't 28 days. Add or subtract the difference — a 35-day cycle, for example, shifts the estimate about 7 days later, since ovulation likely happened later in that longer cycle.
4. Use your conception date instead, if you know it precisely, and count forward 266 days (38 weeks) rather than 280.
5. Confirm with an ultrasound at your first prenatal visit, especially if your cycles are irregular or you're not fully sure of your last period date.

If your cycles are irregular, or you're coming off hormonal birth control, breastfeeding, or fertility treatment, LMP-based dating is less reliable across the board — an early ultrasound is worth prioritizing in those situations. If you're also tracking ovulation to help pin down a conception date, our guides on how to calculate ovulation and the fertility window can help you narrow things down further.

True Labor vs. False Labor

As your due date approaches, contractions become a regular topic of conversation with your care team. Not every contraction means labor has started, and learning to tell the difference can save you an unnecessary trip to the hospital — or help you recognize when it's time to go.

Braxton Hicks (False Labor) vs. True Labor

False labor is another name for Braxton Hicks contractions — a tightening of the uterus that's often described as your body "practicing" for the real thing. Unlike true labor, Braxton Hicks contractions don't open your cervix. (Cleveland Clinic)

The 5-1-1 Rule: When to Call Your Provider

Many care teams use a simple guideline called the 5-1-1 rule: contractions coming every 5 minutes, each lasting about 1 minute, continuing for at least 1 hour. When you hit that pattern, it's generally time to call your provider or head to your birthing center. Some practices use a slightly stricter 4-1-1 version instead — your provider will tell you which one applies to your situation.

Four contractions in 10 minutes generally falls within the normal range once labor is established — most guidelines describe 3 to 5 contractions every 10 minutes as typical active labor, while more than 5 in 10 minutes sustained over half an hour is what providers watch for. If your contractions match that pattern and the 5-1-1 rule above, it's a reasonable time to check in with your care team.

Signs Your Cervix May Be Dilating

Dilation is the gradual opening of the cervix, measured from 0 to 10 centimeters, and the only way to measure it exactly is a cervical exam by your provider. You generally can't feel the opening itself, but a few signs often show up alongside it:

• Losing your mucus plug — a thick discharge that has sealed the cervix during pregnancy
• "Bloody show" — pink or blood-tinged mucus discharge
• Increasing pelvic pressure, cramping, or backache
• Contractions that grow stronger and more regular

Self-checking for dilation at home isn't considered reliable and carries a small infection risk, so it's best left to a trained provider during a prenatal visit or once you're in labor.

Going Past Your Due Date

Given that only 4 to 5% of babies arrive exactly on time, plenty of pregnancies continue past the due date without anything being wrong. Still, once you cross into late term and postterm territory, your care team will usually step up monitoring.

At 41 weeks, many providers begin more frequent check-ins, sometimes including fetal heart rate monitoring or an assessment of amniotic fluid levels. By 42 weeks, induction is commonly recommended, since risks like reduced amniotic fluid and placental changes increase somewhat the further a pregnancy extends beyond term. Some providers also discuss elective induction around 39 weeks for low-risk, first-time pregnancies — this is a decision made individually with your provider, weighing your specific health history and preferences.

Most people who go past their due date have straightforward deliveries. Going over isn't, on its own, a sign that something is wrong — it's simply a reminder that your due date was always an estimate, not a deadline your body was obligated to meet.

Common Misconceptions About Due Dates

• Myth: Your due date is the day you'll give birth. Fact: It's a statistical estimate. Only about 4 to 5% of babies are born on their exact due date.
• Myth: An ultrasound date is always more accurate, no matter when it's done. Fact: Accuracy depends heavily on timing — first-trimester scans are far more precise than third-trimester ones.
• Myth: Going past your due date means something is wrong. Fact: Birth anywhere from 37 to 42 weeks is considered within the normal range.
• Myth: Labor always starts with your water breaking. Fact: For most people, regular contractions start first; the amniotic sac often doesn't rupture until later in labor.
• Myth: If you're a few centimeters dilated, labor is imminent. Fact: Some people walk around dilated for days or weeks before labor actually begins.

Tips for the Final Weeks

A few practical habits can make the last stretch before your due date feel less like a waiting game and more like preparation.

1. Pack your hospital bag by around 36 weeks, earlier if you're expecting twins or have a higher-risk pregnancy.
2. Keep a simple note of contraction timing once they start, so you can quickly tell your provider the frequency and duration when you call.
3. Track fetal movement in the third trimester, and mention any noticeable slowdown to your provider promptly.
4. Confirm your birth plan and hospital route with your support person well before your window opens, not after contractions start.
5. Keep an eye on nutrition and weight gain through the final weeks — 100 Calculator's Pregnancy Weight Gain Calculator and Nutrition Calculator for Women can help you check you're on a healthy track, alongside your provider's guidance.

Once your baby arrives, our Baby Growth Memory Calendar is a simple way to keep track of early milestones.

When to Call Your Doctor

Alongside the labor signs covered above, a few symptoms are worth contacting your provider about regardless of how close you are to your due date:

• Your water breaks, whether as a gush or a trickle
• Vaginal bleeding that's heavier than light spotting
• A noticeable decrease in your baby's movement
• Contractions matching the 5-1-1 pattern described above
• Severe headache, vision changes, or sudden swelling
• You've reached 41 weeks without labor starting

More From Women's Health Guides

Still building your knowledge of cycles, fertility, and pregnancy timing? These related guides dig deeper into the topics covered above.

Frequently Asked Questions