Due Date Calculator

Written by Ash K · Last updated: February 2026 · Sources cited below

Pregnancy planning starts with one question: when will your baby arrive? While most expectant parents focus on that single magic date circled on the calendar, the reality is more nuanced. Your due date calculator is a powerful tool, but understanding how it works—and what it can and cannot tell you—matters just as much as the number itself.

This guide walks you through how due dates are calculated, why they sometimes shift during pregnancy, and how to use this information to prepare confidently for your baby's arrival. Whether you're calculating from your last menstrual period (LMP), a confirmed conception date, or an IVF transfer, we'll help you understand what those numbers really mean.

How Is Your Due Date

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Key Takeaway: According to ACOG, the estimated due date (EDD) is calculated as 280 days (40 weeks) from the first day of your last menstrual period (LMP), assuming a 28-day cycle. This is known as Naegele's Rule. Only about 5% of babies are born on their exact due date — it is an estimate, not a deadline. Calculated?

The standard method for calculating your due date is called Naegele's rule, developed in the 1800s and still the foundation of modern pregnancy dating. It's elegantly simple: take your last menstrual period (LMP), count forward 280 days, and you have your estimated due date. That's exactly 40 weeks, or what we call a "term" pregnancy.

But why 40 weeks instead of nine months? Human gestation isn't measured in calendar months because they vary in length. Forty weeks provides a consistent timeline that accounts for the full development cycle from fertilization to birth.

Pregnancy begins around day 14 of your cycle (when ovulation typically occurs), which is about two weeks after the first day of your last period.

So while you count from your LMP, your baby is technically only 38 weeks old at your due date—but we still say "40 weeks pregnant" to keep everyone on the same calendar page.

Here's what's important to understand: only 4-5% of babies are actually born on their exact due date. This statistic surprises many parents, but it perfectly illustrates why thinking in terms of a "due window" rather than a fixed date is more accurate.

According to research published in medical journals, most babies arrive between 37 and 42 weeks of pregnancy. A term pregnancy officially runs from week 37 to week 42. Before 37 weeks is considered preterm; after 42 weeks is considered overdue.

Your "due date" is really your estimated due date (EDD)—a target week rather than a target day. Think of it as the center of a window, with healthy delivery possible several weeks on either side.

This reframing helps many parents feel less anxious if they haven't delivered by their exact due date. According to ACOG Practice Bulletin No. 175, most variation in delivery timing is completely normal and doesn't indicate any problem with your pregnancy.

How to Use

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Tip: Enter the first day of your last period, not the day you think you conceived. If your cycles are irregular (shorter or longer than 28 days), the calculator adjusts — but an early ultrasound (6-9 weeks) provides a more accurate due date according to ACOG guidelines. This Calculator

A quality due date calculator gives you flexibility because pregnancy dating depends on the information you have available. Most pregnancies are dated using one of three methods, each with specific advantages and accuracy levels.

Method 1: Last Menstrual Period (LMP) This is the most common starting point. If you know the first day of your last period, that becomes your reference point. The calculator adds 280 days (40 weeks) to arrive at your estimated due date.

This method works well if your cycles are regular (typically 28 days), but becomes less reliable if your cycles are irregular or if you can't remember your exact LMP date.

Method 2: Conception Date If you tracked ovulation or know the exact day you conceived, the calculator counts forward 266 days (38 weeks) to your due date. This method is more precise than LMP-based dating because it skips the variability of cycle length.

However, many people don't know their exact conception date unless they were actively tracking fertility or using fertility treatments.

Method 3: IVF Transfer Date For parents who conceived through in vitro fertilization, due date calculation accounts for whether a 3-day or 5-day embryo was transferred. A 5-day blastocyst transfer adds 263 days to your transfer date; a 3-day embryo transfer adds 265 days.

Your fertility clinic typically provides your estimated due date at the time of transfer, but this calculator lets you double-check or recalculate if you want to adjust for an updated ultrasound finding.

Which method is best to use? Start with whichever date you're most confident about. If you have an early ultrasound (ideally before 13 weeks), your provider may adjust your due date based on what they see.

Here's an important point: early ultrasound dating is actually more accurate than LMP dating, especially if your cycles are irregular or if your ultrasound suggests your baby is measuring differently than expected.

According to ACOG Committee Opinion No. 700, a first-trimester ultrasound can establish gestational age within ±5 days—much more precise than LMP alone.

Pregnancy Trimesters — ACOG Timeline

Understanding Your Results

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Note: According to published obstetric research, a "full term" pregnancy is 39-40 weeks. Early term is 37-38 weeks, and late term is 41 weeks. ACOG considers 42+ weeks "post-term." Most providers discuss induction options between 41-42 weeks based on individual risk factors.

When you use a due date calculator, you get more than just a single number. A comprehensive result shows you several pieces of useful information:

Your Estimated Due Date (EDD) This is your primary reference point—the week your baby is most likely to arrive. Mark this on your calendar, but remember: it's a week, not a day. Your baby may come earlier or later and still be completely on schedule.

Current Gestational Age The calculator tells you exactly how many weeks and days along you are right now. This matters because different milestones happen at specific gestational ages.

Your anatomy scan happens around 20 weeks; viability (the point where a baby could survive outside the womb with medical support) is generally around 24 weeks. Knowing your exact gestational age helps you anticipate these important moments.

Trimester Breakdown with Key Milestones Understanding where you are in your pregnancy helps you prepare mentally and practically. The first trimester brings organ formation and, for many, intense nausea. The second trimester is often called the "golden period"—many women feel their best, and you'll see your baby's anatomy on ultrasound.

The third trimester involves rapid growth and nesting instincts as you prepare for labor.

How Far Along as a Percentage Some calculators show your progress as a percentage of full term. This can be psychologically helpful as you move through pregnancy. At 20 weeks, you're halfway there; at 30 weeks, you're firmly in the final stretch.

Now that you understand your due date and current gestational age, you can track other important health metrics throughout your pregnancy. For example, if you're concerned about healthy weight gain, our Pregnancy Weight Gain Calculator helps you track whether you're gaining at a healthy pace for your stage of pregnancy.

Healthy weight gain varies by trimester, and having a personalized range makes the process less stressful.

Due Date Accuracy — Why Only 5% of Babies Arrive on Time

The Three Trimesters

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Note: According to ACOG, the three trimesters are divided as: First trimester (weeks 1-12), Second trimester (weeks 13-27), Third trimester (weeks 28-40). Each trimester has distinct developmental milestones and prenatal care schedules recommended by medical guidelines.

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Note: According to ACOG, the first trimester (weeks 1-13) is when organs form and miscarriage risk is highest. The second trimester (weeks 14-27) is often called the "golden period" — energy returns and nausea typically subsides. The third trimester (weeks 28-40) is focused on fetal growth and delivery preparation. — What Happens When

Understanding your due date becomes much more meaningful when you know what's happening in your body and your baby's body at each stage. Pregnancy is divided into three trimesters, each with distinct developments and experiences.

Trimester	Weeks	Key Baby Milestones	Key Appointments
First	1–12	Heartbeat (week 6), all major organs forming, ~15g by week 12	First prenatal visit, dating ultrasound (8–13 weeks)
Second	13–27	Anatomy visible on scan (week 20), movement felt, ~900g by week 27	Anatomy scan (18–22 weeks), glucose screening (24–28 weeks)
Third	28–40	Rapid weight gain (~230g/week), lungs maturing, ~3.4 kg at birth	Group B strep test (35–37 weeks), weekly visits from week 36

First Trimester (Weeks 1-12)

Your first trimester starts with the first day of your last menstrual period, even though you likely don't know you're pregnant for another two weeks. This period is marked by rapid cell division and organ formation.

By the end of week 8, your baby has a heartbeat (visible on ultrasound around week 6). All major organ systems begin developing in these early weeks, making this the most critical period for fetal development.

Many people experience significant nausea, fatigue, and food aversions during this time—a phenomenon so common it's sometimes called "morning sickness," though it can happen any time of day. This is driven by rapidly rising hormone levels, particularly human chorionic gonadotropin (hCG).

While uncomfortable, nausea often signals that your pregnancy hormones are rising appropriately. If you're tracking your hCG levels in early pregnancy, our HCG Doubling Time Calculator can help you understand whether your levels are progressing as expected.

You'll typically have your first prenatal ultrasound during the first trimester, usually between 8-13 weeks. This scan does several things: it confirms your pregnancy, checks for a heartbeat, estimates your due date, and screens for any obvious concerns.

If this first-trimester ultrasound differs from your LMP-based due date by more than a few days, your provider will likely adjust your official due date to match the ultrasound finding.

Second Trimester (Weeks 13-27)

The second trimester is often called the "golden period" of pregnancy. Morning sickness usually fades, your energy returns, and you're visibly pregnant in a way that feels exciting rather than uncomfortable. This is when many people share their pregnancy news with the world and begin the fun of planning and shopping.

Around week 20, you'll have your anatomy scan—the detailed ultrasound where your baby's organs are carefully examined. This is often the first time parents see their baby's face in detail and can learn the baby's sex if they choose. It's an emotional milestone for many families.

During the second trimester, you'll likely feel your baby moving for the first time, typically between weeks 16-25 depending on where your placenta is positioned and whether you've been pregnant before. These movements—first like flutters, then like butterflies, finally like definite kicks—mark a shift in how pregnancy feels. Your baby becomes tangibly real, not just a concept.

Developmentally, your baby is growing rapidly, developing fingerprints, hearing sounds, and opening their eyes. The brain is developing its neural connections. By the end of the second trimester, your baby weighs around 1-2 pounds and, though not yet viable outside the womb, is beginning to develop the ability to survive with medical support.

Third Trimester (Weeks 28-40+)

The third trimester brings your baby from relative viability to full-term readiness. At 28 weeks (the start of the third trimester), your baby would have a reasonable chance of survival with intensive medical support if born. By 32 weeks, most babies would do well with minimal support.

At 37 weeks, they're considered "term" and fully ready to navigate the outside world independently.

The third trimester is when most dramatic growth happens. Your baby nearly triples in weight, their lungs mature, their brain continues developing.

You may experience more intense symptoms: shortness of breath as your baby moves upward and crowds your lungs, back pain from the added weight, sleep disruption from difficulty finding a comfortable position, and intense nesting instincts as your body prepares for labor.

You'll have more frequent prenatal visits during the third trimester, and around week 36, your provider checks your baby's position. Most babies flip head-down by this point, though some don't, which may affect your delivery options. You'll also be screened for signs of preeclampsia and gestational diabetes if not previously screened.

Trimester Overview

Trimester	Weeks	Key Milestones	Common Symptoms
First	1–12	Heartbeat (week 6), all organs formed (week 12)	Nausea, fatigue, food aversions
Second	13–27	Anatomy scan (week 20), first movements felt	Energy returns, visible bump, less nausea
Third	28–40	Viability (week 28), full-term (week 37)	Back pain, shortness of breath, nesting

Key Development Milestones

Week 6: Fetal heartbeat visible on ultrasound
Week 12: All major organs have formed; risk of miscarriage drops significantly
Week 20: Anatomy scan typically occurs; detailed examination of baby's development
Week 24: Viability threshold—with intensive care, a baby born at this point could survive
Week 28: All major organ systems are functional; third trimester begins
Week 37: Term pregnancy begins; baby is considered full-term if born now
Week 40: Estimated due date—the center of your delivery window

Why Due Dates Change

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Note: According to ACOG guidelines, if a first-trimester ultrasound disagrees with the LMP-based date by more than 7 days, the ultrasound date is used. Early ultrasound dating is accurate to ±5 days, while LMP-based dating assumes a 28-day cycle with ovulation on day 14 — which varies significantly between women.

If you've had multiple ultrasounds during your pregnancy, you may have noticed your due date shifting slightly from what was calculated at your first appointment. This isn't a mistake—it's actually how we refine our dating as we get more information. Understanding why this happens helps you trust the process.

Early Ultrasound Is More Accurate Than LMP

According to ACOG Committee Opinion No. 700 on Methods for Estimating Due Date, ultrasound measurements are more accurate than menstrual dating, especially in early pregnancy.

Here's why: while you can remember the first day of your last period fairly reliably, you may have irregular cycles, and your cycle length may not be the standard 28 days everyone assumes. An ultrasound actually measures your baby, which is objective and precise.

Accuracy Depends on Timing

The earlier in pregnancy you have an ultrasound, the more accurate it is for dating purposes:

Before 13 weeks: Accurate to within ±5 days
14-20 weeks: Accurate to within ±7-10 days
After 20 weeks: Accurate to within ±2-3 weeks

This is why your first-trimester dating ultrasound is so important. The further along you are when you have your ultrasound, the less reliable it is for dating purposes, because normal variation in baby size becomes more pronounced.

When Your Ultrasound Date Differs from Your LMP Date

If your dating ultrasound differs from your LMP-calculated date by more than 5 days in the first trimester, or more than 10 days in the second trimester, your provider will typically adjust your official due date to match the ultrasound measurement.

This isn't because the ultrasound is always "right"—rather, it's because at that point in pregnancy, it's more reliable.

If you conceived on a date different from what your LMP suggested, or if your cycle is unusually long or short, the ultrasound provides a more accurate picture of how far along you actually are.

Later Adjustments

It's less common for due dates to change in the third trimester, because ultrasound dating becomes less precise. However, if you didn't have an early ultrasound, a late-pregnancy scan might adjust your date.

Your provider wants to be as accurate as possible because delivering too early (before 37 weeks is considered preterm) or too late (after 42 weeks carries increased risks) impacts your baby's health.

When to Consult

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Warning: According to ACOG, a pregnancy that continues beyond 42 weeks (post-term) carries increased risks including placental insufficiency and meconium aspiration. Most providers discuss induction options between 39-41 weeks per current guidelines. Contact your provider if you pass 41 weeks.

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Bottom Line: Your due date is an estimate based on Naegele's Rule or ultrasound measurement per ACOG standards. Only about 5% of babies arrive on the exact date. Focus on the "due month" rather than the due date. Discuss any concerns about timing with your healthcare provider. a Doctor

While a due date calculator is a helpful starting point, it has important limitations. There are situations where medical dating is preferable to calculator-based estimates, and times when reaching out to your healthcare provider is important rather than relying solely on a calculator.

Limitations of Calculator-Based Estimates

A calculator gives you an estimate based on the information you provide. If you're unsure of your last menstrual period, if your cycles are irregular, or if you have conditions like polycystic ovary syndrome (PCOS) that affect ovulation timing, calculator results may be less reliable.

Additionally, calculators can't account for individual variations in fetal growth, maternal factors, or health conditions that your provider knows about.

When Medical Dating Is Preferable

Contact your healthcare provider for pregnancy dating if:

You're unsure of your last menstrual period or have very irregular cycles
You have a medical condition affecting fertility or ovulation
Your pregnancy resulted from fertility treatments (your clinic typically provides dating at the time of transfer)
You want to confirm or adjust a due date before sharing widely
You're in early pregnancy and haven't had an ultrasound yet

Signs That Warrant Immediate Consultation

Certain symptoms require immediate medical attention, regardless of what a calculator tells you:

Vaginal bleeding or spotting
Severe abdominal pain or cramping
Chest pain or shortness of breath
Severe headache or vision changes
Vaginal fluid leakage (possibly amniotic fluid)
Decreased or absent fetal movement (in third trimester)
Signs of preeclampsia: swelling, high blood pressure, protein in urine

Your healthcare provider has access to your full medical history and can provide personalized dating and monitoring that a calculator cannot. Use the calculator as a tool to understand your pregnancy, but always confirm your due date with your care team, especially in early pregnancy.

Frequently Asked Questions

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Bottom Line: The due date is an estimate, not a deadline. According to published obstetric data, only 5% of babies arrive on their EDD. The normal range is 37-42 weeks. First-trimester ultrasound provides the most accurate dating per ACOG guidelines.

1. How accurate is a due date calculator?

A due date calculator is accurate to within about ±2 weeks when using your LMP, assuming regular 28-day cycles. If you have irregular cycles, the margin of error is larger. An ultrasound measurement in the first trimester is accurate to within ±5 days, making it more precise than calculator estimates.

Keep in mind that even with the most accurate dating, about 50% of babies are born before their due date and 50% after—variation is completely normal.

2. Why is my ultrasound due date different from my LMP due date?

This happens when your actual cycle differs from the standard 28-day assumption, or when you ovulated earlier or later than typical. If ultrasound shows your baby measuring earlier than your LMP suggests, you may have conceived earlier than you thought, or your cycle may be shorter than 28 days.

Your provider will adjust your official date to match the ultrasound, which is more reliable at that point in pregnancy. A difference of a few days is not concerning.

3. How do I calculate my due date with IVF?

With IVF, your due date is calculated from your transfer date, adjusted for whether you transferred a 3-day or 5-day embryo. For a 5-day blastocyst transfer, add 263 days; for a 3-day embryo transfer, add 265 days. Your fertility clinic calculates this at the time of transfer.

You can verify the calculation with a calculator, but your clinic's date is your primary reference point. Confirm with your OB provider at your first prenatal visit.

4. Can my due date change during pregnancy?

Yes, due dates most commonly shift during the first and early second trimester based on ultrasound measurements, especially if your early dating was based on an uncertain LMP. Changes are less likely in the third trimester.

If your due date adjusts by a few days, this is normal and doesn't indicate any problem. Significant changes might prompt a conversation with your provider about why the adjustment is happening.

5. What if I don't know my last menstrual period?

If you can't remember your LMP, an ultrasound is essential for dating your pregnancy. Ideally, this happens in the first trimester for maximum accuracy. Your healthcare provider can perform an ultrasound to determine your due date based on your baby's measurements.

Some people also remember when they conceived, which can serve as an alternative reference point if paired with an ultrasound confirmation.

Sources & Medical References

ACOG Committee Opinion No. 700: Methods for Estimating the Due Date. American College of Obstetricians and Gynecologists, 2017.
ACOG Practice Bulletin No. 175: Ultrasound in Pregnancy. American College of Obstetricians and Gynecologists, 2016.
Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: Ultrasonographic biometry versus certain menstrual dates. American Journal of Obstetrics and Gynecology. 1996;174(2):556-559.
Bottomley C, Bourne T. Dating and growth in the first trimester. Best Practice & Research Clinical Obstetrics & Gynaecology. 2009;23(4):439-452.

Final Thought

Your due date is one of the most anticipated pieces of information in pregnancy, but it's worth remembering what it actually represents: the middle of a window, not a deadline. Most babies arrive within several weeks of their due date, and that variation is completely healthy.

As you prepare for your baby's arrival, stay connected with your healthcare provider, trust your body, and try to embrace the uncertainty. Pregnancy won't last forever, and before you know it, you'll be counting weeks of your baby's life instead of weeks of pregnancy.

If you're expecting, you likely want to monitor more than just your due date. Check out our other pregnancy calculators to track everything from weight gain to baby growth, ensuring you're prepared at every stage.