Free Due Date Calculator
Find out when your baby is due — using your last period date or your conception date
🍼 Due Date Calculator
Find out when your baby is due — choose your preferred method below!
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📅 Your Pregnancy Milestones
How Is My Due Date Calculated?
Your due date is an estimate of when your baby is most likely to arrive — about 40 weeks (280 days) from the first day of your last menstrual period (LMP). This calculator offers three methods:
- Last Period (LMP): The most common method. We add 280 days to your last period's first day, adjusting for your cycle length (Naegele's Rule).
- Conception Date: If you know when you conceived or ovulated, we add 266 days.
- Ultrasound: The most accurate, especially in early pregnancy. We calculate based on how far along you were at your scan.
Understanding the Three Trimesters
🌱 First Trimester (Weeks 1–12)
Major organs form, and you may experience morning sickness, fatigue, and tender breasts. This is a critical period of development.
🌸 Second Trimester (Weeks 13–27)
Often called the "honeymoon phase" — energy returns, you'll likely feel baby's first kicks, and the anatomy scan (around 20 weeks) can reveal the gender.
🌼 Third Trimester (Weeks 28–40)
Baby grows rapidly and gains weight. You'll have more frequent checkups as you prepare for delivery. Full term begins at 37 weeks.
How Accurate Is a Due Date?
Only about 5% of babies are born on their exact due date. Most arrive within two weeks before or after. A "normal" full-term delivery can happen anytime between 37 and 42 weeks. Think of your due date as a helpful guide, not a deadline. 💗
Tips for a Healthy Pregnancy
- 🥗 Eat a balanced diet rich in folate, iron, and calcium
- 💊 Take a daily prenatal vitamin
- 💧 Stay hydrated and get gentle exercise
- 😴 Prioritize rest and manage stress
- 🩺 Attend all prenatal appointments
- 🚭 Avoid alcohol, smoking, and limit caffeine
Frequently Asked Questions
Try Our Other Free Tools
Your Pregnancy Timeline
How is due date calculated?
We use Naegele's Rule — the same formula your OB-GYN uses. It adds 280 days (40 weeks) to the first day of your last menstrual period. This accounts for the 2 weeks before conception in a standard 28-day cycle.
How accurate is this calculator?
This calculator gives you a reliable estimate. Only about 5% of babies are actually born on their exact due date. Most arrive within 2 weeks before or after. Your doctor will confirm with an ultrasound, especially in the first trimester.
What are the three trimesters?
Your pregnancy is divided into three trimesters. The first trimester is weeks 1–12, the second is weeks 13–27, and the third is weeks 28–40. Each trimester brings different development milestones and symptoms.
When should I see a doctor?
Book your first prenatal appointment as soon as you get a positive pregnancy test — ideally before week 8. Your doctor will confirm your due date with an early ultrasound and start your prenatal care plan.
- A due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period, assuming a 28-day cycle with ovulation on day 14.
- This is an estimate, not a guarantee — only a small percentage of babies are born on their exact due date.
- First-trimester ultrasound is the most accurate dating method, with a margin of error of about ±5–7 days.
- Knowing your conception date can refine your estimate further.
- Due dates are sometimes adjusted after an early ultrasound if it differs from the LMP-based estimate.
- Full-term pregnancy is generally 37 to 42 weeks — a five-week range, not a single fixed day.
- Irregular cycles can shift your actual due date away from the standard calculation.
- Ovulation Calculator — Not sure of your LMP or cycle length? Estimate your ovulation date first.
- Pregnancy Week Tracker — See what's happening week by week once you have your due date.
- IVF Due Date Calculator — A dedicated version of this calculator for IVF transfer dates.
- Period Tracker — Log your cycles to get a more accurate LMP-based estimate.
The most common method adds 280 days (40 weeks) to the first day of your last menstrual period (LMP), assuming a standard 28-day cycle with ovulation on day 14 — a method known as Naegele's rule. If you know your conception date, 266 days is added instead. Early first-trimester ultrasound (before 14 weeks) is considered the most accurate method overall, with a margin of error of roughly ±5–7 days, compared to ±7–14 days for LMP-based dating.
One of the very first things most people want to know after a positive pregnancy test is: when will the baby actually arrive? A due date calculator gives you a fast estimate, but the number it produces is built on a few different possible methods — and knowing which one your estimate is based on (and how accurate that method really is) makes the number much more useful than just a date on a calendar.
This guide explains exactly how due dates are calculated, compares the accuracy of each method using guidelines from the American College of Obstetricians and Gynecologists (ACOG), and answers the questions people ask most once they have their estimated due date in hand.
Table of Contents
- How Is a Due Date Calculated?
- The 4 Dating Methods, Compared
- How Accurate Is a Due Date, Really?
- Why Your Due Date Might Change
- What If Your Cycle Isn't 28 Days?
- Due Date vs. Full-Term Range
- Frequently Asked Questions
1. How Is a Due Date Calculated?
The most widely used method is called Naegele's rule, named after the 19th-century obstetrician who developed it. It works by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a textbook 28-day cycle with ovulation occurring around day 14.
If you know your exact conception date instead — for example, from ovulation tracking or an IVF embryo transfer — the calculation shifts slightly to 266 days from that date, since it skips the two-week estimate for when ovulation happened.
Did You Know? Naegele's rule has been the standard method for estimating due dates for nearly 200 years, but it was designed before ultrasound existed. Today it's usually treated as a helpful starting estimate rather than the final word, especially once an early ultrasound is available.
Not yet sure when you ovulated? Start with the Ovulation Calculator to pin down your conception window first.
2. The 4 Dating Methods, Compared
| Method | How It Works | Typical Margin of Error |
|---|---|---|
| Last Menstrual Period (LMP) | 280 days from first day of LMP, assumes 28-day cycle | ±7–14 days |
| First-Trimester Ultrasound (CRL) | Measures crown-rump length up to 13 weeks 6 days | ±5–7 days (most accurate) |
| Conception Date | 266 days from known conception/ovulation date | ±7–10 days |
| IVF Embryo Transfer | 266 days from transfer date, adjusted for embryo age | Most precise — exact date known |
| Second-Trimester Ultrasound | Measures BPD, HC, AC, femur length (14–28 weeks) | ±10–14 days |
| Third-Trimester Ultrasound | Same measurements, later in pregnancy | ±21–30 days (least accurate) |
According to ACOG Committee Opinion No. 700, first-trimester ultrasound dating is generally considered the most reliable method available, and the organization recommends using it to adjust or confirm the LMP-based estimate whenever the two differ by more than about 5–7 days in the first trimester.
Expert Tip: If you had an early ultrasound and your provider gave you a due date that's different from what an online calculator estimated using your LMP, that's completely normal — the ultrasound date is generally considered more reliable and typically takes precedence once you're being seen for prenatal care.
3. How Accurate Is a Due Date, Really?
It's worth saying clearly: a due date is an estimate, not a prediction of the exact day labor will start. Research comparing LMP dating, ultrasound dating, and their combination — referenced in detail on Perinatology.com's clinical due date calculator — has consistently shown a meaningful spread around the estimated date, which is part of why full-term pregnancy is defined as a five-week range (37–42 weeks) rather than a single day.
Statistics Snapshot:
- Only a small minority of babies are born on their exact calculated due date.
- LMP-based dating alone has a typical margin of error of ±7–14 days.
- First-trimester ultrasound narrows that margin to roughly ±5–7 days.
- Third-trimester-only ultrasound dating has the widest margin of error, at ±21–30 days.
Myth vs. Fact
Myth: Your due date is a firm prediction of when labor will start.
Fact: It's a statistical estimate of the middle of the range in which labor is likely to begin naturally — most healthy pregnancies deliver anywhere within a couple of weeks on either side of it.
4. Why Your Due Date Might Change
It's common for a due date to shift slightly once more information becomes available, and this isn't a sign that anything is wrong. The most frequent reason is a first-trimester ultrasound: if the ultrasound-based gestational age differs from the LMP-based estimate by more than about 5–7 days, providers typically use the ultrasound date going forward, since it's generally more reliable, particularly for anyone with irregular cycles or uncertainty about their exact LMP.
Common Mistake: Assuming a changed due date means the original calculation was "wrong" or that something is off with the pregnancy. In most cases, it simply means a more precise dating method became available, which is a normal and expected part of early prenatal care.
Once your due date is confirmed, providers generally don't revise it again later in pregnancy except in rare circumstances, since fetal size becomes a less reliable dating tool as pregnancy progresses.
5. What If Your Cycle Isn't 28 Days?
Naegele's rule assumes a standard 28-day cycle, but cycle length varies significantly between individuals — anywhere from about 21 to 35 days is considered typical. If your cycles run longer or shorter than 28 days, your actual ovulation and conception dates may fall earlier or later than the standard calculation assumes, which can shift your real due date away from the basic LMP estimate.
Quick Checklist for a More Accurate Estimate:
- Know your average cycle length (not just your period start date)
- If you track ovulation, use your ovulation date to calculate rather than LMP alone — try the Ovulation Calculator
- If you're unsure of your LMP or have irregular cycles, an early ultrasound will likely be more accurate than any calendar-based method
- Log your cycle history in a Period Tracker to spot your typical pattern over time
6. Due Date vs. Full-Term Range
A "full-term" pregnancy isn't a single date — it's a five-week window, generally defined as 37 weeks 0 days through 41 weeks 6 days, a definition used consistently by the NHS pregnancy due date calculator and other national health bodies. Babies born anywhere in that range are considered full-term, which is one more reason the calculated due date works better as a midpoint estimate than a fixed deadline.
Pro Insight: Many care providers find it more useful to think in terms of a due range rather than a due date — roughly two weeks on either side of the calculated date — when setting expectations for how pregnancy is likely to unfold.
Related FeminaTrack Resources
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- How to Calculate What Week You Are In Your Pregnancy
- hCG Levels in Early Pregnancy: What Is Normal Week by Week?
- 12 Early Pregnancy Symptoms Before a Missed Period
- What Is a Fertile Window and How Do You Calculate Yours?
Conclusion
A due date calculator gives you a fast, useful starting estimate — but the real value comes from understanding which method it's based on and how much that estimate can naturally shift. LMP-based dating is a reasonable starting point, first-trimester ultrasound sharpens the estimate considerably, and knowing your actual conception date (through ovulation tracking or IVF) can refine things even further. Whatever method you start with, treating your due date as the middle of a likely range — rather than a fixed deadline — tends to match how pregnancy actually unfolds. Tools like FeminaTrack's Ovulation Calculator and Pregnancy Week Tracker can help you refine and follow that estimate as your pregnancy progresses.
This content is for educational purposes only and should not replace professional medical advice, diagnosis, or treatment.