The first time my wife and I tried to figure out her due date, we made a couple of mistakes. We first counted nine months from her positive pregnancy test. That was wrong! Pregnancy is measured from your last period, not when you think you conceived. Then, we counted forty weeks from that wrong date and landed on a public holiday. Oops!
On our third try, we used a proper calculator. Now I understand why doctors always ask for the “first day of your last menstrual period” right away. It’s the starting point for everything!
This article will show you every way doctors figure out a due date. You’ll learn why your due date might change by 5 to 10 days depending on the method. We’ll also see how to use all these methods together. Our free pregnancy due date calculator uses all three methods for you. Let’s compare them by hand below.
Method 1 — Naegele’s Rule (the classic way)
This is the first formula every OB-GYN learns. It’s also the basis for most “9 months” due date guesses.
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Estimated due date = LMP + 7 days − 3 months + 1 year
LMP stands for the first day of your last menstrual period. This rule assumes a few things:
- Your period cycle is 28 days long.
- You ovulate on day 14 of your cycle.
- Your pregnancy will last 280 days (which is 40 weeks).
Let’s try an example: Your LMP was January 1, 2026.
- Add 7 days: January 8.
- Subtract 3 months: October 8, 2025 (yes, you go back into the previous year).
- Add 1 year: October 8, 2026. This is your estimated due date! ✅
This gives you that standard 40-week estimate.
Why Naegele’s Rule is usually “close enough”
A study in 2013 looked at many pregnancies. It found that, on average, first-time mothers are pregnant for about 40 weeks and 5 days. For women who have had babies before, it’s usually a little less. So, Naegele’s rule often predicts a due date a few days earlier than average. But it’s close enough that we still use it as the main public date. Keep in mind, only about 5% of babies are born on their exact due date.
Method 2 — Adjust for your cycle length
Naegele’s rule works best if your cycle is exactly 28 days. If your cycle is shorter or longer, your ovulation day changes. This also changes your due date. Here’s the updated formula:
EDD = LMP + 280 days + (your cycle length − 28)
| Cycle length | Adjustment | Due date shift |
|---|---|---|
| 25 days | −3 days | Due about 3 days earlier |
| 28 days | 0 | Same as Naegele’s rule |
| 32 days | +4 days | Due about 4 days later |
| 35 days | +7 days | Due about 7 days later |
This adjustment is really important for women with PCOS or other reasons for irregular cycles. Make sure to use the cycle-length field in our due date calculator to get a more accurate answer.
Method 3 — Dating from conception (if you know it)
Sometimes you know the exact day you conceived. This might be from IVF, IUI, or a super accurate ovulation tracking app. If so, the math is even simpler:
EDD = Conception date + 266 days
There’s a 14-day difference between this and Naegele’s rule. That’s because Naegele’s rule counts the two weeks between your last period and when you ovulate in a “perfect” 28-day cycle. When you have reliable conception data, this method is usually the most accurate. You can use our ovulation calculator to help find your fertile window.
Method 4 — Ultrasound dating
Your first-trimester dating ultrasound is usually done between 8 and 12 weeks. This is considered the best way to date a pregnancy. During this scan, the technician measures the baby’s crown-rump length (CRL). This is the length from the top of its head to its bottom. They then compare this measurement to a growth chart.
| Gestational age | Approx. CRL |
|---|---|
| 6 weeks | 4 mm |
| 7 weeks | 10 mm |
| 8 weeks | 17 mm |
| 10 weeks | 33 mm |
| 12 weeks | 55 mm |
Doctors today often change your due date from the LMP-based one to the ultrasound-based one if they are more than 5 to 7 days apart. The American College of Obstetricians and Gynecologists (ACOG) has clear rules for this. An ultrasound between 8 and 13 weeks and 6 days is accurate within 5 days. Between 14 and 20 weeks, it’s accurate within 10 days. After 20 weeks, it’s much less accurate because babies start growing at their own unique pace.
Trimesters explained
A 40-week pregnancy is split into three parts, called trimesters.
| Trimester | Weeks | What’s happening |
|---|---|---|
| First | 1–13 | Baby’s organs develop, highest risk of miscarriage, morning sickness often starts. |
| Second | 14–27 | Often called the “honeymoon trimester.” You might feel the baby move, and you’ll likely have an anatomy ultrasound. |
| Third | 28–40 | Baby grows quickly, lungs finish developing, and your body gets ready for labor. |
The first day of your last period is officially counted as “Week 1 Day 1.” This is true even though you haven’t actually conceived yet. That’s why you’re considered about 4 weeks pregnant by the time a home urine test shows positive.
Important dates to mark in your calendar
- Week 7–8: Your first prenatal visit and a dating ultrasound.
- Week 10–13: NIPT (non-invasive prenatal testing) is an option.
- Week 12: The end of the first trimester. The risk of miscarriage drops a lot.
- Week 16–20: Anatomy scan (this is often when you can find out the baby’s gender).
- Week 24: Viability threshold. Babies born now can survive with special care in the NICU.
- Week 28: Glucose tolerance test to check for gestational diabetes.
- Week 35–37: Group B strep test.
- Week 37: Full term officially begins.
- Week 39–41: Most babies are born naturally during this time.
- Week 42: Post-term. Doctors usually suggest inducing labor at this point.
Common questions about due dates
“How accurate is my due date, really?”
Only about 5% of babies arrive on their exact due date. Roughly 60–70% are born within 10 days before or after. Most doctors see a pregnancy as normal if the baby arrives between 37 and 42 weeks. They only step in if the baby comes before 37 weeks (premature) or after 42 weeks (post-term).
“My first ultrasound changed my due date by a week. Which date is correct?”
In almost every case, the ultrasound date is more accurate. LMP dating needs you to remember an exact start date. It also assumes a perfect 28-day cycle and ovulation on day 14. These are often not quite right. A first-trimester ultrasound measures your baby’s actual size. It’s usually accurate within a few days.
“My doctor changed the due date twice. Should I be worried?”
Probably not. It’s normal for due dates to be adjusted in the first trimester. It’s rare for them to change after week 20. After 22 weeks, doctors stick with the earliest reliable estimate (either your LMP or the first ultrasound). They stop changing it because babies start growing at different rates after that.
Estimating the conception date in reverse
If you know your due date and want to guess when conception happened:
Approximate conception date = EDD − 266 days
This is good enough for your personal journal or for telling “we conceived on holiday” stories. But it’s not precise enough for legal matters or paternity questions. For those, you’d need a first-trimester ultrasound with a crown-rump length measurement.
Special cases for due dates
Twins
Twin pregnancies usually last about 37 weeks, not 40. Triplets are often born even earlier, around 33–35 weeks. The due date for twins is still set at 40 weeks for medical records. However, the plan for delivery is made knowing they will likely come earlier.
IVF pregnancies
With IVF, you know the exact transfer date. To get your due date, add 266 days. Then, subtract the age of the embryo when it was transferred (either a 3-day or 5-day blastocyst). Most IVF calculators handle this math for you automatically.
Irregular cycles or PCOS
If you can’t remember your last period’s start date, or if your cycles vary by more than 10 days, your first-trimester dating ultrasound is the most reliable way to get a due date. In these cases, it’s best to skip the LMP method and wait for your ultrasound.
Frequently asked questions
How is the due date calculated if I have irregular periods?
You’ll need a confirmed ovulation date. This can come from tracking your basal body temperature, using ovulation strips, or a fertility app. Otherwise, you’ll need to wait for your first-trimester ultrasound. Using Naegele’s rule with an irregular cycle is basically just a guess.
Can the due date be wrong even after an ultrasound?
A first-trimester ultrasound is accurate within 5 to 7 days. After the first trimester, babies start to grow at their own unique pace. This means later ultrasounds are less accurate for dating. The very early scan is the most precise.
How long is a “normal” pregnancy?
A full-term pregnancy is officially between 37 and 42 weeks. If a baby is born before 37 weeks, it’s considered preterm. If it’s after 42 weeks, it’s post-term, and doctors usually recommend inducing labor. A study in 2020 found that the average pregnancy length is 39 weeks and 3 days.
Why does the due date use 280 days and not 9 months?
The length of “9 months” can change because some months have 30 days and some have 31. Also, pregnancy is measured from your last menstrual period. This adds about two weeks before conception actually happens. So, 40 weeks times 7 days equals 280 days. This is roughly 9 calendar months plus one week.
How do online due-date calculators work?
Most online calculators use Naegele’s rule. They also often let you adjust for your cycle length. Some of the better ones allow you to enter ultrasound CRL measurements directly. This lets you override the date that came from your last period. The CalcaTools due date calculator supports all these options.
Wrapping up
Think of your due date as an educated guess, not a fixed appointment. Naegele’s rule gives you a starting point. Your first-trimester ultrasound usually makes it more precise. But in the end, your baby decides the actual day! The best thing you can do with your due date is to plan around the big milestones. This includes tests, scans, and maternity leave. Then, just let your baby surprise you on their arrival day. Use the CalcaTools pregnancy due date calculator along with our pregnancy week-by-week calculator. You’ll have a full timeline of your pregnancy right at your fingertips.
Related calculators and guides
More pregnancy and fertility tools:
- Due Date Calculator — estimate your delivery date.
- Pregnancy Calculator — track week by week.
- Ovulation Calculator — find your fertile window.
- Conception Calculator — estimate the date of conception.
- How Age Is Actually Calculated — years, months and days.
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