The first time my wife and I tried to find her due date, we made two mistakes. We counted nine months from her positive pregnancy test. That was wrong. Pregnancy is measured from your last menstrual period, not from a home test or from when you think conception happened. 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 see why doctors immediately ask for the “first day of your last menstrual period.” That date is the starting point for all standard calculations.
This article explains every common method doctors use to estimate a due date. You will see why different methods can shift the date by about 5–10 days. I also show how to use the methods together. Our free pregnancy due date calculator applies all three methods for you. Below, we compare the methods by hand.
Method 1 — Naegele’s Rule (the classic way)
This formula is the first thing OB-GYNs learn. It is also the basis for most “9 months” due date estimates.
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Estimated due date = LMP + 7 days − 3 months + 1 year
LMP means the first day of your last menstrual period. Naegele’s rule assumes three things:
- Your cycle is 28 days long.
- You ovulate on day 14 of the cycle.
- Your pregnancy lasts 280 days (40 weeks).
Example: Your LMP was January 1, 2026.
- Add 7 days: January 8.
- Subtract 3 months: October 8, 2025 (yes, that goes into the previous year).
- Add 1 year: October 8, 2026. This is your estimated due date! ✅
This gives the standard 40-week estimate.
Why Naegele’s Rule is usually “close enough”
A 2013 study looked at many pregnancies. It found that, on average, first-time mothers are pregnant for about 40 weeks and 5 days. Women who have had pregnancies before tend to be pregnant a little less time. That means Naegele’s rule often predicts a date a few days earlier than the average. Still, the rule is close enough to serve as the usual public due date. Remember, only about 5% of babies are born on their exact due date.
Method 2 — Adjust for your cycle length
Naegele’s rule works best for a 28-day cycle. If your cycle is shorter or longer, your ovulation day shifts. That shift changes your due date. Use this formula instead:
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 matters for people with PCOS or other causes of irregular cycles. Use the cycle-length field in our due date calculator for a more accurate result.
Method 3 — Dating from conception (if you know it)
Sometimes you know the exact day you conceived. That happens with IVF, IUI, or precise ovulation tracking. Then the math is simpler:
EDD = Conception date + 266 days
There is a 14-day difference between this method and Naegele’s rule. Naegele’s rule includes the two weeks between your last period and ovulation in a “perfect” 28-day cycle. When you have a reliable conception date, this method is usually the most accurate. You can use our ovulation calculator to help find your fertile window.
Method 4 — Ultrasound dating
The first-trimester dating ultrasound is usually done between 8 and 12 weeks. It is considered the best way to date a pregnancy. The technician measures the baby’s crown-rump length (CRL). That is the length from the top of the head to the bottom. They 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 |
Today, doctors often change an LMP-based due date to the ultrasound-based date if the two are more than 5 to 7 days apart. The American College of Obstetricians and Gynecologists (ACOG) has clear rules. An ultrasound between 8 weeks and 13 weeks 6 days is accurate within 5 days. Between 14 and 20 weeks it is accurate within 10 days. After 20 weeks the accuracy drops because fetuses grow at different rates.
Trimesters explained
A 40-week pregnancy splits 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.” That is true even though you have not conceived yet. That is why you are usually about 4 weeks pregnant when a home urine test turns 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 it. Most doctors call a pregnancy normal if the baby arrives between 37 and 42 weeks. They intervene 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 depends on remembering an exact start date. It also assumes a perfect 28-day cycle and ovulation on day 14. Those assumptions are often wrong. A first-trimester ultrasound measures the baby’s actual size. It is usually accurate within a few days.
“My doctor changed the due date twice. Should I be worried?”
Probably not. It is normal for due dates to be adjusted in the first trimester. It is rare for them to change after week 20. After 22 weeks, doctors usually keep the earliest reliable estimate (either your LMP or the first ultrasound). They stop changing it because fetuses grow at different rates after that point.
Estimating the conception date in reverse
If you know your due date and want to estimate conception:
Approximate conception date = EDD − 266 days
This is fine for a personal journal or for saying “we conceived on holiday.” It is not precise enough for legal matters or paternity questions. For those, you 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 recorded at 40 weeks in medical records. However, clinicians plan delivery knowing twins will likely arrive earlier.
IVF pregnancies
With IVF, you know the exact transfer date. To get your due date, add 266 days and then subtract the age of the embryo when transferred (either a 3-day or 5-day embryo). Most IVF calculators do this math automatically.
Irregular cycles or PCOS
If you cannot remember the start date of your last period, or if your cycles vary by more than 10 days, the first-trimester ultrasound is the best way to get a due date. In these situations, skip the LMP method and wait for the scan.
Frequently asked questions
How is the due date calculated if I have irregular periods?
You need a confirmed ovulation date. That can come from tracking your basal body temperature, using ovulation strips, or a fertility app. Otherwise, you must wait for a first-trimester ultrasound. Using Naegele’s rule with an irregular cycle is mostly 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, fetuses start to grow at their own pace. Later ultrasounds become less accurate for dating. The very early scan remains the most precise.
How long is a “normal” pregnancy?
Full-term pregnancy is officially between 37 and 42 weeks. If a baby is born before 37 weeks, it is preterm. If it is after 42 weeks, it is post-term, and doctors usually recommend inducing labor. A 2020 study found the average pregnancy length to be 39 weeks and 3 days.
Why does the due date use 280 days and not 9 months?
“Nine months” is imprecise because months vary in length. Pregnancy dating starts from the last menstrual period, which adds about two weeks before conception. So 40 weeks × 7 days equals 280 days. That is roughly nine calendar months plus one week.
How do online due-date calculators work?
Most online calculators use Naegele’s rule. They often let you adjust for cycle length. Some let you enter ultrasound CRL measurements to override the LMP date. The CalcaTools due date calculator supports all these options.
Wrapping up
Treat your due date as an educated guess, not a firm appointment. Naegele’s rule gives a starting point. Your first-trimester ultrasound usually refines that date. In the end, your baby decides the actual day. Use your due date to plan tests, scans, and maternity leave. Then let your baby surprise you on their arrival. Use the CalcaTools pregnancy due date calculator along with our pregnancy week-by-week calculator to get a full timeline.
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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