My Due Date Moved Twice Before My First Ultrasound — Here's How Pregnancy Due Dates Are Actually Calculated
The app I downloaded the week I found out I was pregnant gave me a due date the moment I entered my last period. Two weeks later, my doctor's office gave me a different one — three days earlier. Then, at my 8-week ultrasound, the sonographer measured the fetus and moved it again, this time five days later than either previous guess. Same pregnancy, three "due dates" in six weeks, and nobody at any of those appointments seemed remotely concerned about it.
That's because a due date was never meant to be a prediction of the exact day you'll give birth. It's a statistical estimate built on a 200-year-old formula, and understanding how that formula actually works — and why doctors keep adjusting it — makes the whole process a lot less confusing.
⚠️ Disclaimer
This article is for informational purposes only and does not constitute medical advice. Every pregnancy is different — always follow the dating and care guidance given by your own doctor, midwife, or healthcare provider.
How a Due Date Is Actually Calculated (Naegele's Rule)
A pregnancy due date, formally called the estimated date of delivery (EDD), is calculated by adding 280 days — 40 weeks — to the first day of your last menstrual period (LMP). This method is known as Naegele's rule, named after the 19th-century German obstetrician Franz Naegele, and it has been the standard in obstetric practice since the 1850s.
The quirk almost nobody explains up front: pregnancy is dated from your LMP, not from conception — even though you obviously aren't pregnant yet on day one of your last period. Naegele's rule assumes a textbook 28-day cycle with ovulation on day 14, so by the time conception actually happens, you're already "2 weeks pregnant" by the clock the calculator is running.
If you know your conception date instead — from IVF, an ovulation test, or a single-cycle tracking app — the math shifts to account for that missing two weeks:
Here's the LMP version worked through with real dates. Say your last period started May 5, 2026. Naegele's traditional shortcut for doing this by hand — subtract 3 months, add 7 days, add 1 year — gets you there without counting out 280 days one by one:
| Step | Date |
|---|---|
| Last menstrual period | May 5, 2026 |
| Subtract 3 months | Feb 5, 2026 |
| Add 7 days | Feb 12, 2026 |
| Add 1 year | Feb 12, 2027 |
That lands on the same date as counting 280 days forward from May 5 — Naegele's rule was designed as a mental-math trick for exactly this, decades before calculators existed.
Key Takeaway
A due date is a 40-week estimate from your LMP, not a prediction of the day labour will start. It exists to give your care team a planning anchor — not a deadline.
If you'd rather skip the manual subtraction, our Pregnancy Due Date Calculator runs both the LMP and conception-date methods for you and shows your current trimester alongside the estimate.
Why an Ultrasound Can Move Your Due Date
Naegele's rule has one major weak point: it assumes every person has an identical, regular 28-day cycle with ovulation right on day 14. In reality, cycle length varies widely between individuals and even between cycles for the same person, which means the LMP method can be off by a week or more before anyone measures the fetus directly.
That's why a first-trimester ultrasound is considered the gold standard for pregnancy dating. Ultrasound dating estimates gestational age by measuring the fetus — typically the crown-rump length before 14 weeks — and comparing it against standardized growth charts built from thousands of confirmed pregnancies. The American College of Obstetricians and Gynecologists (ACOG) recommends that when a first-trimester ultrasound and LMP-based dating disagree by more than 5–7 days (depending on gestational age), the ultrasound date should be used instead.
This is exactly what happened to me. My LMP-based due date and my 8-week ultrasound date differed by 5 days — right at the threshold where ACOG guidance says the scan wins. My chart got updated, and that became my "real" due date for the rest of the pregnancy.
💡 Pro Tip
Once a due date is confirmed by a first-trimester ultrasound, it's generally not adjusted again by later scans — third-trimester ultrasounds measure the baby's size, not the pregnancy's true age, and size can vary for reasons that have nothing to do with dating accuracy.
Only about 5% of babies are actually born on their exact estimated due date, according to figures widely cited by ACOG and the UK's NHS. Most arrive within a two-week window either side of it. A birth at 39–40 weeks is classified as full term; 37–38 weeks is early term; 41–42 weeks is late term — labels that matter more to your care team's monitoring plan than to any single "deadline" on a calendar.
What "40 Weeks" Really Means
The two-week LMP-to-conception gap causes a second point of confusion: gestational age and fetal age are not the same number, and both get used casually in conversation.
Gestational age is how far along a pregnancy is, counted from the first day of the last menstrual period — this is the number your doctor, midwife, and every due-date calculator use. Fetal age (sometimes called conceptional or embryonic age) is counted from the actual date of conception, and runs roughly two weeks behind gestational age for the same pregnancy.
So when a chart says "8 weeks pregnant," that's gestational age — the embryo itself is developmentally closer to 6 weeks old. Neither number is wrong; they're just answering two different questions, and mixing them up is the single most common source of "wait, that doesn't add up" moments in early pregnancy.
| Milestone | Gestational Age | Fetal Age (Approx.) |
|---|---|---|
| Positive pregnancy test | 4–5 weeks | 2–3 weeks |
| First heartbeat visible | 6 weeks | 4 weeks |
| First-trimester ultrasound | 8–12 weeks | 6–10 weeks |
| Viability milestone | 22–24 weeks | 20–22 weeks |
That viability figure is worth calling out on its own: fetal viability — the point at which a baby born prematurely has a reasonable chance of survival with intensive medical support — is generally considered to begin around 22–24 weeks gestational age, with survival odds improving meaningfully with each additional week, per data cited by major neonatal and obstetric bodies.
If you want to track exactly where you are week to week rather than recalculating by hand, the Weeks Pregnant Calculator converts either your LMP or conception date into your current gestational age and trimester automatically.
US vs UK: Same Math, Different Names
The underlying formula — 280 days from LMP — is identical in the US and the UK, and Naegele's rule is used internationally as the default starting point. What differs is the language and the scan schedule built around it.
In the US, care teams typically refer to the EDD (estimated date of delivery) and often confirm dating at the first prenatal visit, which ACOG suggests scheduling around 8–10 weeks gestational age. In the UK, the NHS refers to it as the EDD too, but nearly every pregnancy gets a dedicated dating scan between 10 and 14 weeks specifically to confirm gestational age via crown-rump length — it's a distinct, scheduled appointment rather than something folded into a general check-up.
| Term / Practice | United States | United Kingdom |
|---|---|---|
| Due date name | EDD | EDD |
| First dating check | ~8–10 weeks, at first prenatal visit | Dedicated dating scan, ~10–14 weeks |
| Governing body | ACOG | NHS / RCOG |
| Adjustment threshold | LMP vs. ultrasound discrepancy of 5–7+ days | Similar; ultrasound generally takes priority |
Whichever country you're in, the practical guidance converges on the same point: an early ultrasound is more reliable than LMP alone, and once it's confirmed, that date becomes the one your care team plans around — appointment scheduling, screening windows, and the definition of "term" all key off it.
⚠️ Note
If your cycles are irregular, run late, or you're not certain of your LMP date, tell your provider early — it's exactly the situation ultrasound dating exists to correct, and the sooner it's confirmed, the more reliable every later milestone and screening window will be.
If you're also tracking recommended weight gain alongside your due date, the Pregnancy Weight Gain Calculator uses the same trimester breakdown to show a healthy range based on your pre-pregnancy BMI.
Frequently Asked Questions
Why does my due date keep changing between appointments?
Early estimates from an app or your LMP date are based on an assumed 28-day cycle, which doesn't match everyone. Once a first-trimester ultrasound measures the fetus directly, that measurement is more accurate and typically becomes the date your care team uses going forward — it isn't usually revised again after that.
How accurate is a due date calculated from my last period?
The LMP method is generally accurate to within about ±2 weeks for people with regular cycles, and less reliable with irregular cycles or an uncertain LMP date. A first-trimester ultrasound narrows that window considerably and is considered the gold standard for dating.
What if my cycles are longer or shorter than 28 days?
Naegele's rule assumes a 28-day cycle, so a consistently longer or shorter cycle shifts your actual ovulation date away from the "day 14" assumption — moving your true due date without changing the formula's output. Your provider will factor this in, and an early ultrasound is the most reliable way to correct for it.
Is it normal to not go into labour on my due date?
Yes — very normal. Only around 5% of babies are born on their exact estimated due date, and most arrive within roughly two weeks either side of it. The due date is a planning estimate for your care team, not a deadline for the baby.
What's the difference between gestational age and fetal age?
Gestational age is counted from the first day of your last menstrual period and is the number used in routine pregnancy dating. Fetal (or conceptional) age is counted from the actual date of conception, roughly two weeks less than gestational age at any given point.
Try It Yourself
A due date is a 40-week estimate from your last period, refined by an ultrasound if your dates and cycle don't line up with the textbook assumption — not a promise about the exact day you'll meet your baby. Run your own numbers with the Pregnancy Due Date Calculator, track your current week with the Weeks Pregnant Calculator, and check your recommended weight gain range with the Pregnancy Weight Gain Calculator.
