Has your due date changed after an ultrasound? Or does the date from your scan not match what you calculated from your last period or conception date? This is one of the most common questions we hear in our Dublin clinics — and there is almost always a simple explanation.
Your due date matters for more than planning baby's arrival. It sets the schedule for prenatal tests like the nuchal translucency scan (best at 11–14 weeks), helps your doctor interpret blood results, and is the baseline for spotting growth concerns later on. With so much depending on it, it's worth understanding where the number comes from — and why it sometimes moves.
The Three Ways a Due Date Is Calculated
| Method | How it works | Typical accuracy |
|---|---|---|
| Last menstrual period (LMP) | Adds 280 days (40 weeks) to the first day of your last period | Assumes a 28-day cycle and day-14 ovulation — often off by days or weeks |
| Conception or ovulation date | Adds 266 days (38 weeks) to the date of conception or ovulation | Better if the date is certain, but ovulation is hard to pin down exactly |
| Early ultrasound (dating scan) | Measures the embryo's crown rump length (CRL) at ~8 weeks | Accurate to within ± 5 days |
You can compare all three yourself with our free due date calculator — it works from your last period, conception date, or IVF transfer date.
The Problem with LMP Calculations
The traditional method of calculating due dates assumes:
- A 28-day menstrual cycle
- Ovulation on day 14
However, normal menstrual cycles vary between 21 and 35 days — and some fall outside even that range. Regular-as-clockwork cycles are less common than most people think. If your cycle is longer or shorter than average, or you ovulated earlier or later than expected, your LMP-based due date can be off — sometimes by more than a week, occasionally by as much as four.
We still start with your LMP because the first day of a period is an identifiable event, while ovulation and fertilisation are not. It's a guideline, not a verdict.
Due Date Based on Conception or Ovulation
If you know your conception or ovulation date — from ovulation tracking, or a single possible conception date — you can count 266 days (38 weeks) forward instead. This removes the cycle-length guesswork but still relies on knowing exactly when ovulation happened, and sperm can fertilise an egg several days after intercourse. That's why even a confident conception date can disagree with an early scan by a few days.
For IVF pregnancies the maths is different and more precise: the due date is calculated from the egg collection or embryo transfer date (adjusted for a day-3 or day-5 transfer), not from your LMP. Our due date calculator has a dedicated IVF option for this.
The Gold Standard: an Early Dating Scan
An early pregnancy scan at approximately 8 weeks gives the most accurate due date prediction. Your sonographer measures the crown rump length (CRL) — the distance from the top of the embryo's head to its bottom. At this stage embryos grow at a very predictable rate, which is what makes the measurement so reliable.
Measuring the embryo at this stage is accurate to within ± 5 days.
Ultrasound dating gets less precise as pregnancy progresses
- 8–13 weeks: ± 5 days — the most accurate window
- Second trimester: roughly ± 1–2 weeks
- Third trimester: roughly ± 2–3 weeks
This is why a due date is rarely changed based on a later scan: by then, normal variation in baby's size is bigger than the dating uncertainty. If you're past the early window, a later dating scan between 11 and 24 weeks can still confirm dates and growth.
When Will Your Due Date Actually Change?
It is very common for an early scan to disagree with menstrual dates — sometimes by more than a week, occasionally by as much as four weeks. But a new estimated due date is assigned only when the CRL measurement falls outside the ± 5-day window around your LMP-based date. If the difference is smaller than that, your original date stands, because the scan can't prove the LMP date wrong.
So after your scan there are two possible outcomes:
- Your dates agree (within 5 days): the LMP due date is kept.
- They disagree by more than 5 days: the ultrasound date replaces it, because the embryo's measured size is the more reliable evidence.
Why an Accurate Due Date Matters
Precise dating is essential for:
- Scheduling the nuchal translucency screening (only valid at 11–14 weeks)
- Interpreting blood test results, which are gestation-dependent
- Identifying growth concerns at later growth scans
- Timing decisions around delivery, if they're ever needed
Frequently Asked Questions
Can my due date change after an ultrasound?
Yes — if the early scan measurement differs from your LMP-based date by more than 5 days, the ultrasound date is adopted. This happens often and simply means the scan has produced better information than the calendar could.
How accurate is an ultrasound due date?
At 8–13 weeks, to within about ± 5 days. Accuracy decreases later in pregnancy, which is why the earliest scan's date is the one that sticks for the rest of your pregnancy.
My due date from conception doesn't match my scan — which is right?
Usually the scan. Even a well-known conception date involves some biological uncertainty about when fertilisation actually occurred, while the embryo's measured size follows a very predictable growth curve in early pregnancy.
Do only about 4–5% of babies arrive on their due date?
Roughly, yes — a due date is the middle of a range, not an appointment. Most babies arrive within two weeks either side of it.
What if I don't know my LMP at all?
Book an early dating scan. The CRL measurement alone can establish your due date without any menstrual history. Our due date calculator can then keep you oriented week by week, alongside our week-by-week pregnancy guide.
Get accurate dating with an early pregnancy scan at Ultrasound Ireland — or browse all our pregnancy scans in Dublin. Call 01 210 0232.
This article is for general information only and is not medical advice. It is not a substitute for professional diagnosis or treatment — always speak to your GP or another qualified healthcare provider about your own situation.