Has your due date changed after an ultrasound? This is more common than you might think, and there's a good reason why.
Why Due Dates Differ
Discrepancies between ultrasound-based due dates and those calculated from your last menstrual period (LMP) are actually quite common.
The Problem with LMP Calculations
The traditional method of calculating due dates assumes:
- A 28-day menstrual cycle
- Ovulation on day 14
However, menstrual cycles vary between 21-35 days, making LMP calculations potentially unreliable. If your cycle is longer or shorter than average, or if you ovulated earlier or later than expected, your LMP-based due date may be off.
The Gold Standard: Early Ultrasound
Early pregnancy ultrasounds at approximately 8 weeks provide the most accurate due date predictions. This is done by measuring the Crown Rump Length (CRL) - the distance from the top of the embryo's head to its bottom.
How Accurate Is It?
Measuring the embryo on ultrasound is accurate to within plus or minus five days of the due date. This precision is invaluable for:
- Prenatal testing schedules
- Monitoring fetal development
- Planning for delivery
When Will Your Due Date Change?
A new estimated due date is assigned only when CRL measurements fall outside the five-day accuracy range from the LMP calculation. If the difference is minimal, your original date may be kept.
Why Accuracy Matters
Precise dating is essential for:
- Scheduling nuchal translucency screening (best at 11-14 weeks)
- Interpreting blood test results
- Identifying growth concerns
- Planning induction if needed
The Reassurance
Finding a different due date than expected is normal and not a cause for concern. It simply means your ultrasound has provided more accurate information than the LMP calculation alone.
Get accurate dating with an early pregnancy scan at Ultrasound Ireland. Call 01 210 0232.