How Accurate Are Due Dates in the First 6 Weeks?
If you’ve just learned you’re pregnant and used a calculator to estimate a due date, you might wonder how reliable that number is. The short answer: it’s an estimate, not a guarantee. Here’s what that means in practice.
Estimates, not certainties
Due dates in the first 6 weeks are almost always based on the first day of your last menstrual period. That method assumes a typical cycle length and ovulation timing. In reality, cycles vary. Ovulation can shift. So the “exact” due date you see is best understood as the center of a range.
Many providers and calculators use a confidence window: the due date plus or minus several days (often around ±5 to ±14 days). That range reflects real variation in how pregnancy develops. Research cited by organizations like ACOG suggests that LMP-based dating can be off by a week or more in a meaningful number of pregnancies.
What affects accuracy
Several factors influence how accurate an early due date is:
- Cycle regularity. Regular cycles make LMP-based dating more reliable. Irregular cycles introduce more uncertainty.
- Ovulation timing. The standard rule assumes ovulation around cycle day 14. If you ovulate earlier or later, the estimate may be off.
- Recall. The first day of full flow (not spotting) is what matters. Approximate dates are often used when the exact day isn’t clear.
When dates may change
An early ultrasound often provides a different date. Many providers will adjust the due date based on that scan if it differs significantly from the LMP calculation. That doesn’t mean the original estimate was wrong; it means early dating has limits and ultrasound adds information.
The NHS and CDC both note that early scans are commonly used to refine dates. If your provider changes your due date, they can explain why and what to expect going forward.
It’s also worth knowing that some providers stick with the LMP date if it’s consistent with the ultrasound. Others prefer the ultrasound date when there’s a discrepancy. There isn’t a single universal rule; practices vary. What matters is that your provider uses a consistent date for your care.
What you can do
Treat early due dates as estimates. Use a confidence window if one is shown. Avoid planning around a single day. When you have your first appointment, your provider can discuss which date to use and whether an early ultrasound would be helpful.