Why Your Due Date Might Change: LMP vs. Ultrasound Dating

It’s common to get one due date from a period-based calculation and a different one at your first ultrasound. This isn’t an error, it’s actually the more accurate number replacing an estimate. Here’s what’s really going on.

How the first due date is usually calculated

The standard starting method, Naegele’s Rule, adds 7 days to the first day of your last menstrual period (LMP) and subtracts 3 months. This is exactly the calculation used in our pregnancy due date calculator.

Why LMP dating isn’t always precise

Naegele’s Rule assumes a 28-day cycle with ovulation on day 14. According to the American College of Obstetricians and Gynecologists (ACOG), this doesn’t account for irregular cycles, inaccurate recall of the LMP, or variation in ovulation timing, and roughly half of women don’t recall their LMP with precision.

Why ultrasound dating often wins

ACOG identifies first-trimester ultrasound (up to 13 weeks 6 days) as the most accurate method for establishing gestational age, since it measures the fetus directly rather than relying on cycle assumptions. In one clinical study, 40% of women had their estimated due date adjusted because of a discrepancy of more than 5 days between ultrasound and LMP dating.

When doctors will actually change your due date

ACOG guidance generally supports revising the due date when there’s a meaningful discrepancy between LMP and first-trimester ultrasound dating, since ultrasound measurement is considered more reliable in that window. Once a due date is confirmed by early ultrasound, it typically isn’t changed again except in unusual circumstances.

What this means practically

• A due date shift after your first ultrasound is normal and expected, not a sign something is wrong.
• The earlier the ultrasound (ideally in the first trimester), the more reliable it is for dating purposes.
• If you have irregular cycles or aren’t sure of your LMP, expect ultrasound dating to be more trustworthy than the calendar-based estimate.
• Only about 5% of babies are actually born on their exact calculated due date; most arrive within a week or two of it either way.

Common mistakes

• Treating the LMP-based due date as final before an early ultrasound has a chance to confirm or adjust it.
• Assuming a due date change means a mistake was made, when it usually just reflects more precise information becoming available.
• Panicking over normal variation in delivery timing, since full-term delivery has a wide, medically normal range around the due date.

If you’re tracking early symptoms alongside your dates, see our list of early signs of pregnancy before a missed period for what’s normal in these first few weeks.