LMP Dating vs. Ultrasound Dating: Which Due Date Should You Trust?

How last-period dating and early ultrasound dating can produce different due dates, and why doctors often adjust to whichever is more reliable for your situation.

It's common to get two slightly different due dates — one from a calculator based on your last period, and one from your first ultrasound. Both are legitimate estimates; they just start from different assumptions.

LMP dating

Naegele's Rule assumes a standard 28-day cycle with ovulation on day 14. It's simple to calculate at home and requires no appointment, but it's only as accurate as your memory of your last period's start date and how close your actual cycle was to 28 days that month.

Ultrasound dating

An early ultrasound (typically 8-13 weeks) measures the embryo or fetus directly — usually crown-rump length in the first trimester — and compares it to standardized growth charts to estimate gestational age. Because it's based on actual measured size rather than a remembered date, it's generally considered the more accurate method, especially for people with irregular cycles or an uncertain LMP date.

When doctors adjust the due date

If an early ultrasound estimate differs from LMP dating by more than about 5-7 days (in the first trimester), most providers will adjust the official due date to match the ultrasound. Smaller discrepancies are usually left as-is, since some natural variation in fetal size is expected.

Use the Due Date Calculator to get an LMP- or conception-based estimate to plan around, then treat your ultrasound-confirmed due date from your provider as the more authoritative number going forward.