You worked out a due date from your last period, then the first ultrasound came back with a different one. This happens all the time, and it is almost never cause for worry. It just means your pregnancy is now being dated more precisely.
Two ways to date a pregnancy
The first estimate leans on your last period (LMP) and assumes a 28-day cycle with ovulation on day 14. The second comes from an early ultrasound, which measures the embryo itself. When the two disagree, it really comes down to which one to trust. The ultrasound vs LMP tool lays the difference out side by side.
Why the scan usually wins
Between 8 and 13 weeks, embryos grow at remarkably similar rates, so a scan in this window can pin the dates to within about five days either way. If your cycle ran longer or shorter than 28 days, or you ovulated late, the last-period estimate is off in a predictable direction, and the scan quietly corrects it. The usual rule of thumb: if the scan and the LMP date differ by more than about 5 to 7 days, your provider goes with the scan.
What the new date means for you
If the due date moved later, you are a touch less far along than you thought. If it moved earlier, a touch more. Either way, everything downstream shifts with it: screening windows, the anatomy scan, and the due date itself. It is worth re-checking your timeline in the due date calculator and confirming your current week in the pregnancy week calculator using whatever date your provider settled on.
When to ask questions
A big gap between the two dates, or a later scan showing growth well outside what was expected, is worth raising with your provider, who may want to follow up.