Can your due date change?

Why the first estimate can be off

Calculating from a last menstrual period assumes two things: that you remember the date accurately, and that ovulation happened at a typical point in your cycle. Both are reasonable assumptions and both are frequently wrong by a few days.

Ovulating later than usual in the cycle you conceived would make the pregnancy less far along than the dates suggest, and the scan will show it.

What an early scan measures

In the first trimester, an ultrasound measures the crown-rump length, the length of the embryo from head to bottom. Early growth is consistent enough across pregnancies that this measurement dates the pregnancy closely.

Where that measurement disagrees with the calculated date by more than a small margin, care teams typically adopt the scan's date. Thresholds vary by guideline and by how early the scan was, and your care team will explain what they are using.

Why later scans are not used for dating

As pregnancy progresses, growth varies much more between individuals. A third-trimester scan measuring a larger or smaller baby is telling you about growth, not about dates.

This is why a due date set in the first trimester generally stands for the rest of the pregnancy, even if later measurements look ahead of or behind it.

What a change means in practice

A revised due date shifts the schedule of tests and appointments, which is the main practical effect. Some screening tests are only valid in particular week ranges, so accurate dating matters for them.

A change of a few days is administrative, not medical. It does not mean anything has gone wrong, and it does not change how the pregnancy is progressing.

Once you have a revised date, use it in these calculators rather than your last period. Entering the new due date keeps every figure in step with your notes.