GUIDE深度 · Deep dive
Due Date vs Ultrasound
A due date is an estimate, not an exact delivery day. The LMP method adds 280 days; an early ultrasound adjusts by fetal size. When they disagree, the early scan usually wins.
The LMP method
Add 280 days to the first day of the last period; adjust for cycle length if it is not 28 days.
If periods are irregular or the date is uncertain, LMP alone can be far off.
How ultrasound adjusts
An early scan (about 8 to 13 weeks) using crown-rump length is most accurate; later scans are less precise and usually do not change the date.
Clinicians use a discrepancy threshold, so not every scan changes the due date.
FAQ
Will I deliver on the due date?
Only about 4 percent of babies arrive on the due date; two weeks either side is common.
What if my cycles are irregular?
Use an early ultrasound; it is more reliable than LMP alone.