Why the two dates differ
- Ovulation may not have happened on day 14. The period method assumes it did, adjusted only for average cycle length.1
- The remembered period date may be off, or the bleed may not have been a normal period.
- Measurements have error. Early on, babies are very similar in size, so a first-trimester scan is accurate to within about 5 to 7 days; later, natural differences in size make scans less precise for dating.2
The thresholds clinicians use
ACOG, AIUM and SMFM recommend using the ultrasound date when it differs from the period date by more than these amounts, depending on gestational age (by the period date) at the scan.2
| Gestational age at the scan | Use the ultrasound date if the difference is more than |
|---|---|
| Through 8 weeks 6 days | 5 days |
| 9 weeks 0 days to 13 weeks 6 days | 7 days |
| 14 weeks 0 days to 15 weeks 6 days | 7 days |
| 16 weeks 0 days to 21 weeks 6 days | 10 days |
| 22 weeks 0 days to 27 weeks 6 days | 14 days |
| 28 weeks 0 days and beyond | 21 days |
The rule says “more than”, so a difference exactly equal to the threshold doesn’t trigger a change. Clinicians also consider how sure you are of your period date. Our ultrasound calculator applies these thresholds if you enter both dates.
Once set, the date should rarely change
The due date should be set as early as possible and documented. After that, it should be changed only in rare circumstances. Later scans that show a baby measuring ahead or behind are information about growth, not age; moving the date to match a late scan could hide a growth problem.2
For pregnancies conceived with IVF or other assisted reproduction, the due date comes from the IVF dates, not from ultrasound.2
Questions to ask your care team
- Which date are you using as my due date, and why?
- Was it set from my period, a scan, or both?
- If a later scan shows a different size, what does that mean for my baby’s growth?
In the UK, the dating scan is offered between 11 weeks 2 days and 14 weeks 1 day.3 In the US, timing varies by practice.