Why the dates differ
LMP dating assumes you ovulated on day 14 of a 28-day cycle. In reality, ovulation can occur several days earlier or later – even in women with regular cycles. If you ovulated on day 19, your baby is five days “younger” than LMP dating assumes, and will measure small on an early scan. Other reasons include uncertain recall of the LMP, bleeding in early pregnancy mistaken for a period, and irregular cycles.
Why early scans are so accurate
In the first trimester, embryos grow at a remarkably uniform rate. Measuring crown-rump length (CRL) between about 7 and 13+6 weeks predicts gestational age to within about ±5–7 days. Genetics and nutrition haven’t yet had time to make one baby noticeably bigger than another.
The re-dating rules
ACOG recommends changing the due date to the ultrasound date when the difference exceeds:
| GA by LMP at scan | Threshold |
|---|---|
| ≤ 8w 6d | > 5 days |
| 9w 0d – 15w 6d | > 7 days |
| 16w 0d – 21w 6d | > 10 days |
| 22w 0d – 27w 6d | > 14 days |
| ≥ 28w 0d | > 21 days |
What about later scans?
From the second trimester, babies grow at different rates. A third-trimester scan that says your baby is “two weeks ahead” usually means a bigger baby, not a wrong due date. Pregnancies without any scan before 22 weeks are considered “suboptimally dated”, and your doctor will interpret later measurements with more caution.
Got your first-trimester scan report? Enter it in the Ultrasound tab of the due date calculator.
Frequently asked questions
My scan says I’m less pregnant than my LMP suggests. Is something wrong?
Usually not – most often you simply ovulated later than day 14. Your doctor will decide whether to change your dates and may repeat the scan.
Can a due date change in the third trimester?
Rarely. Once a due date has been confirmed by an early scan, it isn’t changed because of later growth scans.
Medical disclaimer: The results and information on this page are estimates for general education only. They are not a diagnosis and do not replace advice from your gynaecologist, obstetrician or midwife. If you have pain, bleeding, reduced baby movements or feel unwell, contact your doctor or call 108 immediately. Read the full disclaimer.
Sources & further reading
- American College of Obstetricians and Gynecologists (ACOG). Committee Opinion No. 700: Methods for Estimating the Due Date. acog.org