Two ways to date a pregnancy
LMP dating counts from the first day of the last period. By convention, the estimated due date is 280 days after that day.1 It needs only a calendar, which is why it is usually the first estimate anyone has.
Ultrasound dating measures the baby instead. The sonographer estimates when the baby is due based on the baby's measurements.2 In early pregnancy (usually before 13 weeks) the scan measures the length from the baby's head to its bottom, called the crown-rump length, and compares it with population data to calculate the due date. Later in pregnancy, measurements of the head, thigh bone and abdomen are used.3
Why the scan date and the LMP date can differ
LMP dating assumes a regular 28-day cycle with ovulation on day 14, so it does not account for an LMP that is remembered inaccurately, for irregular cycle lengths, or for ovulation that happens earlier or later than usual.1 It has been reported that only about half of women accurately recall their LMP.1
A scan does not depend on any of those, so differences are common. In one study, 40% of the women who had a first-trimester ultrasound had their due date adjusted because the scan and LMP dates differed by more than 5 days, compared with 10% of a group whose first scan was in the second trimester.1 Irregular periods and your due date looks at the cycle side of this in more detail.
How accurate is ultrasound dating?
It depends on when the scan is done. Measurements of the crown-rump length are more accurate the earlier in the first trimester the scan is performed,1 and accuracy falls as pregnancy goes on:
| Weeks of pregnancy at the scan | Accuracy of the dating |
|---|---|
| Up to 13 weeks 6 days | ±5 to 7 days |
| 14 weeks 0 days to 21 weeks 6 days | ±7 to 10 days |
| 22 weeks 0 days to 27 weeks 6 days | ±10 to 14 days |
| 28 weeks 0 days and later | ±21 to 30 days |
Source for the table: ACOG Committee Opinion 700.1 Third-trimester ultrasound is the least reliable way to date a pregnancy.1
Scan dating also predicted the birth date better in a study of 34,249 pregnancies in which the mothers were sure of their LMP: births fell within 7 days of the due date in 55.2% of cases using ultrasound alone, against 49.5% using the LMP alone, and within 10 days in 70.3% against 64.1%. Scan dating also cut the number of pregnancies classed as postterm by 70%.4
When a due date is changed after a scan
ACOG, with the American Institute of Ultrasound in Medicine and the Society for Maternal–Fetal Medicine, publishes single cut-offs so that dating is consistent between clinics. If the scan and LMP dates differ by more than the amount below, the guidance is to change the due date to match the scan:1
| Weeks of pregnancy at the scan | Difference between scan and LMP dating |
|---|---|
| Before 9 weeks 0 days | More than 5 days |
| 9 weeks 0 days to 13 weeks 6 days | More than 7 days |
| 14 weeks 0 days to 15 weeks 6 days | More than 7 days |
| 16 weeks 0 days to 21 weeks 6 days | More than 10 days |
| 22 weeks 0 days to 27 weeks 6 days | More than 14 days |
| 28 weeks 0 days and later (first scan of the pregnancy) | More than 21 days |
Before 9 weeks, ACOG describes a difference of more than 5 days as an appropriate reason for a change. For smaller differences, clinicians weigh how early the scan was and how reliable the LMP date is. In the third trimester, decisions are guided by the whole clinical picture, because of the risk of redating a small baby who may not be growing as expected.1
Your care team makes this decision
These cut-offs are guidance for clinicians, shown here so the process makes sense. This site cannot compare your scan with your dates. If your due date has changed, your doctor or midwife can explain why.3
Once the due date is set
ACOG's guidance is to set the due date as soon as the LMP, the first accurate ultrasound, or both are known, and to reserve later changes for rare circumstances.1 If a first-trimester scan was done, especially one that agreed with the LMP, the guidance is that, with rare exceptions, the dating should not be adjusted because of a second-trimester scan.1
Pregnancy, Birth and Baby puts it the same way for patients: with a regular cycle and an LMP-based due date, or after an early dating scan, the due date is unlikely to change later in pregnancy. Without an early dating scan, a second- or third-trimester scan can be used to date the pregnancy.3 ACOG considers a pregnancy without a scan that confirms or revises the due date before 22 weeks 0 days to be suboptimally dated.1
When dating scans happen
Timing differs by country and care setting. In England, hospitals offer at least 2 ultrasound scans during pregnancy;2 the dating scan happens at around 10 to 14 weeks and is used to work out how many weeks pregnant you are and your due date.5 The 20-week scan follows between 18 and 21 weeks.2
If you don't know the date of your LMP, dating is based on ultrasound, ideally at or before 13 weeks 6 days, with the earliest crown-rump measurement treated as the most reliable.1 What if you don't know your LMP? covers what to do in that situation.
What this means for the calculators on this site
The calculators here use only the first day of your LMP. They give an LMP-based estimate, the starting point before any scan. If your care team has given you a due date from a scan, that is the date that applies to your care.
To follow your weeks from a scan-based due date, count back 280 days to the start of that count (see how to work out your LMP from your due date) and enter that date in the Pregnancy Weeks Calculator.
Start with your LMP-based estimate
Enter the first day of your last period to see the due date the 280-day count gives, the date a scan is compared with.
Common questions
Which is more accurate, an ultrasound due date or an LMP due date?
An ultrasound in the first trimester is the most accurate method to establish or confirm gestational age.1 In the study of 34,249 pregnancies, scan dating alone predicted the birth date better than the LMP, even when the LMP date was considered certain.4
Why did my due date change after my ultrasound?
Usually because the scan and LMP dates differed by more than the margin for the week of the scan, shown in the table above.1 Your doctor or midwife can tell you which applied to you.
Can my due date change again later?
Rarely. Once a due date has been set from the LMP and an early scan, later changes are reserved for rare circumstances.1
Is a later scan as good for dating as an early one?
No. Accuracy falls from about ±5 to 7 days in the first trimester to about ±21 to 30 days in the third.1
Sources
- American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date (Committee Opinion No. 700). 2017; reaffirmed 2025.
- NHS (England). Ultrasound scans in pregnancy. reviewed November 2023.
- Pregnancy, Birth and Baby (Healthdirect Australia, funded by the Australian Government). Working out your due date. reviewed August 2024.
- American Journal of Obstetrics and Gynecology. Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates. 174(1 Pt 1):278–81. 1996.
- NHS (England). 12-week scan. reviewed November 2023.