Ultrasound vs LMP: Which Due Date Should You Use?

Two ways to date a pregnancy

From your last menstrual period (LMP)

By convention, the estimated due date is 280 days after the first day of the last menstrual period.¹ It needs nothing more than a date you may already know, which is why it’s usually the first due date you get. Its weakness is its assumptions: a regular 28-day cycle with ovulation on day 14. It doesn’t account for an inaccurately remembered date, irregular cycles or variation in when ovulation happened.¹ The full method is explained in how a due date is calculated.

From an ultrasound scan

A dating scan uses ultrasound to estimate how far along the pregnancy is. Ultrasound measurement up to and including 13 weeks 6 days is the most accurate method to establish or confirm how far along a pregnancy is.¹ In the UK, the NHS notes that the 12-week scan can more accurately estimate how many weeks pregnant you are.²

Whichever method is used, the result is expressed the same way: as gestational age counted from the first day of the last period, in completed weeks and days.³ That’s why a scan result such as “12 weeks 3 days” can be compared directly with the date from your period.

Which is more accurate: ultrasound or LMP?

For establishing or confirming how far along a pregnancy is, an ultrasound up to 13 weeks 6 days is described as the most accurate method.¹ That doesn’t make the period-based date useless. It is available from the start, it is the date the scan is compared against, and when the two agree closely, guidance keeps the original date.¹

Can an ultrasound change your due date?

Yes, but only when the difference is large enough. ACOG guidance describes comparing the ultrasound with the LMP date and changing the due date when the difference is larger than a set number of days. That number gets larger the later the scan takes place:¹

This table is shown to explain the guidance. It isn’t something to apply to your own scan: your midwife or doctor takes your full history into account when deciding which date to use.

Why your scan and your period may give different dates

A difference between the two dates doesn’t in itself mean something is wrong. The period-based date rests on assumptions that won’t fit every pregnancy:¹

• Cycle length. If your cycles are longer, shorter or irregular, ovulation may not have been around day 14. See due dates with irregular periods.
• Ovulation timing. Even in regular cycles, the day of ovulation varies.
• Remembering the date. The first day of a period can be hard to recall exactly, especially weeks later.

The scan provides an independent estimate. If the two are close, the original date stays; if they’re further apart than the threshold for that stage, guidance says the due date should change.¹

Once a due date is set, does it keep changing?

No — it shouldn’t. Once the due date has been set from the LMP and/or the first accurate ultrasound, later changes should be reserved for rare circumstances.¹ So a later scan that measures differently doesn’t usually move your due date. If you have questions about a later scan result, your midwife or doctor can explain what it means for you.

What about IVF pregnancies?

If the pregnancy resulted from assisted reproduction, the ART-derived gestational age should be used to set the due date.¹ That date comes from the embryo age and transfer date: 261 days after transfer for a day-5 embryo and 263 days for a day-3 embryo.¹ Your fertility clinic can confirm the dates that apply to you.

A worked example

Suppose your last period started on 9 July 2026. By the 280-day convention, your estimated due date is 15 April 2027, and on that basis you would reach 13 weeks 6 days on 14 October 2026. If you have a scan before then, it is compared with this period-based date using the thresholds above. Whether the date changes, and to what, is decided by the professionals who carried out and interpreted your scan.

Which due date should you use?

If your healthcare team has given you a due date, use that one — it may already take a scan into account. You can enter it in the Pregnancy Weeks Calculator to see how far along you are today. If you haven’t had a scan yet, you can calculate your estimated due date from your last period as a starting point until your dates are confirmed.

Frequently asked questions

Can my due date change at the 12-week scan?

It can. A scan up to 13 weeks 6 days is the most accurate way to confirm how far along you are, and under ACOG guidance the due date is changed if the scan differs from your period-based date by more than 7 days between 9 weeks 0 days and 13 weeks 6 days.¹

Will a later scan change my due date?

If your due date has already been set from your period and/or an earlier accurate scan, later changes should be rare.¹ The thresholds for a first scan later in pregnancy are also wider — more than 10 days between 16 weeks 0 days and 21 weeks 6 days, for example.¹

Is the ultrasound date always the one that’s used?

Not automatically. The scan is compared with your period-based date, and the date changes only if the difference exceeds the threshold for that stage of pregnancy.¹ For IVF pregnancies, the transfer-based date is used.¹

Can I use my scan due date in the calculators?

Yes. Enter it in the weeks calculator to see how far along you are, or in the conception date calculator for a conventional conception estimate that matches it.

Medical statements in this article are linked to the numbered sources listed below: published clinical guidance and peer-reviewed research. It has not been reviewed by a named clinician. Read how content is written in our editorial policy, and see our medical disclaimer.