Ultrasound Due Date vs LMP Due Date: Why They Differ and Which One Counts

If your dating scan gives a different due date from the one you worked out from your last period, the scan usually wins, but only when the gap is bigger than a set number of days. In early pregnancy (up to about 9 weeks) the scan date replaces the period date if the two differ by more than 5 days. Between roughly 9 and 14 weeks, the gap has to be more than 7 days. Smaller gaps are ignored and the period-based date stays. Once a due date is fixed, later scans do not change it.

That rule surprises many women. You may have been told "eight weeks" at home and "seven weeks and three days" in the scan room, and nobody explained why one number mattered more than the other. This guide explains where the difference comes from, how doctors decide, and what it means for the rest of your pregnancy.

Why the two dates can disagree

Your period-based date (LMP date) assumes three things: you remember the first day of your last period correctly, your cycle is 28 days long, and you ovulated on day 14. Each of those can be wrong.

1. You ovulated later (or earlier) than day 14

This is the most common reason. Ovulation timing varies from cycle to cycle even in women whose periods arrive on time. If you ovulated on day 21, the baby is a week younger than the LMP suggests.

2. Your cycle isn't 28 days

A 35-day cycle puts ovulation near day 21, as we explained in our guide to calculating the due date. Count from the LMP alone and you will be a week ahead of the real pregnancy.

3. The LMP is uncertain

Many women are not sure of the date, especially if periods are irregular, if there was recent spotting, or if the last period was a lighter bleed than usual. Implantation bleeding is sometimes mistaken for a period.

4. You conceived soon after stopping the pill, a miscarriage or breastfeeding

Cycles after these events are often irregular, and ovulation can happen before a normal period returns. There may be no real "last period" to count from.

5. The baby is naturally a little bigger or smaller

Embryos grow at almost the same pace in the first trimester. Later, genetics, the mother's build and placenta function all change how fast a baby grows, which is why late scans are poor at dating.

Why the early scan is trusted more

In the first trimester, the sonographer measures the crown-rump length (CRL), the distance from the top of the baby's head to the bottom of the torso. At this stage nearly all healthy embryos are almost the same size at the same age, so the measurement predicts gestational age to within roughly 5 to 7 days. A review of due date estimation in StatPearls (National Library of Medicine) gives that accuracy for early CRL, compared with a margin of several weeks for dating by LMP and clinical examination alone.

After about 14 weeks, dating moves to other measures: head circumference, abdominal circumference and femur length. These are less precise because babies of the same age differ more in size, and the error widens week by week. By the third trimester, the margin can be about three weeks either way.

The rules doctors use to change a due date

The American College of Obstetricians and Gynecologists (ACOG) publishes the thresholds most clinics follow. If the scan and the LMP differ by more than the number below, the scan date becomes the official one.

| Gestational age at the scan | Change the due date if the difference is more than | |---|---| | Up to 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 and later | 21 days |

The tolerance grows because ultrasound gets less exact as pregnancy advances. A 5 day gap at 8 weeks is a real signal. A 5 day gap at 30 weeks is simply normal variation in baby size.

Four worked examples

All four use an LMP of 10 March 2026, which gives an LMP due date of 15 December 2026 (280 days later).

Example 1: a small gap, no change

The scan is on 5 May 2026. By LMP you are exactly 8 weeks 0 days. The CRL measures 7 weeks 3 days. The difference is 4 days. That is within 5 days, so the LMP date stays: 15 December 2026.

Example 2: a larger gap, the date moves

Same LMP, same scan day. This time the CRL measures 6 weeks 5 days, which is 8 days behind your LMP age. Eight days is more than 5, so the scan wins. Your new due date is 8 days later: 23 December 2026. From now on, "how many weeks am I?" is counted from this new date.

Example 3: the gap sits right on the line

You have a scan at 12 weeks 0 days by LMP. The CRL says 11 weeks 0 days, a 7 day gap. The 9 to 14 week rule says change if the difference is more than 7 days. Exactly 7 does not meet that, so the LMP date stays.

Example 4: a long cycle explains the gap

Your cycles run 35 days. By LMP you are 8 weeks 0 days on 5 May, but the CRL says 7 weeks 0 days. The gap is 7 days, which is over the 5 day limit at this stage. The new due date is 22 December 2026. It makes sense, too: ovulation on day 21 puts conception about 7 days later than the standard calculation, which pushes the true due date a week later.

"My scan says smaller than my dates." Is the baby too small?

Not at a dating scan in the first trimester. The measurement is used to set the dates, not to judge growth. If the baby measures behind your LMP, the likeliest explanation is later ovulation, not a growth problem.

The situation is different if your dates were already fixed and a later scan finds the baby smaller than expected. Then the doctor compares the baby's size with the established due date and may recommend growth scans or Doppler studies. In that setting, "small for dates" means something, and it is the reason doctors do not re-date later in pregnancy: changing the dates to match the baby would hide a growth problem.

What if your dates are fixed by IVF?

If you conceived through IVF, the embryo transfer date gives an exact start point, so the due date is calculated from that (transfer date plus 261 days for a day-5 blastocyst, or plus 263 days for a day-3 embryo). Ultrasound does not override it in the usual way, because the conception date is known to the day.

What changes when the due date changes

A new due date alters:

• The weeks you are told you are at, today.
• The timing of screening tests such as the first-trimester scan, the anomaly scan (usually done around 18 to 20 weeks) and the glucose tolerance test (usually at 24 to 28 weeks).
• When vaccines are due. The whooping cough (Tdap) vaccine is usually offered in the third trimester, and the timing is counted from your weeks.
• When your doctor discusses induction if you go past term.
• The date you apply for maternity leave. Check the official due date in your hospital records.

Because so many appointments hang on the date, ask your doctor which date is on your file, and use it consistently.

Which date should you celebrate?

Both. The LMP date is a fine guide for planning. The scan date is the working date for care. Remember that only about 4 to 5 percent of babies arrive on the due date, and most arrive within two weeks either side. A date that moved by a week matters far less than it feels like in the moment.

Common worries

• "The doctor changed my date, and now I'm a week behind." You aren't behind. Your baby is exactly the age the early scan says.
• "The dates are different on two scans." Dates do not get re-set at later scans. Different machines and operators give slightly different measurements, but the due date on your file stays put unless a doctor deliberately re-dates.
• "The online calculator gave me a different date." Online tools use the LMP. Your doctor uses the best available evidence, which is usually the early scan.
• "My cycle is irregular and my doctor wants a scan." That's the right call. An early scan is the most reliable way to date such pregnancies.

How to prepare for your dating scan

• Note the first day of your last period and, if you track it, your usual cycle length.
• Bring any earlier test results, such as a home pregnancy test date or ovulation test results.
• Tell the sonographer if you used fertility treatment.
• Drink water as advised by your clinic if a full bladder is required for a transabdominal scan.
• Expect a vaginal scan if you are under about 8 weeks, since it gives a clearer view in early pregnancy.

Frequently asked questions

Why is my ultrasound due date different from my LMP?

Most often because you ovulated earlier or later than day 14, or your cycle is not 28 days. The early scan measures the embryo directly, so it reflects its real age.

Which due date is more accurate, ultrasound or LMP?

An ultrasound in the first trimester, which dates pregnancy to within about 5 to 7 days. LMP dating is only as good as your memory and your cycle regularity.

How many days difference before the due date is changed?

More than 5 days up to 9 weeks, more than 7 days from 9 to 15 weeks, more than 10 days from 16 to 22 weeks, and wider gaps after that.

Can the due date change at the 20 week scan?

Usually it does not, if the due date has already been set by an earlier scan. If there was no early scan, a mid-pregnancy scan can set it, but with a wider margin of error.

Is it normal for the ultrasound to be a week behind the LMP?

Yes. It's common, and often points to later ovulation.

Does the due date matter if the baby comes when it comes?

It sets the schedule for tests, vaccines and the timing of any discussion about induction. It isn't a deadline for the baby.

When to see a doctor

Book an early scan as soon as you have a positive test if you have irregular cycles, an uncertain LMP, or a history of miscarriage or ectopic pregnancy. Seek urgent care for heavy bleeding, severe one-sided pain, dizziness or fainting at any stage.

Care at Prakash Hospital Noida

Whether you live in Sector 18, Sector 62, Greater Noida West, or anywhere nearby, Prakash Hospital Noida is a trusted name for antenatal care and obstetric ultrasound in Noida. Our doctors confirm your dates carefully and explain every change in plain words.

To book a consultation, call the number.

Sources: Estimated Date of Delivery. StatPearls, National Library of Medicine (NBK536986). ACOG Committee Opinion No. 700, Methods for Estimating the Due Date, 2017.

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