LMP vs Ultrasound Due Date: Why Might They Differ?
- An estimated due date based on your last menstrual period may differ from the date estimated during an ultrasound because the two methods use different information.
- An uncertain LMP, irregular cycles, earlier or later ovulation, and the timing of the ultrasound can all contribute to a difference.
- A difference between the estimates does not automatically mean one date is wrong.
- Your health professional will document the estimated due date used in your care.
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An estimated due date based on your last menstrual period may differ from the date estimated during an ultrasound because the two methods use different information. LMP dating begins from the first day of your last period, while ultrasound dating estimates gestational age from pregnancy measurements.
An uncertain LMP, irregular cycles, earlier or later ovulation, and the timing of the ultrasound can all contribute to a difference. Your health professional will document the estimated due date used in your care.
What Is an LMP Due Date?
LMP means the last menstrual period before the pregnancy.
An LMP-based due date is normally estimated from the first day bleeding began:
This places the estimated due date at around 40 weeks from the first day of the last period.
LMP dating is generally more useful when the first day of the last period is known and menstrual cycles are reasonably regular.
The calculation does not identify the exact date of ovulation, conception or fertilisation.
What Is an Ultrasound Due Date?
During the scan, a sonographer takes pregnancy measurements. In early pregnancy, these measurements can be used to estimate gestational age and calculate an EDD.
A dating scan may be particularly useful when menstrual dates are uncertain, cycles are long or irregular, or pregnancy dating needs clinical confirmation.
An ultrasound-based estimate does not reveal the exact date of conception. It estimates pregnancy timing from the measurements available when the scan is performed.
LMP vs Ultrasound Due Date Comparison
| Attribute | LMP-based due date | Ultrasound-based due date |
|---|---|---|
| Main information used | First day of the last menstrual period | Ultrasound measurements |
| What it estimates | When pregnancy reaches approximately 40 weeks | Gestational age and an estimated due date |
| Most useful when | LMP is known and cycles are reasonably regular | Menstrual dates are uncertain or confirmation is needed |
| Main assumptions | Menstrual and ovulation timing reasonably reflect the standard model | Measurements appropriately represent the pregnancy stage |
| Main limitations | Uncertain LMP, irregular cycles and variable ovulation | Dating precision varies with scan timing and measurement conditions |
| Identifies exact conception? | No | No |
| Who determines the EDD used in care? | Health professional | Health professional |
A difference between the estimates does not automatically mean one date is wrong.
Why Might the Due Dates Be Different?
An LMP date and an ultrasound date may differ for several reasons.
Menstrual-date uncertainty
It may be difficult to remember the exact first day of the last period. Bleeding may also have been mistaken for a menstrual period.
Standard LMP dating depends on the entered date accurately representing the menstrual period immediately before the pregnancy.
Cycle and ovulation timing
Standard LMP dating assumes a reasonably predictable relationship between the period, ovulation and conception.
That relationship may be less reliable when cycles are irregular. A cycle can also be regular but consistently longer or shorter than the standard 28-day model.
Ovulation may therefore have occurred earlier or later than the calculation assumes, even in someone with generally regular cycles.
Learn about due-date estimation with irregular periods.
Ultrasound timing and measurement variation
An ultrasound estimates pregnancy timing from measurements taken on the day of the scan.
The stage of pregnancy and normal measurement variation can affect the estimate. This means that different starting information can produce a different gestational-age estimate and EDD without either method containing a simple arithmetic error.
Which Due Date Should You Follow?
Follow the EDD documented by your health professional.
Do not independently choose whichever date appears more accurate or better matches an expected conception date.
Once a clinical EDD has been established, continue using that date unless your health professional tells you otherwise.
Does Ultrasound Timing or Measuring Ahead Affect Your Due Date?
Ultrasound dating generally provides greater precision earlier in pregnancy than later.
A later ultrasound can still provide important information about fetal size and growth. However, a measurement that appears ahead or behind does not automatically mean the original pregnancy dating was incorrect or that the EDD should change.
Ask your health professional what a measurement means in the context of your pregnancy. This article cannot interpret an ultrasound result or determine whether a due date should change.
What If an Online Calculator Gives a Different Date?
An online calculator provides an educational estimate based on the information entered.
The Perdays calculator uses the standard 280-day method in LMP mode and can also calculate from a known EDD. It does not interpret ultrasound measurements or compare clinical dating evidence.
Enter the first day of your last menstrual period for a standard estimate or use an EDD already documented by your health professional.
A calculator result should not override the date recorded in your health professional care.
Medical Information Notice
This article provides general educational information. It does not establish or change an individual EDD, interpret ultrasound measurements, diagnose fetal-growth concerns, identify an exact conception date or replace antenatal care.
Speak with your health professional if:
- Your LMP and ultrasound dates differ.
- Your LMP is unknown or uncertain.
- Your menstrual cycles are irregular.
- You do not know which EDD is recorded.
- You have questions about an ultrasound.
- You are concerned about your pregnancy or your baby's growth.
References
- Pregnancy, Birth and Baby: Working out your due date
- Pregnancy, Birth and Baby: Dating scan
- Safer Care Victoria: Accurate pregnancy dating: estimated due date
- American College of Obstetricians and Gynecologists: Methods for estimating the due date
- American College of Obstetricians and Gynecologists: Management of suboptimally dated pregnancies
