Ultrasound Due Date: How Accurate Is Dating by Scan?

When you have your first pregnancy ultrasound, the sonographer measures your baby and estimates how far along you are—and when you'll give birth. This estimated due date (EDD), calculated from the scan, is often different from the one calculated from your last menstrual period (LMP). Which one is more accurate? The answer depends on when the scan was done, and understanding the science behind it helps you interpret your results with confidence.

How ultrasound dating works

Ultrasound dating works by measuring specific fetal structures and comparing them to reference charts based on thousands of normal pregnancies. In the first trimester, the primary measurement is crown-rump length (CRL)—the distance from the top of the baby's head to the bottom of the buttocks. This is the most accurate measurement available for dating because fetal growth in early pregnancy is remarkably consistent across individuals.

In the second trimester, when the baby is curled and CRL is no longer measurable, sonographers switch to a combination of measurements: biparietal diameter (BPD, the width of the skull), head circumference (HC), abdominal circumference (AC), and femur length (FL). These four measurements, combined into a composite gestational age estimate, provide the best second-trimester dating accuracy.

Accuracy by trimester

LMP vs ultrasound dating: which wins?

LMP-based dating assumes a 28-day cycle with ovulation on day 14—Naegele's rule. In reality, menstrual cycles vary significantly (21–35+ days is normal), ovulation timing varies, and many women have irregular cycles or do not remember their LMP accurately. Studies consistently show that LMP overestimates gestational age in women with long cycles and underestimates it in those with short cycles.

When a first-trimester CRL ultrasound is available, it is uniformly more accurate than LMP dating for women with irregular or uncertain cycles. A 2015 Cochrane review confirmed that routine ultrasound dating reduces the rate of post-term pregnancies (pregnancies that go past 42 weeks)—which means ultrasound-based EDDs are more reliable anchors for obstetric management.

When should the due date be changed?

Clinical guidelines from ACOG (American College of Obstetricians and Gynecologists) and NICE (National Institute for Health and Care Excellence) provide specific thresholds for changing a previously established EDD:

If you had a reliable first-trimester CRL scan, subsequent scans that show a 'different' size do not mean the due date should be recalculated—they reflect normal variation in fetal growth, not a changed gestational age. Many parents worry unnecessarily when a 20-week scan gives a slightly different estimate than the 12-week scan. This is expected and not clinically significant unless the difference exceeds the thresholds above.

IVF pregnancies and dating

In pregnancies conceived via IVF or ICSI, the exact date of fertilization or embryo transfer is known precisely. This makes dating extremely accurate—typically ±1–2 days. IVF dating adds a fixed number of days to the egg retrieval or transfer date (for day-3 embryo transfers: +266 days to retrieval; for day-5 blastocysts: +261 days). Ultrasound is still done for fetal wellbeing, but the EDD is usually not altered.

Why accurate dating matters

An accurate EDD is not just about circle-the-date planning. It affects clinical management at almost every step of prenatal care: the timing of first-trimester screening blood tests (optimal between 10–13 weeks), the interpretation of Down's syndrome risk (which is age- and gestation-specific), the decision to induce labor for post-term pregnancy, and the assessment of fetal growth restriction in later scans. Getting gestational age right early makes every subsequent decision more accurate.

Frequently asked questions

My 12-week scan gave a different date than my 8-week scan. Which is correct?

If both were CRL measurements performed by a qualified sonographer, the 8-week scan is generally considered more accurate because CRL variation at that stage is minimal. However, if the difference is 5 days or less, it is within normal measurement error and either date is clinically equivalent. Consult your midwife or obstetrician if the discrepancy is larger.

Can my due date change after the 20-week scan?

Only under specific circumstances. If your 20-week scan shows a gestational age more than 10 days different from your established EDD (from a reliable first-trimester scan), your clinician may revise the date. But if you had a first-trimester CRL scan, that date is usually kept even if later scans produce different estimates.

Is an 8-week ultrasound more accurate than a 12-week scan?

Both are highly accurate. An 8–9-week transvaginal CRL has slightly better precision (±3–4 days vs ±5–7 days at 12 weeks), but both are vastly more accurate than LMP-based dating alone. The 12-week scan is preferred in most health systems because it coincides with first-trimester screening and NT measurement.

This article is for educational purposes only and does not replace the advice of your obstetric provider. Gestational age assessment and due date management should be overseen by a qualified healthcare professional.