Pregnancy Calculator Due Date Accuracy Ultrasound Dating vs Last Menstrual Period — Why the Two Methods Often Disagree

You enter your last menstrual period (LMP) into a pregnancy calculator. It gives you a due date: October 15. At your first ultrasound, the doctor measures the fetus and calculates a different due date: October 20 — five days later. Both dates are based on standard medical practice. Both are "correct" in the sense that they follow established guidelines. But they disagree. Which one is more accurate?

The short answer: early ultrasound dating is more accurate than LMP dating — but only when performed in the first trimester. Here is why the two methods disagree, what the research says about their accuracy, and why the difference matters for pregnancy management.

LMP Dating: The Traditional Method (and Its Limitations)

LMP-based dating assumes that conception occurred 14 days after the first day of your last menstrual period — the "standard" 28-day cycle with ovulation on day 14. The due date is calculated as LMP + 280 days (40 weeks). This is Naegele's rule, named after the German obstetrician Franz Naegele who proposed it in 1812.

The limitation: most women do not have a 28-day cycle with ovulation on day 14. Cycle length varies from 21 to 35 days. Ovulation can occur anywhere from day 10 to day 21. If you ovulate on day 21 (a week later than the standard assumption), the LMP-based due date will be a week too early. The pregnancy is actually a week younger than the calculator assumes.

Additional sources of error: irregular cycles (the standard formula assumes regularity), uncertain LMP date (many women do not track their periods precisely), and early pregnancy bleeding (which can be mistaken for a period, throwing off the LMP date entirely).

LMP dating is accurate to within about ±2 weeks for most pregnancies. That is a wide range. The due date from LMP is a rough estimate, not a precise prediction. This is why obstetricians use ultrasound to confirm or adjust the LMP-based date.

Ultrasound Dating: The More Accurate Method

First-trimester ultrasound measures the crown-rump length (CRL) — the distance from the top of the fetus's head to the bottom of its torso. The CRL grows at a highly predictable rate during the first trimester, corresponding to gestational age with an accuracy of ±5-7 days. This is significantly more precise than LMP dating.

The accuracy is highest between 8 and 14 weeks of pregnancy. After 20 weeks, ultrasound dating becomes less accurate because fetal growth varies more between individuals — some babies are naturally larger, some are naturally smaller. The ultrasound measures size, not age. In the first trimester, size and age are closely correlated. Later in pregnancy, the correlation weakens.

The American College of Obstetricians and Gynecologists (ACOG) recommends using the LMP-based due date unless the ultrasound date differs by more than: 7 days in the first trimester (if the ultrasound date differs by more than a week, use the ultrasound date), or 14 days in the second trimester (if the ultrasound date differs by more than two weeks, use the ultrasound date).

Why the Difference Matters

A due date that is off by a week can affect: induction timing (inducing labor at 41 weeks instead of 42), prematurity classification (a baby born at 36 weeks and 6 days is "preterm" while one at 37 weeks is "term"), and the timing of prenatal tests that are only valid within specific gestational age windows.

Use the pregnancy calculator for the LMP-based estimate. Then confirm with your doctor's ultrasound. The calculator gives you the traditional estimate. The ultrasound gives you the more accurate date. Both together give you a range — and the range is more honest than a single date. Only about 4% of babies are born on their actual due date. The due date is an estimate. The baby decides the real date.