What makes a due date accurate?
There are two main ways to date a pregnancy: from the first day of the last period, and from ultrasound measurements. The period method is simple but depends on remembering the date correctly and on ovulation happening at the expected time. An ultrasound measures the embryo or fetus directly. In the first trimester, babies grow at a very similar rate, so the crown-rump length (CRL) measured on a scan is generally considered the most accurate way to estimate gestational age for a naturally conceived pregnancy.
How the ultrasound method works
A scan report typically says something like “8 weeks 3 days” on the day of the scan. That is the gestational age on that date. The calculator works out how many days were left until 40 weeks and adds them to the scan date:
Due date = scan date + (280 days − gestational age on the scan in days)
Example: a scan on June 1 shows 8 weeks 3 days (59 days). 280 − 59 = 221 days, and June 1 + 221 days = January 8. Choose “Ultrasound” above and enter your own numbers.
When is a due date changed after a scan?
If the scan and the period-based date disagree, healthcare professionals do not always switch to the scan date — small differences are expected. Guidance from the American College of Obstetricians and Gynecologists (Committee Opinion 700) suggests changing the due date to the ultrasound date when the difference is larger than a set threshold that grows with gestational age:
| Gestational age at the scan | Redate if the difference is more than |
|---|---|
| Up to 8 weeks 6 days | 5 days |
| 9 weeks 0 days – 13 weeks 6 days | 7 days |
| 14 weeks 0 days – 15 weeks 6 days | 7 days |
| 16 weeks 0 days – 21 weeks 6 days | 10 days |
| 22 weeks 0 days – 27 weeks 6 days | 14 days |
| 28 weeks 0 days and later | 21 days |
Your own provider may follow different local guidance, and these decisions are always made by your care team. Once a due date is set from an early scan, it is usually not changed again by later scans. Pregnancies conceived by IVF are dated from the transfer instead — see the IVF Due Date Calculator.
Which dating method is most accurate?
| Method | Typical precision | Why |
|---|---|---|
| IVF / FET transfer | Highest | The embryo’s age is known exactly |
| First-trimester ultrasound | High | Early embryos grow at a very similar rate |
| Known ovulation or conception date | Good | Avoids assumptions about cycle length |
| Last period, regular cycle | Good | Reliable when ovulation is close to day 14 |
| Last period, irregular or uncertain | Lower | Ovulation timing is harder to predict |
| Later ultrasound (second/third trimester) | Lower for dating | Babies’ sizes vary more as pregnancy progresses |
This is why most care teams compare your period-based date with an early scan and choose one official due date. No method can predict the actual day of birth — they estimate the 40-week point that your pregnancy is measured against.
Getting the most accurate result from your period dates
If you have not had a scan yet, you can still improve the estimate: use the first day of real bleeding (not spotting), and enter your average cycle length if it is not 28 days. The Estimated Due Date Calculator explains the cycle adjustment, and once you have an official due date you can plug it into the Reverse Due Date Calculator to get the rest of your dates.