The standard 40-week due date most people are given is a widely used estimate based on a simple calculation, not a precise prediction. Understanding how it's calculated — and its built-in assumptions — explains why it can shift after an early ultrasound.
The standard calculation: Naegele's rule
280 days is 40 weeks, the standard length used for a full-term pregnancy count. This method, commonly called Naegele's rule, assumes a typical 28-day menstrual cycle with ovulation occurring around day 14.
Worked example: a last menstrual period starting January 1:
| Calculation | Result |
|---|---|
| January 1 + 280 days | October 8 |
Why this is an estimate, not a prediction
Only a small percentage of babies are actually born exactly on their calculated due date — most arrive within a window of roughly two weeks before or after it, and that's considered entirely normal. The 280-day figure is a population-level average, and Naegele's rule specifically assumes a standard 28-day cycle with ovulation on day 14, an assumption that doesn't hold for everyone with irregular or naturally longer or shorter cycles.
Why an ultrasound date can differ
A first-trimester ultrasound measures the developing fetus directly (typically via crown-rump length) rather than relying on the last-menstrual-period assumption. For someone with irregular cycles, or who isn't certain of their last period's exact start date, this direct measurement can provide a more individually accurate estimate — and healthcare providers will often adjust the estimated due date based on early ultrasound findings when it differs meaningfully from the LMP-based calculation.
How trimesters break down
| Trimester | Approximate weeks |
|---|---|
| First | 1–13 |
| Second | 14–27 |
| Third | 28–40 |
A note on this calculator
This tool provides a general educational estimate based on Naegele's rule and is not a substitute for medical advice. Due dates, gestational age, and pregnancy monitoring should be confirmed and tracked with a qualified healthcare provider, who can incorporate ultrasound findings and individual factors this simple calculation doesn't account for.
Try the calculator
The pregnancy due date calculator applies this calculation to your last menstrual period date and shows how far along you are.
Frequently asked questions
How is a pregnancy due date calculated?
The standard method, known as Naegele's rule, adds 280 days (40 weeks) to the first day of the last menstrual period. This assumes a typical 28-day cycle with ovulation around day 14.
How accurate is a due date calculated this way?
It's an estimate, not a precise prediction. Only a small percentage of babies are born exactly on their calculated due date — most arrive within a window of about two weeks before or after it. A first-trimester ultrasound often provides a more accurate estimate, especially for people with irregular cycles.
Why might ultrasound dating differ from the last-menstrual-period calculation?
Naegele's rule assumes a standard 28-day cycle with ovulation on day 14, which doesn't apply to everyone. Ultrasound measures the size of the developing fetus directly, which can provide a more individualized estimate, particularly for those with irregular or longer/shorter cycles.
How are pregnancy trimesters divided?
The first trimester covers roughly weeks 1-13, the second trimester weeks 14-27, and the third trimester weeks 28 through delivery (around week 40).