How to Calculate Your Due Date (3 Methods)
A due date is an estimate of when your baby is most likely to arrive — not a fixed appointment. Only about 1 in 20 babies is born on the exact date. Still, knowing your estimated due date (EDD) helps you and your provider track the pregnancy, schedule scans, and plan. There are three common ways to calculate it.
1. From Your Last Menstrual Period (Naegele's Rule)
This is the standard method. It counts 280 days (40 weeks) forward from the first day of your last menstrual period (LMP). A quick shortcut: take the first day of your last period, subtract three months, and add seven days.
The catch: this assumes a regular 28-day cycle with ovulation on day 14. If your cycles are longer, shorter, or irregular, the LMP estimate can be off by several days.
2. From Your Conception Date
If you know roughly when you conceived — for example, if you were tracking ovulation — you can count 266 days (38 weeks) forward from conception. Because conception happens about two weeks after your LMP, this and the LMP method usually land close together.
3. From an Early Ultrasound (Most Accurate)
A dating ultrasound in the first trimester (ideally 8–13 weeks) measures the baby and is the most accurate way to establish your due date. If the ultrasound date differs significantly from your LMP estimate, your provider will usually go with the ultrasound.
Why Your Due Date Might Change
It's completely normal for your provider to adjust your due date after your first scan. The goal isn't a perfect prediction — it's a reliable reference point for monitoring your baby's growth.
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Due Date Calculator →Frequently Asked Questions
How accurate is a due date?
It's an estimate. Most babies arrive within two weeks either side of the date, and only about 5% arrive on the date itself. An early ultrasound is the most accurate way to set it.
Can my due date change?
Yes. Providers often adjust the due date after an early dating ultrasound, especially if it differs notably from the last-period estimate.
What if I don't know my last period date?
An early ultrasound can date the pregnancy by measuring the baby, so you don't need to know your LMP for an accurate estimate.