Due Date by Conception Date: How It Works
Pregnancy is dated in a few different ways, and they do not always agree. Here is how a due date is estimated from conception, your last period, or IVF.
A due date is only ever an estimate — fewer than 1 in 20 babies actually arrive on it. But the estimate matters, because it sets the clock for prenatal care. There are a few ways to calculate it, and they start counting from different moments.
The three common methods
1. From your last menstrual period (Naegele’s rule)
The traditional method, Naegele’s rule, adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). It assumes a regular 28-day cycle with ovulation on day 14, so pregnancy is "counted" from about two weeks before you actually conceived. That is why you can be "4 weeks pregnant" only two weeks after conception.
2. From conception date
If you know roughly when you conceived — for example from tracking ovulation — the estimate is conception date + 266 days (38 weeks), because it skips that pre-ovulation fortnight that the LMP method includes.
3. From an IVF transfer
IVF dates are the most precise, because the embryo’s age is known exactly. A day-5 (blastocyst) transfer is dated as transfer date + 261 days; a day-3 transfer adds 263 days.
Your ultrasound has the final say According to ACOG, a first-trimester ultrasound is the most accurate way to date a pregnancy. If your scan date differs from a calculator, your care team will use the scan. Always confirm dates with your OB-GYN or midwife.
Why the methods disagree
The LMP method assumes textbook-regular cycles. If you ovulate earlier or later than day 14, or your cycles run long or short, the LMP estimate can be off by several days — while the conception-based estimate may be closer. This is exactly why early ultrasound dating exists, and why the number can shift slightly at your first scan.
- Track your cycle so you know your real ovulation day with the Ovulation Calculator.
- Once you have a due date, follow your progress with the Pregnancy Week-by-Week tool.
- Treat every date as an estimate and let your clinician confirm it.
People also ask
How is a due date calculated from conception?
Add 266 days (38 weeks) to the conception date. This is two weeks less than the last-period method, which counts from before ovulation.
Why is my due date based on my last period, not conception?
Most people know their last period date but not their exact conception date, so the 40-week (LMP + 280 days) method is the practical standard. An early ultrasound then refines it.
How accurate is a due date calculator?
It gives a solid estimate, but a first-trimester ultrasound is more accurate and takes precedence. Only about 1 in 20 babies are born on the estimated date.