Due date by LMP vs. conception date: which is right?

Two due-date calculations circulate for the same pregnancy, and they don’t always land on the same day. One counts from the first day of your last menstrual period (LMP) and adds 280 days. The other counts from a known conception dateand adds 266 days. Both are legitimate, both are used clinically, and the gap between what they predict is the single most common source of “why did my due date change” confusion.

The math behind each method

LMP dating uses Naegele’s rule: add 280 days (40 weeks) to the first day of the last period. It’s the default because most people know their LMP date more reliably than their exact conception date, and it doesn’t require knowing when ovulation happened.

Conception-date dating adds 266 days (38 weeks) to a known or estimated conception date, which is meant to represent the day of ovulation and fertilization directly, skipping the assumption baked into LMP dating.

The 14-day gap between 280 and 266 is exactly the assumed length of the follicular phase — the time from LMP to ovulation in a standard cycle. That's the hidden assumption LMP dating makes, and it's also the reason the two methods diverge for anyone whose cycle isn’t textbook-regular.

When the two methods disagree

If your cycle is close to 28 days and ovulation lands near day 14, LMP and conception dating will produce due dates within a day or two of each other. If your cycles run longer (say, 32 or 35 days), ovulation likely happened later than day 14 — which means the LMP-based due date is probably too early, and a conception-based estimate (if you know the date) is more trustworthy.

This is also why clinicians routinely check LMP-based dating against a first-trimester ultrasound estimate, and revise the due date if the two disagree by more than about a week. Ultrasound measures fetal size directly rather than relying on a recalled date and an assumed cycle length, which is why ACOG treats it as the most reliable of the three inputs when available.

Which one should you enter?

• Know your conception date precisely (single encounter, ovulation-tracked, or IVF transfer date)? Use conception-date math — it removes the 14-day assumption entirely.
• Only know your LMP, and your cycles run close to 28 days? LMP dating is a solid default and is what most people use without a second thought.
• Irregular or unusually long/short cycles? Treat an LMP-based due date as a starting estimate to be confirmed by ultrasound, not a fixed number.

Either way, a due date from either method is a statistical estimate, not a deadline — only about 1 in 20 babies actually arrives on the calculated day.