How Is Your Pregnancy Due Date Calculated?

The standard estimated due date, or EDD, is 40 weeks (280 days) from the first day of your last menstrual period, using a method called Naegele’s rule. If you know your conception date instead, it is about 266 days from there. An early ultrasound can refine the estimate. A due date is only an estimate, so your OB-GYN, midwife, or healthcare provider gives you your official one.

How Is a Due Date Calculated?

The short answer is that your due date is calculated by counting 280 days, which is 40 weeks, from the first day of your last menstrual period. This starting point is called the LMP, and it is used because most people know that date more reliably than the exact day they conceived. The result is your estimated due date, often written as EDD.

Counting from the LMP may feel odd, because you were not pregnant on that day. Health providers use it anyway as a shared, consistent reference point. Conception usually happens about two weeks after the LMP, so the 280-day count already builds in that gap. Every pregnancy is different, and this is why the number is called an estimate rather than a fixed date. To see the count applied to your own dates, the Pregnancy Calculator does the arithmetic for you once you enter the first day of your last period.

Your OB-GYN, midwife, or healthcare provider is the one who gives you your official due date. They may start with the LMP method and then adjust it based on an ultrasound or your medical history, so treat the do-it-yourself number as a helpful preview rather than the final word.

Naegele’s Rule: The 280-Day Count

Naegele’s rule is the classic formula behind the LMP method, and it reaches the same 280-day answer with simple calendar steps. Starting from the first day of your last menstrual period, you add one year, subtract three months, and then add seven days. The date you land on is your estimated due date.

Here is a worked example. Suppose the first day of your last period was March 1, 2026. Add one year to reach March 1, 2027. Subtract three months to reach December 1, 2026. Add seven days, and you arrive at December 8, 2026. That final date is the estimated due date under Naegele’s rule.

The rule assumes a regular 28-day cycle with ovulation around day 14. If your cycles are longer, shorter, or irregular, the plain LMP count can be slightly off, which is one reason an ultrasound is so useful. The timeline below shows how the same pregnancy is measured from two different starting points.

Calculating From a Known Conception Date

If you know the day you conceived, perhaps because you were tracking ovulation or used a fertility treatment with a precise date, you can count from there instead. From the conception date, the estimated due date is about 266 days, which is 38 weeks. That is 14 fewer days than the 280-day LMP count, and the difference is exactly the two-week gap between the LMP and conception.

Both counts point to the same due date when your cycle is a typical 28 days. The conception-date method can be a little more precise for people with irregular or unusually long cycles, because it skips the assumption about when ovulation happened. If you want to learn how ovulation timing is estimated in the first place, our guide on the fertile window and ovulation walks through it. Keep in mind that even a known conception date produces an estimate, since the length of pregnancy naturally varies from person to person.

How an Early Ultrasound Refines the Estimate

An ultrasound in the first trimester is often the most accurate way to date a pregnancy. Early in pregnancy, babies grow at a very similar rate, so measuring the size of the embryo lets a provider estimate how far along the pregnancy is and adjust the due date if needed. This dating scan is usually most reliable in the first trimester, before individual growth differences widen.

When the ultrasound estimate and the LMP estimate disagree by more than a certain amount, providers often use the ultrasound date. This is common when the LMP is uncertain or cycles are irregular. Your provider decides which method gives the best estimate for your pregnancy, so the official due date on your chart may differ from a simple LMP count, and that is normal.

Comparing the Three Methods

Each method answers the same question in a slightly different way and needs different information from you. The table below sums up what each one uses and how precise it tends to be. Every pregnancy is different, so treat the accuracy notes as general guidance, not a guarantee.

In practice, a provider frequently combines these. They may start with the LMP, confirm or adjust with an early ultrasound, and factor in a known conception date when one is available. The goal is a single best estimate for planning, not a perfect prediction of the birthday.

Why a Due Date Is Only an Estimate

It helps to remember what the due date is really for. It marks 40 weeks and gives your care team a reference for tracking growth, scheduling check-ups, and watching for anything that needs attention. It is not a countdown to a guaranteed arrival. Only about 1 in 20 babies, roughly five percent, are actually born on their exact due date.

A full-term pregnancy spans a range, generally counted as 37 to 42 weeks. Babies born within that window are considered full term, and arriving a week or two on either side of the due date is common and usually expected. The chart below shows how births spread across that range rather than clustering on a single day.

Because arrival is a range, it can help to think in trimesters and milestones rather than one single day. Our overview of pregnancy by trimester lays out what each stage covers. Whatever the estimate says, your provider tracks your pregnancy week by week and is the right person to explain what your particular due date means for you.

FAQs About Due Dates

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Educational note: This article is general health information, not medical advice, and it is not a diagnosis or a personalized care plan. Every pregnancy is different, and a due date is only an estimate. Your OB-GYN, midwife, or healthcare provider gives you your official due date and is the right person to answer questions about your pregnancy. If you have any concerns, contact your provider. Reviewed for accuracy by Dr. Abdullah Khalil, MBBS, as part of our editorial review process. Content last reviewed September 10, 2026.