How Is a Due Date Calculated?

The 280-day rule, step by step

The standard way to estimate a pregnancy due date needs only one date: the first day of your last period. By convention, the estimated due date (EDD) is 280 days after that day.¹ 280 days is exactly 40 weeks, which matches the average length of a singleton pregnancy measured from the same starting point.²

1. Find the first day of your last menstrual period — the day bleeding started, not the day it ended.
2. Count forward 280 days (40 weeks) on the calendar.
3. The date you land on is your estimated due date.

For example, if your last period started on 9 July 2026, counting 280 days forward gives an estimated due date of 15 April 2027. Counting by hand is easy to get wrong across months of different lengths, so you can calculate your estimated due date instead — it does the same count on your device.

Why is the due date counted from your last period?

Pregnancy is dated from the first day of the last normal menstrual period, not from the day of conception, and its length is expressed in completed weeks and days.³ That gives everyone the same, recognisable starting point.

It also explains a question many people ask: why are you already “2 weeks pregnant” at conception? The dating convention assumes ovulation on day 14 of a regular 28-day cycle.¹ So by convention, the first two weeks of the 40 are counted before conception. In the example above, a conception date estimated by convention would be 23 July 2026 — a point at which the pregnancy is already dated as 2 weeks 0 days.

Because the count starts on the first day of the period, that first day is day zero, not day one.³ If you want to see how far along that makes you today, you can find out how many weeks pregnant you are.

What the 280-day rule assumes

The convention is simple because it makes assumptions about your cycle. It assumes a regular 28-day cycle with ovulation on day 14, so it does not account for:¹

• a last period date that was remembered inaccurately
• cycles that are longer, shorter or irregular
• variation in when ovulation actually happened

This is why a due date worked out from your last period is a starting estimate rather than a measurement of your own pregnancy. If your cycles don’t follow a 28-day pattern, read about due dates with irregular periods.

Four ways a due date can be worked out

Depending on what you know, the due date can be counted from different starting dates. In this example pregnancy, each method lands on the same estimated due date:

From your last period

This is the default method described above: first day of the last period plus 280 days.¹

From a conception date

If you know when conception is likely to have happened, a due date can be estimated by convention. The convention places ovulation 14 days after the first day of the last period,¹ so 280 − 14 gives an interval of 266 days (38 weeks) from an estimated conception date to the due date. This interval is derived from the LMP convention rather than stated as a separate clinical rule — and a conception date is itself never known exactly. For the reverse calculation, see how to estimate your conception date.

From an IVF embryo transfer

For pregnancies conceived through IVF, the due date is assigned from the embryo age and the transfer date: 261 days after transfer for a day-5 embryo and 263 days after transfer for a day-3 embryo.¹ When a pregnancy resulted from assisted reproduction, this ART-derived date should be used to set the due date.¹ Your fertility clinic can confirm the dates that apply to your treatment.

From an early ultrasound scan

Ultrasound measurement up to and including 13 weeks 6 days is the most accurate method to establish or confirm how far along a pregnancy is.¹ A scan doesn’t simply replace the period-based date: healthcare professionals compare the two and change the due date only when the difference is larger than a threshold that depends on how far along the pregnancy is.¹ The details are in ultrasound vs LMP dating.

Is a due date 40 weeks — or 37, or 42?

The estimated due date is the day you reach 40 weeks 0 days. But a pregnancy isn’t expected to end on that exact day. The NHS says pregnancy normally lasts from 37 to 42 weeks from the first day of the last period,⁴ and ACOG divides births at term into these categories:²

So when people ask whether a due date is “38 or 40 weeks”, the answer is that the due date marks 40 weeks, while full term spans 39 weeks 0 days to 40 weeks 6 days. Measured from a conception date, the same due date is 38 weeks (266 days) away — which is where the two numbers get mixed up.

Why a due date isn’t guaranteed

A due date is an estimate of when 40 weeks will be reached, not a prediction of the day labour will start. In one study, only 5% of births happened on the estimated date.⁵ Pregnancy length also varies naturally: in a study that identified ovulation with hormone tests, the time from ovulation to birth varied over a 37-day range even after excluding preterm births.⁶ There’s more on this in how accurate a due date is.

When to check your dates with a professional

• You don’t know your last period. If you don’t know or aren’t sure of the first day of your last period, speak to a midwife or GP.⁴
• You conceived through IVF. The transfer-based date should be used; your clinic can confirm it.¹
• You’ve had an early scan. If your healthcare team has given you a due date based on a scan, use theirs.

Frequently asked questions

Why is a pregnancy due date 280 days?

A singleton pregnancy lasts an average of 40 weeks (280 days) from the first day of the last period to the estimated date of delivery,² and by convention the due date is set 280 days after that first day.¹

Why do doctors add 2 weeks to pregnancy?

Nothing is added. Pregnancy is counted from the first day of the last period, and the convention assumes ovulation on day 14,¹ so the first two weeks counted come before conception.

Does my cycle length change the due date?

The standard calculation assumes a 28-day cycle and doesn’t account for longer or shorter cycles.¹ This site’s calculators don’t adjust for cycle length, because no adjustment formula has been verified against the sources it uses. An early scan is the more accurate way to confirm dates.

What does EDD stand for?

Estimated due date.⁴ Read more about what EDD means.

Medical statements in this article are linked to the numbered sources listed below: published clinical guidance and peer-reviewed research. It has not been reviewed by a named clinician. Read how content is written in our editorial policy, and see our medical disclaimer.