How to Calculate the Pregnancy Due Date (EDD): Naegele's Rule, Ultrasound and Examples
11 November 2026 · 5 min read
The expected date of delivery (EDD), or due date, is the first date a doctor works out in a pregnancy. It decides when each antenatal test and scan is due, when the pregnancy reaches full term, and when it becomes post-term.
This guide explains how the due date is calculated, with worked examples, and when the date from an ultrasound should be used instead.
This guide is for general information only. It is not medical advice. The due date and the antenatal plan should always be confirmed by the treating doctor.
If you only need the answer, use the free pregnancy due date calculator.
How is the due date calculated from the LMP?
The standard method is Naegele's rule. A pregnancy is taken to last 280 days (40 weeks) from the first day of the last menstrual period (LMP), so:
EDD = first day of LMP + 280 days
A quick way to do this by hand is to add 7 days to the LMP, subtract 3 months, and add 1 year. This shortcut can be a day or two off from counting exactly 280 days, because months have different lengths, so calculators count the days.
Worked example
- First day of LMP: 1 March 2026
- Add 280 days: 6 December 2026
On 11 November 2026, this pregnancy would be 255 days along, which is 36 weeks 3 days.
Why is pregnancy counted from the last period?
Pregnancy is counted from the LMP, not from conception, because the LMP is a date most women know, while the exact day of conception is usually not known. In a 28-day cycle, ovulation and conception happen about two weeks after the LMP. This is why a woman is already said to be "2 weeks pregnant" on the day of conception.
What if the cycle is not 28 days?
Naegele's rule assumes a 28-day cycle. When the usual cycle is longer, ovulation happens later, so the due date moves later by the same number of days. When it is shorter, the due date moves earlier.
Adjusted EDD = LMP + 280 days + (cycle length − 28 days)
For example, with an LMP of 1 March 2026 and a 35-day cycle, the due date moves 7 days later, to 13 December 2026.
When cycles are irregular, a date based on the LMP is less reliable, and an early ultrasound is usually preferred.
How is the due date calculated from the conception date?
When the conception date is known, such as in IVF or when there was a single possible date, the due date is:
EDD = conception date + 266 days (38 weeks)
This gives the same result as the LMP method for a 28-day cycle, because conception is taken as 14 days after the LMP. For IVF, the treating doctor may use the embryo transfer date and embryo age instead.
How is the due date calculated from an ultrasound?
An ultrasound report gives the gestational age on the day of the scan, for example 8 weeks 4 days. To find the due date:
- Convert the gestational age to days: 8 weeks 4 days = 60 days.
- Count back that many days from the scan date to find the start of pregnancy.
- Add 280 days.
Worked example
- Scan on 1 May 2026, gestational age 8 weeks 4 days (60 days)
- Start of pregnancy: 60 days before 1 May = 2 March 2026
- Add 280 days: 7 December 2026
LMP or ultrasound: which date should be used?
An early ultrasound, particularly in the first trimester, is generally considered more reliable than the LMP, especially when:
- the LMP is not remembered clearly
- periods are irregular
- the woman was breastfeeding or recently stopped hormonal contraception
- the scan and LMP dates differ by more than a few days
When both are available and they differ, it is the treating doctor's decision which date to follow. Once a scan-based due date is chosen, it is usually kept for the rest of the pregnancy rather than changed at each scan.
Key weeks after the due date is known
Once the start of pregnancy is fixed, other dates follow from it:
- First trimester: up to 13 weeks 6 days
- Second trimester: 14 weeks to 27 weeks 6 days
- Third trimester: from 28 weeks
- Full term: from 37 weeks
- Due date: 40 weeks
- Post-term: 42 weeks and beyond
Antenatal tests and scans, such as the NT scan, the anomaly scan and the sugar test, are each done in a particular window of weeks, so their dates also come from the due date.
Will the baby be born on the due date?
Usually not exactly. The due date is an estimate, and only a small share of babies are born on that day. Most are born in the weeks around it. This is why doctors talk about full term as a range, from 37 weeks onwards, rather than a single day.
Calculate it in seconds
The free pregnancy due date calculator works out the EDD from the LMP (with cycle length), the conception date or an ultrasound scan. It also shows weeks pregnant today, the trimester dates and the usual antenatal test schedule. Nothing you enter is saved or sent anywhere.
For gynecologists: keep it in the patient's record
A calculator is useful for a quick answer. In a clinic, the due date needs to stay with the patient, along with ANC visits, test dates and risk factors. Doctopath's gynecology case paper calculates the EDD and gestational age at every visit and tracks the full antenatal schedule. Read more in software for gynecologists: pregnancy card, ANC visits and Form F.