The EDD — Estimated Due Date — is one of the first things a pregnant patient wants to know. And one of the first things a doctor gets asked to provide. Whether you’re writing a booking letter, completing antenatal notes, or simply answering a patient’s question in clinic, knowing how to calculate and explain the EDD accurately is a core clinical skill.

But here’s what most people don’t realise: only about 4–5% of babies are born on their exact due date. The EDD is not a deadline. It’s the midpoint of a probability window — and understanding how it’s derived makes you a better clinician and helps you counsel patients far more accurately.

In this guide, we’ll walk through every method used to calculate an EDD — from the classic Naegele’s rule to IVF-specific protocols — and explain when each one applies, and when ultrasound should take over.

What Does EDD Stand For?

EDD stands for Estimated Due Date — sometimes also written as EDC (Estimated Date of Confinement), though EDD is the modern preferred term. It represents the date at which a pregnancy is estimated to reach 40 completed weeks of gestation. You’ll see it written in antenatal notes, booking letters, scan reports, and discharge summaries.

It’s important to emphasise the word estimated. The EDD is calculated from population-level data, not an individual prediction. Around 80% of births occur within 10 days either side of the EDD. The remaining 20% fall outside that window entirely — and that’s completely normal.

What Exactly Is Gestational Age?

Before we talk about due dates, let’s be precise about what we’re measuring. Gestational age is calculated from the first day of the last menstrual period (LMP) — not from the date of conception. This feels counterintuitive, but it’s the universal clinical standard.

Why count from a period rather than conception? Because the exact date of ovulation and fertilisation is rarely known with certainty, even in natural conceptions. The LMP is a reliable date the patient can usually recall. So in practice, a pregnancy is already considered “2 weeks along” at the point of conception.

A full-term pregnancy is 40 weeks from LMP, or 38 weeks from conception.

Naegele’s Rule: The Standard Calculation

Developed by Franz Naegele, a German obstetrician, in 1812, this rule has remained the foundation of due date calculation for over two centuries. The formula is simple:

Example: LMP was 1st January 2026. Go back 3 months → 1st October 2025. Add 7 days → 8th October 2025. Add 1 year → 8th October 2026.

Naegele’s rule assumes a 28-day menstrual cycle with ovulation on day 14. This works well for women with regular 28-day cycles — but in practice, many women have longer or shorter cycles, which changes when ovulation actually occurs.

Cycle Length Adjustment: Why It Matters

If a woman has a 35-day cycle, she ovulates around day 21 — a full 7 days later than assumed by Naegele’s rule. This means conception happened later, and the due date should be shifted forward accordingly. If you don’t adjust for cycle length, you’ll systematically underestimate the EDD in women with longer cycles — and potentially mislabel a normal pregnancy as post-dates.

The adjustment is straightforward:

So for a 35-day cycle: add 7 days to the Naegele EDD. For a 21-day cycle: subtract 7 days.

Ultrasound Dating: When It Overrides Everything

No matter how precise the LMP calculation, ultrasound is the most accurate method of dating a pregnancy — particularly in the first trimester. This is because embryonic growth is highly consistent in early pregnancy, before genetic and environmental factors cause variation in size.

The key measurement is Crown-Rump Length (CRL), measured between 6 and 14 weeks. The accuracy of ultrasound dating varies by trimester:

The NICE guideline recommendation is clear: the EDD should be determined by the first trimester ultrasound (ideally at the 10–14 week dating scan) and should only be revised by a later scan if there is a significant discrepancy.

In general, if the ultrasound EDD differs from the LMP EDD by more than 5 days in the first trimester, or more than 10 days in the second trimester, the ultrasound date takes precedence.

IVF Due Date Calculation

IVF pregnancies require a different approach because the date of fertilisation — or embryo transfer — is known precisely. There is no need to estimate ovulation. ACOG (2024) recommends the following:

These calculations are based on a 266-day gestational period from conception (38 weeks). IVF dating is generally more reliable than LMP-based dating because conception date is exact — though first trimester ultrasound is still recommended to confirm fetal growth.

Understanding the Trimesters

Once you have the EDD, it’s useful to understand where each trimester begins and ends — particularly when counselling patients or completing documentation.

Term Definitions: What “Full Term” Actually Means

Many patients — and some junior doctors — use the word “term” loosely. The ACOG definition is worth knowing precisely:

• Preterm: Before 37+0 weeks
• Early term: 37+0 to 38+6 weeks
• Full term: 39+0 to 40+6 weeks — the optimal delivery window
• Late term: 41+0 to 41+6 weeks
• Post-term: 42+0 weeks and beyond — associated with increased perinatal risk

This matters clinically. A baby delivered at 37 weeks is technically “term” but has meaningfully higher risks of respiratory distress, temperature instability, and feeding difficulties compared to a 39-week baby. When documenting delivery timing or counselling patients on elective caesarean section dates, be specific about gestational age — not just “term.”

Key Pregnancy Milestones From EDD

Once you have the EDD, you can back-calculate every major milestone. Here’s a quick reference:

Common Mistakes in Due Date Calculation

These are errors that come up regularly — in GP surgeries, on labour wards, and in antenatal clinic letters.

1. Using the LMP date when ultrasound disagrees

If the 12-week scan puts the pregnancy a week behind the LMP calculation, the scan date should be used. Sticking rigidly to the LMP EDD when a high-quality first trimester scan says otherwise is incorrect practice — and can lead to unnecessary post-dates induction.

2. Forgetting cycle length adjustment

A patient with a 35-day cycle who is dated purely by Naegele’s rule will appear to be 7 days further along than she actually is. This is a common cause of “incorrectly post-dates” pregnancy.

3. Re-dating in the third trimester

A growth scan at 32 weeks showing a small baby is not a reason to change the EDD. It’s a reason to investigate for growth restriction. These are different clinical problems.

4. Confusing gestational age with fetal age

Gestational age (from LMP) is always approximately 2 weeks more than fetal age (from conception). When speaking to patients, be explicit about which system you’re using.

A Note for Patients Reading This

If you’ve landed here as a patient rather than a clinician — welcome. The key things to know are:

• Your due date is an estimate. Most babies arrive between 38 and 42 weeks.
• The dating scan in your first trimester is the most reliable way to confirm your due date — more reliable than period dates alone.
• Only around 1 in 20 babies are born on their exact due date. Don’t fixate on a single day.
• If you want to calculate your own due date based on your LMP, conception date, or IVF transfer, you can use our free calculator — it also shows your current week, trimester, and key milestones.

Summary

• EDD = Estimated Due Date — the date at which pregnancy reaches 40 completed weeks
• Gestational age is counted from LMP — approximately 2 weeks before conception
• Naegele’s rule gives EDD as LMP + 280 days, assuming a 28-day cycle
• Adjust for cycle length by adding or subtracting days relative to 28
• First trimester ultrasound (CRL) is the gold standard for dating — accurate to ±5 days
• IVF pregnancies use transfer date: Day 3 = +263 days, Day 5 = +261 days
• Full term is 39+0 to 40+6 weeks — not simply “37 weeks and above”
• Do not re-date after 20 weeks based on scan findings alone

References

1. NICE. Antenatal care for uncomplicated pregnancies. Clinical guideline CG62. Updated 2021.
2. American College of Obstetricians and Gynecologists. Methods for Estimating the Due Date. Committee Opinion No. 700. 2017 (reaffirmed 2024).
3. Naegele FC. Erfahrungen und Abhandlungen aus dem Gebiete der Krankheiten des weiblichen Geschlechtes. 1812.
4. Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates. Am J Obstet Gynecol. 1996;174(1):278-281.
5. Royal College of Obstetricians and Gynaecologists. Induction of Labour at Term in Older Mothers. Scientific Impact Paper No. 34. 2013.