How to Calculate Your Due Date: LMP, Conception, and Ultrasound Methods

Source: NHS, American College of Obstetricians and Gynecologists (ACOG) | Last reviewed: April 2026

One of the first things every pregnant person wants to know is: when is the baby due? A due date — technically called the Estimated Date of Delivery (EDD) or Estimated Due Date (EDD) — gives you a timeline for antenatal care, maternity leave planning, and preparing for birth.

This guide explains how due dates are calculated, how accurate they are, and what to do when different methods give different answers.

The Three Methods for Calculating a Due Date

Method 1: Last Menstrual Period (LMP) — Naegele's Rule

The most widely used method is Naegele's rule, named after Franz Naegele, the 19th-century German obstetrician who formalised it. The calculation is:

Due date = First day of LMP + 280 days (40 weeks)

Or using the original Naegele's rule steps:

1. Take the first day of your last menstrual period
2. Add 1 year
3. Subtract 3 months
4. Add 7 days

Worked example:

• LMP: 1 January 2026
• Add 1 year: 1 January 2027
• Subtract 3 months: 1 October 2026
• Add 7 days: 8 October 2026

This assumes a 28-day cycle with ovulation on day 14. Most healthcare providers use this method as the starting point.

Method 2: Conception Date

If you know when conception occurred — through ovulation tracking, a positive ovulation test, or assisted reproduction (such as IVF) — you can calculate your due date directly:

Due date = Conception date + 266 days (38 weeks)

The reason this uses 266 rather than 280 days is that conception typically occurs around 14 days after the LMP in a standard cycle. The two methods converge on the same result in most cases.

Worked example:

• Conception date: 15 January 2026
• Add 266 days: 8 October 2026 (same as the LMP example above, assuming 14-day cycle)

This method is more precise when ovulation was tracked or IVF was used, because it removes the uncertainty about when in the cycle conception occurred.

Method 3: Ultrasound Dating

An early ultrasound — ideally between 8 and 14 weeks — allows the sonographer to measure the embryo or fetus directly. The crown-rump length (CRL) in the first trimester is closely correlated with gestational age and is typically accurate to within 5–7 days.

If the ultrasound estimate differs from the LMP-based date by more than 7–10 days, most healthcare providers will revise the due date to match the ultrasound. This is standard practice because ultrasound measurement in early pregnancy is more reliable than a recalled LMP date.

When ultrasound is the primary dating method:

• When the LMP date is unknown or uncertain
• When cycles are irregular
• When the pregnancy was conceived via IVF (in which case the embryo transfer date is known precisely)

Adjusting for Cycle Length

The standard 28-day cycle assumption does not apply to everyone. If your cycles are:

A longer cycle shifts ovulation later, which means conception occurred later than the 28-day model assumes, which moves the due date later. Our Pregnancy Due Date Calculator allows you to input your actual cycle length for a personalised estimate.

How Accurate Are Due Date Calculations?

All due date methods are estimates. Key facts about accuracy:

• Only about 5% of babies are born on their exact calculated due date
• About 80% of babies are born within 2 weeks before or after the due date
• A birth is considered at full term if it occurs between 39 and 40+6 weeks
• Early term: 37–38+6 weeks
• Post-term: 42+ weeks

The due date is best understood as a midpoint in a 5-week window of normal delivery (37–42 weeks), not a deadline.

Gestational Age vs. Fetal Age

A source of common confusion: gestational age counts from the LMP, not from conception. This means:

• At "4 weeks pregnant," the embryo is actually about 2 weeks old from fertilisation
• The "40-week pregnancy" includes approximately 2 weeks before conception occurred
• When a doctor says a baby is "38 weeks gestational age," the actual developmental age from fertilisation is approximately 36 weeks

This matters when reading about fetal development milestones — most medical references use gestational age (from LMP), not fetal age (from conception).

When Your Due Date May Be Revised

Your due date may be changed by your healthcare provider in the following circumstances:

After a dating scan: If the crown-rump length measurement gives a gestational age that differs from your LMP calculation by more than a week, the scan result takes precedence.

After IVF: The embryo transfer date is known precisely, allowing an exact due date to be calculated. Your fertility clinic typically provides this.

After a later scan: Second-trimester scans (18–22 weeks) are less accurate for dating but still used to confirm estimated gestational age. A revised date at this stage is less common.

A revised due date is not cause for concern — it simply reflects the most accurate available information about gestational age.

What to Do With Your Due Date

Once you have an estimated due date:

1. Tell your midwife or GP — they will confirm the date and arrange your antenatal care schedule
2. Book your dating scan (typically offered at 11–14 weeks in the UK, or as an early ultrasound from 8 weeks privately)
3. Calculate your maternity/paternity leave start date — most policies allow leave from 11 weeks before the due date
4. Note your antenatal appointment schedule — these are typically at 8–10, 16, 20, 25, 28, 31, 34, 36, 38, 40, and 41 weeks (NHS schedule, varies by trust)

Related Guides

• LMP vs Conception Date: Which Should You Use? — when the two methods give different answers
• Why Due Dates Change — understanding date revisions
• Pregnancy Trimesters Explained — what happens at each stage
• Pregnancy Due Date Calculator — calculate your EDD instantly
• Ultrasound Dating vs LMP: Which Is More Accurate? — how scan-based dating works and when it overrides the LMP calculation

This guide is for general information only, based on NHS and ACOG guidance. Always confirm your due date with your midwife, GP, or obstetrician. Revised due dates from ultrasound take precedence over calculated dates.