What Is an NHS Due Date Calculator?

An NHS Due Date Calculator is a tool that estimates when your baby is likely to be born � your Estimated Due Date (EDD). In the UK, pregnancy is usually dated as 40 weeks (280 days) from the first day of your last menstrual period, but NHS teams also adjust this based on:

• Early dating scan (ultrasound) measurements
• IVF transfer or egg retrieval dates
• Known conception or ovulation (for example, from fertility tracking)
• Cycle length if your periods are not exactly 28 days

Our Free NHS Due Date Calculator mirrors these clinical rules as closely as possible. It lets you choose the method that fits your situation and then shows not only your due date but also your current gestational age, trimester, and important pregnancy milestones such as the dating scan window, anomaly scan window, and the typical full-term period.

How the NHS Calculates Your Due Date (Overview)

In NHS maternity care, your gestational age and due date are usually assessed in two stages:

Stage 1 � Initial estimate from LMP

At first, your midwife or GP often uses the first day of your last period (LMP) to give a quick estimated due date:

EDD = LMP + 280 days (40 weeks)

This is simple and works well for many people, especially when cycles are regular and around 28 days, but it does not always reflect your actual ovulation date.

Stage 2 � More accurate dating from ultrasound

As part of routine antenatal care, you are usually offered a dating scan at around 11�14 weeks. The ultrasound technician measures your baby and calculates gestational age based on those measurements. If this scan date differs from the LMP-based calculation, NHS guidance typically recommends using the scan-based EDD.

Our NHS Due Date Calculator supports both approaches: you can calculate using LMP or enter your dating scan details to see how they compare.

Method 1 � LMP (Last Menstrual Period) & Cycle Length

The LMP method is the starting point for many pregnancy calculations. You simply use the first day of your most recent period and count ahead:

EDD = LMP + 280 days

This works on the assumption that your cycle is 28 days long and that you ovulate around day 14. However, many people have cycles that are shorter or longer, which can shift ovulation and affect the due date.

Adjusting for different cycle lengths

If your average menstrual cycle is not 28 days, the calculator can automatically adjust your due date using a simple rule:

Adjustment = (cycle length in days ? 28)

The calculator adds or subtracts this difference from the LMP-based estimate to better reflect when ovulation might have occurred.

• Example � Longer cycle (32 days): 32 ? 28 = +4 days ? EDD moved 4 days later
• Example � Shorter cycle (26 days): 26 ? 28 = ?2 days ? EDD moved 2 days earlier

When the LMP method is useful

The LMP method works well when:

• You clearly remember the first day of your last period
• Your cycles are fairly regular
• You have not had an early ultrasound yet

If your cycles are irregular or you are unsure of your LMP, a dating scan will usually be more accurate than an LMP estimate alone.

Method 2 � NHS Dating Scan (Ultrasound)

The NHS dating scan, usually done between 11 and 14 weeks, is considered one of the most accurate ways to date a pregnancy. During this scan, the sonographer measures your baby and records gestational age, often written as something like �12+3� on your report.

Our NHS Due Date Calculator lets you enter two pieces of information from your scan report:

• Scan date � the day the ultrasound was performed
• Gestational age at scan � weeks and days (for example, 12 weeks + 3 days)

How the scan method calculates your due date

Once you enter the scan date and the gestational age printed on the report, the calculator counts forward until 40 weeks (280 days) of gestation. This gives you a scan-based EDD that matches how clinicians interpret your ultrasound results.

Why the scan is often preferred to LMP

The scan is usually more accurate than LMP because:

• It measures your baby directly using standard growth charts
• It is not affected by late ovulation or unusual cycles
• It avoids errors from misremembered period dates

If your scan EDD and LMP-based EDD do not match, NHS teams typically choose the EDD based on the dating scan. Our calculator reflects this by giving you a clear scan-based due date when you use this method.

Method 3 � IVF & Assisted Conception Due Date

For pregnancies conceived through IVF or other forms of assisted reproduction, the NHS Due Date Calculator uses formulas that align with how fertility clinics estimate due dates. IVF offers a unique advantage: clinicians know the exact day of embryo transfer or egg retrieval.

Using embryo transfer date

If you know the date your embryo was transferred and whether it was a day-3 or day-5 embryo, the calculator uses:

• Day-3 embryo transfer: EDD = transfer date + 263 days
• Day-5 blastocyst transfer: EDD = transfer date + 261 days

These formulas produce an EDD that lines up with the standard way pregnancy is dated for naturally conceived pregnancies (40 weeks from LMP).

Using egg retrieval / fertilisation date

If your clinic focuses on the day of egg retrieval or fertilisation, the calculator can use:

EDD = retrieval/fertilisation date + 266 days

Why IVF dates are very precise

In IVF, there is no guesswork about when sperm and egg met or when embryos were transferred. Because these dates are known exactly, IVF due date estimates tend to be very accurate, and many clinics consider them more reliable than LMP alone.

Method 4 � Conception, Ovulation & IUI

Some people know their exact conception or ovulation date thanks to fertility tracking, ovulation predictor kits, basal body temperature charts, or an IUI procedure booked on a specific day. In these cases, the calculator uses:

EDD = conception/ovulation date + 266 days

When this method is ideal

Use the conception/ovulation method when you are confident about the day ovulation or fertilisation happened. It can be more reliable than LMP if:

• Your cycles vary from month to month
• You ovulate earlier or later than day 14
• You use digital ovulation tests or fertility tech
• You underwent insemination (IUI) on a known date

Even with a known conception date, your midwife may still adjust your EDD based on ultrasound measurements later in pregnancy.

How to Use the NHS Due Date Calculator Step by Step

Our Free NHS Due Date Calculator is designed to be simple to use while still reflecting how UK clinicians think about pregnancy dating.

Step 1 � Choose your method

Select one of the four methods:

• LMP � if you know the first day of your last period
• Dating scan � if you have an ultrasound report with weeks+days
• IVF � if you know your transfer or retrieval date
• Conception/ovulation � if you know exactly when fertilisation likely happened

Step 2 � Enter your dates

Fill in the date fields (for example, LMP date, scan date, transfer date) and any additional details such as cycle length or embryo age for IVF.

Step 3 � Click �Calculate�

Hit the �Calculate� button and the NHS Due Date Calculator will instantly show your:

• Estimated Due Date (EDD)
• Gestational age today (in weeks and days)
• Days until due date
• Current trimester
• Key milestones such as dating scan, anomaly scan, and full-term period

Step 4 � Use the results as a guide, not a promise

Remember that a due date is an estimate � not a fixed appointment. Most babies arrive within a range around the EDD, commonly between 37 and 42 weeks. Always follow your midwife�s or doctor�s advice regarding appointments, scans, and any changes to your dates.

All Calculator Outputs Explained

The NHS Due Date Calculator doesn�t just show your due date. It also calculates several helpful values that mirror what you might hear in appointments.

Estimated Due Date (EDD)

This is the day your baby is expected to arrive based on the method you chose (LMP, scan, IVF, or conception). It is usually set at 40 weeks of gestation in NHS-style dating.

Gestational Age Today

Gestational age tells you how many weeks and days pregnant you are right now. For example, 14 weeks + 6 days means you have completed 14 full weeks and are 6 days into week 15.

Days Until Due

This number counts down the number of days between today and your estimated due date. If the EDD has passed, the calculator can also show how many days past your due date you are.

Current Trimester

The calculator uses standard trimester boundaries:

• 1st trimester: conception to 13 weeks
• 2nd trimester: 14 weeks to 27 weeks + 6 days
• 3rd trimester: 28 weeks onwards

Knowing your trimester can help you understand which symptoms are typical at that stage and which tests or appointments may be due soon.

Milestones Table

The milestones table summarises important points in your pregnancy, such as:

• Estimated LMP (if working backwards from scan)
• End of 1st trimester (~13 weeks)
• Start of 3rd trimester (~28 weeks)
• Viability threshold (around 24 weeks, context dependent)
• Typical dating scan window (about 11�14 weeks)
• Typical anomaly scan window (about 18�21 weeks)
• Full-term period (around 39�40+6 weeks)

These are standard reference windows and your own NHS trust may schedule them slightly differently.

Typical UK Pregnancy Milestones & NHS Windows

While every pregnancy is unique, many NHS trusts use similar windows for key scans and milestones. Our NHS Due Date Calculator follows these approximate timings to give you a realistic overview.

Dating Scan (Around 11�14 Weeks)

The dating scan is often your first detailed ultrasound. It checks how many weeks pregnant you are, confirms the EDD, and sometimes combines with screening tests. Many EDDs are adjusted after this scan.

Anomaly (Anatomy) Scan (Around 18�21 Weeks)

The anomaly scan is a mid-pregnancy ultrasound that looks closely at your baby�s development and anatomy. It may be scheduled slightly earlier or later depending on the hospital.

Viability Threshold (~24 Weeks)

Around 24 weeks, a baby may be considered viable in some contexts. This does not guarantee survival, but it often marks a point where doctors may discuss the chances of intensive care if very early birth occurs.

Full-Term Window (39�40+6 Weeks)

Although pregnancy is often described as 40 weeks, babies are considered full term roughly from 39 weeks up to 40 weeks + 6 days. Your baby may arrive earlier or later, but this window is often used as a reference when discussing planned delivery.

LMP vs Ultrasound � Which Date Is More Accurate?

Many people notice that their LMP-based due date and their scan-based due date are not the same. This can be confusing, but it is very common.

Why LMP can be off

LMP assumes:

• You ovulate exactly 14 days after your period starts
• Your cycle is exactly 28 days long
• You remember the first day of your last period correctly

In reality, ovulation can happen earlier or later, cycles can change from month to month, and light spotting can be mistaken for a period. All of this makes LMP only an estimate.

Why NHS trusts usually follow the scan date

The dating scan uses your baby�s measurements, which are compared to standard growth charts. This method is less affected by late ovulation or unusual cycles. For this reason, NHS guidelines typically say to follow the scan-based EDD when there is a difference.

Our NHS Due Date Calculator supports both values so that you can see how they compare, but for medical decisions, always follow your midwife�s or doctor�s advice.

Why Your Due Date Sometimes Changes

It can feel unsettling when your due date is moved, but in practice this is normal. Dates are adjusted when new, more reliable information becomes available.

Common reasons for an EDD change

• Your cycles are longer or shorter than 28 days
• Ovulation happened later than expected
• Implantation took longer than average
• The dating scan suggests a different gestational age
• IVF dates show a more accurate starting point

Small changes of a few days are very common. Sometimes, especially when LMP is uncertain, the difference can be a week or more. The goal is simply to use the best available estimate for monitoring your pregnancy.

Factors That Affect the Accuracy of Your Due Date

Even with a precise NHS Due Date Calculator, there are natural biological differences that affect how accurately we can predict birth timing.

• Individual ovulation pattern: not everyone ovulates on day 14.
• Cycle variability: stress, illness, travel, and medication can shift ovulation.
• Implantation timing: embryos typically implant 6�12 days after ovulation.
• Growth differences: babies naturally grow at slightly different rates.
• Number of babies: twins and multiples are often born earlier than singletons.
• Medical conditions: pre-eclampsia, diabetes, or other issues can lead to earlier planned delivery.

These factors mean that an EDD is a useful guide, but never an exact prediction.

When to Seek Medical Advice Instead of Using a Calculator

While an NHS Due Date Calculator is a helpful tool, it is not a substitute for professional care. Contact your midwife, GP, or maternity unit urgently if you experience:

• Heavy vaginal bleeding or passing clots
• Severe abdominal or pelvic pain
• Sudden swelling, severe headache, or visual changes
• Waters breaking or leaking fluid
• Reduced or absent fetal movements once you feel your baby regularly
• Any symptoms your team has told you to watch out for

If you are worried at any time, it is always better to call your NHS maternity triage, 111, or emergency services depending on the severity and your local guidance.

Limitations & Important Medical Disclaimer

The Free NHS Due Date Calculator is designed for educational use and general information only. It cannot:

• Diagnose pregnancy or confirm viability
• Detect ectopic pregnancy, miscarriage, or complications
• Replace ultrasound scans or blood tests
• Override your clinician�s recommendations

Always follow the personalised advice you receive from your NHS midwife, GP, obstetrician, or fertility specialist. If your calculator results do not match what you were told in clinic, discuss it with your care team before drawing any conclusions.

FAQ � NHS Due Date Calculator, LMP, Scans & IVF

1. How accurate is the NHS Due Date Calculator?

The calculator uses the same principles that NHS tools use: 40 weeks from LMP, scan-based dating, IVF formulas, and conception-based calculations. However, no due date calculator can predict the exact day of birth. Only a small percentage of babies arrive exactly on their due date; most come within a range around it.

2. Does the calculator work if my periods are irregular?

Yes, you can still use it. If your cycles are irregular, the LMP estimate may be less precise, so it is often better to use a dating scan or a known conception/ovulation date if available. The calculator supports all of these methods.

3. My LMP due date and scan due date are different. Which is right?

In NHS practice, the scan-based EDD usually takes priority, especially if the difference is more than a few days. Your medical team will tell you which date they are using officially. Use that date when following clinical advice.

4. Can the NHS Due Date Calculator show how many weeks pregnant I am today?

Yes. The calculator automatically works out your gestational age today based on the method you choose and today�s date, so you can see exactly how many weeks and days pregnant you are.

5. I know the exact date I conceived. Is that better than LMP?

If you are confident about your conception or ovulation date, it can give a very good estimate. The calculator uses conception + 266 days to calculate your EDD, which is equivalent to the standard 40-week pregnancy length when counting from LMP.

6. How does the calculator handle IVF pregnancies?

For IVF pregnancies, you can enter the embryo transfer date and whether the embryo was day-3 or day-5, or use the egg retrieval/fertilisation date. The calculator then applies IVF-specific formulas to match clinical practice and give you an IVF-based EDD.

7. Will my baby definitely be born on the date shown?

No calculator can guarantee a birth date. The EDD is a best estimate. Many babies arrive a little earlier or later than predicted. Your midwife or doctor will monitor you and may discuss induction or closer monitoring if you go significantly past your due date.

8. Can I use this calculator to decide when to travel or work?

You can use the NHS Due Date Calculator as a guide when planning, but always check with your midwife, GP, or occupational health team before making decisions about travel, work, or physical activities during pregnancy.

9. Does the calculator replace my NHS midwife or GP?

No. The calculator is a helpful educational tool, but it does not replace professional healthcare. Always attend your antenatal appointments and follow the advice of the clinicians looking after you.

10. What should I do if my calculator results worry me?

If your results do not match what your midwife or doctor has told you, or if you feel anxious about your pregnancy dates, the best step is to discuss it with your care team. They can check your notes, scans, and blood tests and explain which dates they are using and why.