Due Date From Conception Guide
Work out your estimated due date from your conception date or IVF transfer date, then see how many weeks pregnant you are, your trimester and the key dates ahead.
Due Date From Conception Calculator
Enter the date you conceived, or your embryo transfer date if you had IVF, to see your estimated due date and how far along you are today.
Conception Due Date Calculator
Estimates only. Your midwife will confirm your due date.
Due Dates at a Glance
The basics of pregnancy dating
Add 266 Days
A due date from conception is your conception date plus 266 days, which is 38 weeks.
Why 40 Weeks Is Standard
Pregnancy is normally counted as 40 weeks from your last period. Conception happens about two weeks after that, which is why the numbers differ.
The Date Is an Estimate
Only around 1 in 20 babies arrives on the due date. Most are born between 37 and 42 weeks.
A Scan Can Adjust It
Early ultrasound measurements are used to check your dates. If they differ by more than about a week, your due date may change.
How to Calculate Your Due Date From Conception
The rule behind the calculator, and why your due date is counted the way it is.
The 266-day rule
A full-term pregnancy lasts about 40 weeks from the first day of your last menstrual period, but conception usually happens around two weeks later, at ovulation. That leaves roughly 38 weeks, or 266 days, from conception to birth.
For example, if you conceived on 1 March 2027, your estimated due date is 22 November 2027.
Working back to a “last period” date
Healthcare professionals count pregnancy from the first day of your last period, even if you know your exact conception date. The calculator works out that equivalent date for you.
Gestational age and fetal age
Gestational age counts from the first day of your last period. Fetal age counts from conception, so it is about two weeks less. When your midwife says you are “10 weeks pregnant”, they mean gestational age.
Why the exact date is rarely certain
Sperm can survive for up to five days after sex, ovulation does not always happen on day 14 of a cycle, and the fertilised egg takes several days to implant. Unless you conceived through IVF or another assisted treatment, your conception date is best treated as a well-informed guess.
Due Date Calculation Methods
Different starting points, same aim: a date 40 weeks from the start of your last period. Choose the method that matches what you know.
| If you know | Add this | Notes |
|---|---|---|
| First day of last period | 280 days (40 weeks) | The standard method, most accurate if your cycle is regular at about 28 days. Also known as Naegele’s rule. |
| Conception or ovulation date | 266 days (38 weeks) | Used by this calculator. Only as accurate as your conception date. |
| IVF egg retrieval or fertilisation date | 266 days (38 weeks) | Fertilisation date is known exactly, so this is usually very reliable. |
| IVF day-5 embryo transfer | 261 days | The embryo is already five days old at transfer, so fewer days are added. |
| IVF day-3 embryo transfer | 263 days | The embryo is three days old at transfer. |
| Early ultrasound scan | Based on baby’s size | The sonographer measures the baby and sets the date. Often used to confirm or adjust your due date. |
Conception Date or Last Period?
Both methods give an estimate. Here is when each one is most useful.
Dating From Conception
Best when the date is known
- IVF and assisted conception. The transfer or fertilisation date is exact.
- Irregular cycles. A known ovulation date is more reliable than a last period date.
- One-off intercourse. A single known date narrows the window considerably.
- Tracked ovulation. Ovulation tests or a fertility tracker can pinpoint the day.
Dating From Last Period
Best when cycles are regular
- Regular 28-day cycles. Ovulation is likely around day 14.
- Certain start date. You remember the first day of your last period clearly.
- Standard NHS practice. Maternity teams start from this date before any scan.
- No conception date known. Most people conceive over several days of possible dates.
Trimesters and Term Definitions
Weeks are shown as gestational age, counted from the first day of your last period.
| Stage | Gestational age | What it means |
|---|---|---|
| First trimester | Up to 13 weeks 6 days | Early development. The dating scan usually takes place towards the end of this stage. |
| Second trimester | 14 weeks to 27 weeks 6 days | Many people feel baby’s first movements. The anomaly scan happens in this stage. |
| Third trimester | 28 weeks until birth | Baby grows and gains weight. Preparation for birth and maternity leave starts. |
| Early term | 37 weeks 0 days to 38 weeks 6 days | Considered term, but some babies born now may need extra support. |
| Full term | 39 weeks 0 days to 40 weeks 6 days | The ideal window for birth in an uncomplicated pregnancy. |
| Late term | 41 weeks 0 days to 41 weeks 6 days | Your midwife will usually discuss monitoring and induction at this point. |
| Post-term | 42 weeks 0 days onwards | Extra monitoring is recommended, and most maternity units aim to avoid reaching this stage. |
UK Pregnancy Milestones
Once you have your due date, these are the key points to plan around. Exact timings can vary between NHS trusts, so follow your own appointment letters.
About 10 to 14 weeks: dating scan
Offered in early pregnancy to check your due date, see how many babies you are expecting and confirm the baby’s growth. Your due date may be updated after this scan.
18 to 20 weeks 6 days: anomaly scan
A detailed scan that checks how your baby is developing. It is offered in the NHS between weeks 18 and 21.
From 20 weeks: MATB1 certificate
Your midwife or GP can provide a MATB1 form, which confirms your due date and is used to claim statutory maternity pay or Maternity Allowance.
11 weeks before your due week: earliest maternity leave
Statutory maternity leave can start no earlier than 11 weeks before the week your baby is due. Give your employer proper notice and check the rules on GOV.UK.
41 to 42 weeks: induction discussion
If your pregnancy goes past 41 weeks, your midwife will usually offer a membrane sweep and discuss induction of labour. Your maternity team will guide you based on your circumstances.
Due Date From Conception FAQ
Answers to the questions people ask most about working out a due date.
Add 266 days, which is 38 weeks, to your conception date. For example, if you conceived on 1 March 2027, your estimated due date is 22 November 2027.
Pregnancy is usually counted as 40 weeks, or 280 days, from the first day of your last menstrual period. Conception normally happens about two weeks after that date, so counting from conception gives 38 weeks, or 266 days.
A due date is an estimate. Only around 4% to 5% of babies are born on their due date, and most arrive between 37 and 42 weeks. Your midwife or sonographer may adjust the date after an early ultrasound scan.
Usually not. Sperm can survive in the body for up to five days, ovulation timing varies, and fertilisation can happen up to a day after ovulation. The date is most reliable after IVF or another assisted treatment, where the fertilisation or transfer date is known exactly.
Clinics count from the date the embryo was transferred. Add 261 days to a day-5 embryo transfer or 263 days to a day-3 embryo transfer. If you know the egg retrieval and fertilisation date, add 266 days to that date.
Gestational age counts from the first day of your last period, while fetal age counts from conception. Fetal age is about two weeks less than gestational age. Healthcare professionals in the UK use gestational age when they talk about how many weeks pregnant you are.
It might. If the size of the baby at an early dating scan differs from your date by more than about a week, the sonographer may move your due date. If your dates are clear, such as after IVF, the date is more likely to stay the same.
The earliest you can start statutory maternity leave is 11 weeks before the week your baby is due. Your baby’s expected week of childbirth is based on your due date, so check your dates and your notice requirements with your employer or GOV.UK.
A baby is early term from 37 weeks 0 days to 38 weeks 6 days, full term from 39 weeks 0 days to 40 weeks 6 days, late term from 41 weeks 0 days to 41 weeks 6 days, and post-term from 42 weeks 0 days.
Going past your due date is common, especially in a first pregnancy. Your midwife will usually offer a membrane sweep and discuss induction of labour, typically between 41 and 42 weeks. If you notice your baby’s movements slow down or stop at any stage, contact your midwife or maternity unit straight away.