Pregnancy Due Date Calculator
Estimate your due date, current gestational age and trimester from your last period, conception date, IVF transfer date, or a due date you already have — plus a UK antenatal and maternity leave timeline.
Your Estimated Pregnancy Timeline
Dates are estimates based on the method you selected. Scan-based dates from your antenatal team always take priority over this calculation.
UK Maternity Leave Dates
General guidance based on standard statutory Maternity Leave rules, calculated from your Expected Week of Childbirth (EWC) — always check your own contract and eligibility with your employer, ACAS or GOV.UK.
Statutory Maternity Pay (SMP) has its own separate eligibility conditions and notice requirements, and is not the same as your right to Statutory Maternity Leave. Check your eligibility and notice deadline on GOV.UK or with your employer.
Pregnancy Due Date Calculator UK
Our Pregnancy Due Date Calculator helps you estimate when your baby is due, how far along your pregnancy is, and which trimester you’re in. Calculate from the first day of your last menstrual period (LMP), a known conception date, an IVF embryo transfer date, or a due date already given to you by a midwife or sonographer. You’ll also get an estimated UK antenatal timeline and statutory maternity leave dates. This tool is for general information only and does not replace advice from your midwife, GP or antenatal care team.
How Is a Due Date Calculated?
Most due dates are estimated using Naegele’s rule, which adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a typical 28-day cycle. If your cycle is longer or shorter than 28 days, the estimate can be adjusted accordingly. This is the same method used by the NHS due date calculator. If your dates come from an IVF embryo transfer instead, the calculation is based on the embryo’s known age at transfer rather than an assumed ovulation date, which generally makes it more precise.
| Dating Method | Formula | Typical Accuracy |
|---|---|---|
| Last menstrual period (LMP) | LMP + 280 days (adjusted for cycle length) | Good, refined by your 12-week scan |
| Conception date | Conception + 266 days | Good, if the date is well known |
| IVF Day 3 transfer | Transfer date + 263 days | High — exact embryo age known |
| IVF Day 5 transfer | Transfer date + 261 days | High — exact embryo age known |
| IVF Day 6 transfer | Transfer date + 260 days | High — exact embryo age known |
| 12-week dating scan | Set by sonographer measurement | Considered the most reliable estimate |
How to Use the Pregnancy Due Date Calculator
- Choose your dating method — last period, conception date, IVF transfer date, or a due date you already have.
- Enter the relevant date. For the last-period method, you can also adjust your average cycle length if it isn’t the standard 28 days.
- For IVF, select whether your transfer was a Day 3, Day 5 or Day 6 embryo, as recorded by your clinic.
- Click Calculate to see your estimated due date, current gestational age, trimester, and a full antenatal and maternity leave timeline.
Understanding Your Gestational Age and Trimester
Gestational age is conventionally counted in weeks and days from the first day of your last menstrual period — even if that’s not the actual moment of conception, which usually occurs around two weeks later. Pregnancy is commonly divided into three trimesters: the first trimester runs up to around 13 weeks, the second trimester from around 13 to 27 weeks, and the third trimester from around 28 weeks until birth. Pregnancy normally lasts around 37 to 42 weeks. Clinical terminology distinguishes between early term, full term, late term and post-term pregnancy within this range, and your midwife will usually discuss your options with you if you go beyond your due date.
Your Estimated UK Antenatal Timeline
The NHS offers a broadly similar schedule of antenatal appointments and scans to every pregnant person in the UK, though exact timing can vary by trust and by individual circumstances.
- Booking appointment: Your first, longest antenatal appointment, ideally arranged before 10 weeks. Your midwife will check your health, discuss your care, and calculate an initial due date.
- Dating scan (11–14 weeks): An ultrasound to confirm how many weeks pregnant you are, check your baby’s development, and see if you’re expecting more than one baby. This scan often sets the due date used for the rest of your pregnancy.
- Anomaly scan (18–21 weeks): Sometimes called the 20-week scan, this checks your baby’s physical development and looks for a number of rarer conditions.
- Gestational diabetes screening (24–28 weeks): Offered if you have risk factors for gestational diabetes, not to everyone.
- Typical pregnancy window (37–42 weeks): The range in which most babies are born.
UK Maternity Leave and Pay: Key Dates
UK statutory maternity dates are worked out from your Expected Week of Childbirth (EWC) — the Sunday-to-Saturday week containing your due date — rather than the exact due date itself. You should tell your employer that you’re pregnant, your expected due date, and (when ready) your intended leave start date by the end of the 15th week before the EWC, known as the “Qualifying Week”. The earliest that Statutory Maternity Leave can normally start is the 11th week before the EWC, though it can start later, including automatically the day after the birth if your baby arrives early. Statutory Maternity Pay (SMP) has its own separate eligibility conditions and notice rules, so it isn’t automatically the same as your leave start date. Rules and eligibility can be complex, so always confirm the specifics with your employer, ACAS, or GOV.UK, and use our Salary Calculator to help plan your household budget around your leave.
Why Might My Due Date Change?
- Your 12-week scan: Ultrasound measurements are generally considered more reliable than a date calculated from your last period alone, especially if your cycles are irregular.
- Irregular cycles: If you don’t have a regular 28-day cycle, an LMP-based estimate is inherently less precise — your dating scan will usually correct for this.
- IVF and known conception dates: These tend to give more precise estimates than the LMP method because the timing of conception (or embryo age) is documented rather than assumed.
- Every pregnancy is different: Even with an accurate due date, natural variation in labour timing means the actual birth date is genuinely unpredictable.
Frequently Asked Questions
How accurate is a pregnancy due date calculator?
A due date from a calculator is an estimate, not a guarantee. Calculations based on your last menstrual period assume a standard 28-day cycle and can be adjusted for a different cycle length, but a 12-week dating scan is generally considered the most reliable way to date a pregnancy. A normal, healthy pregnancy can end anywhere from 37 to 42 weeks.
What is Naegele’s rule?
Naegele’s rule is the standard formula used to estimate a due date: it adds 280 days, or 9 calendar months plus 7 days, to the first day of your last menstrual period. It is the method used by the NHS due date calculator and is adjusted for cycle length where that information is known.
How is an IVF due date calculated differently?
For IVF pregnancies, the due date is calculated from the embryo transfer date rather than the last menstrual period, because the embryo’s exact age is already known. A Day 3 (cleavage-stage) transfer adds 263 days to the transfer date, a Day 5 (blastocyst) transfer adds 261 days, and a Day 6 transfer adds 260 days. Fresh and frozen transfers use the same formula, since only the documented embryo age affects the calculation.
What is gestational age and how is it different from fetal age?
Gestational age is counted in weeks from the first day of your last menstrual period, which is roughly two weeks before conception typically occurs. Fetal (or conceptional) age counts from the actual date of conception and runs about two weeks behind gestational age. Doctors and midwives normally use gestational age, which is what this calculator reports.
When should I book my first midwife appointment?
You can self-refer to NHS midwifery services or contact your GP as soon as you know you’re pregnant. The NHS recommends trying to have your booking appointment, which covers your health history and initial checks, before you’re 10 weeks pregnant, as some screening tests need to be arranged early.
What happens at the 12-week and 20-week scans?
The dating scan, offered between 11 and 14 weeks, estimates how many weeks pregnant you are, checks your baby’s development, and confirms whether you’re expecting more than one baby; it may adjust your estimated due date to a more accurate scan-based estimate. The anomaly scan, offered between 18 and 21 weeks, checks your baby’s physical development in detail and screens for a number of rarer conditions.
By when do I need to tell my employer I’m pregnant in the UK?
You generally need to tell your employer that you’re pregnant, your due date and when you want your maternity leave to start by the end of the Qualifying Week, which is the 15th week before your Expected Week of Childbirth. Statutory Maternity Pay has additional eligibility, proof and notice requirements, so check GOV.UK or your employer for the rules that apply to you.
When can I start my Statutory Maternity Leave?
Under standard statutory rules, the earliest Statutory Maternity Leave can start is the 11th week before your Expected Week of Childbirth (EWC) — the Sunday-to-Saturday week containing your due date, not the exact due date itself. You can choose a later start date, and leave will start automatically the day after the birth if your baby arrives before your chosen date. Statutory Maternity Pay has its own separate eligibility conditions, so check your entitlement and start date on GOV.UK or with your employer.
What does “full term” mean, and what if my baby is overdue?
Pregnancy normally lasts around 37 to 42 weeks. Clinical terminology distinguishes between early term, full term, late term and post-term pregnancy within this range. If you go beyond your due date, your midwife or doctor will usually discuss monitoring and, if needed, options such as induction — this calculator cannot advise on that; please speak to your antenatal care team.
Can I use this calculator if I have irregular periods?
You can enter your typical average cycle length to adjust the estimate, but irregular cycles make last-period-based dating inherently less reliable. If your cycles are irregular, a dating scan is particularly important, and the due date given at your 12-week scan should take priority over any date calculated here.