Pregnancy Due Date Calculator UK

How to Use This Calculator

The calculator needs just two pieces of information:

• The first day of your last menstrual period (LMP): select the date your last period started, not when it ended
• Your average cycle length: for most women this is 28 days, but cycles can range from 21 to 35 days or longer. Enter your typical cycle length for a more accurate result

Click Calculate My Due Date to see your estimated due date, how many weeks pregnant you are today, and your key pregnancy milestones.

Just found out you're pregnant? Your next step is to contact your GP surgery or local NHS midwifery service as soon as possible to book your first appointment. You can self-refer directly to a midwife at most NHS trusts without needing to see your GP first.

How Your Due Date Is Calculated : Naegele's Rule

The standard formula for calculating an estimated due date is Naegele's Rule, developed by German obstetrician Franz Naegele in the early 19th century. It works by taking the first day of your last menstrual period, adding 280 days (40 weeks) and adjusting for cycle length if it differs from the standard 28 days. In practice, it works like this:

EDD = First day of LMP + 280 days (adjusted for cycle length)

Your period and ovulation are counted as the first two weeks of pregnancy. If your baby arrives on their due date, they will have been growing for 38 weeks, not 40, but you are still classed as 40 weeks pregnant.

If your cycle is longer than 28 days, ovulation happens later, so the calculator adds extra days accordingly. If your cycle is shorter, it subtracts days. This adjustment makes the estimate significantly more accurate than using a fixed 28-day assumption for everyone.

How Accurate Is a Due Date Calculator?

Your estimated due date (EDD) is a guide, not a guaranteed delivery date.

Only around 4 in 100 babies are born on their due date as it is just an estimate. Pregnancy normally lasts from 37 to 42 weeks from the first day of your last period.

About 50% of first-time mothers go past 40 weeks. The most accurate confirmation of your due date comes from the NHS dating scan, which is accurate to within approximately five days.

Calculator-based estimates are most accurate when:

• You know the exact date your last period started
• Your periods are fairly regular
• You entered your correct average cycle length

They are less accurate if you have irregular periods, have recently come off hormonal contraception or are unsure of your last period date. In all of these cases, your dating scan will establish the most reliable due date.

The NHS Dating Scan

NHS dating scans are performed between 10 and 14 weeks and measure your baby's crown-rump length (CRL), the distance from the top of the head to the bottom of the spine. This measurement is accurate to within approximately five days and is considered the most reliable method for establishing your due date.

At your dating scan you will also find out:

• Whether you are expecting more than one baby
• How your baby is developing
• Your confirmed gestational age in weeks and days

You will be offered your first appointment with a midwife, the booking appointment, by 10 weeks. At that appointment, you will be offered a separate appointment to have an ultrasound dating scan. The due date established at this scan will be used as your official EDD for the rest of your pregnancy.

If your scan date differs from the calculator estimate by more than a few days, the scan measurement takes precedence. This is normal and does not indicate any problem.

Calculating Your Due Date with Irregular Periods

If your periods are irregular, the LMP-based calculation is less reliable, but the calculator can still give a useful starting estimate.

If you had irregular periods before you got pregnant, you can still use the due date calculator. Select your average cycle length, the typical number of days between the first day of one period and the first day of the next.

If your periods are very irregular or unpredictable, your due date will be most accurately determined at your dating scan. Tell your midwife at your booking appointment that your cycles are irregular. They will note this and use the crown-rump length measurement from your scan as the definitive gestational dating method.

If you conceive using ovulation tracking or know your ovulation date precisely, you can calculate your EDD by adding 266 days (38 weeks) from the date of ovulation, as this bypasses the LMP-based estimation entirely.

IVF Due Date Calculation

If you conceived through IVF, the standard LMP-based calculator does not apply directly. Your due date is calculated differently depending on the type of transfer.

• Fresh embryo transfer (Day 3 transfer): Add 263 days (37 weeks and 4 days) from the date of egg retrieval.
• Fresh embryo transfer (Day 5 blastocyst transfer): Add 261 days (37 weeks and 2 days) from the date of egg retrieval.
• Frozen embryo transfer: Add 266 days (38 weeks) from the date of the transfer, adjusted for the developmental stage of the embryo at transfer.

Your fertility clinic will typically provide your estimated due date as part of your post-transfer documentation. Your NHS booking appointment and dating scan will then confirm the gestational age.

We also have a dedicated IVF Due Date Calculator that handles all transfer types automatically.

NHS Antenatal Care Schedule

Your due date determines the timing of every antenatal appointment and scan throughout your pregnancy. Here is the standard NHS schedule for a straightforward first pregnancy:

Women expecting their second or subsequent baby have fewer routine appointments, typically seven rather than ten.

Antenatal care is the health care and support you receive when you are pregnant. It is important for keeping you and your baby well through pregnancy and ready for the birth. You can refer yourself for NHS antenatal care with a midwife, visit your local hospital's website to find out more.

Your Pregnancy Timeline

• Weeks 1 to 4: The fertilised egg implants in the womb lining. A home pregnancy test becomes positive around week 4.
• Weeks 5 to 8: Your baby's heart begins to beat (around week 6). Major organs begin forming. Most women notice early symptoms, nausea, breast tenderness, fatigue and frequent urination.
• Weeks 9 to 12: Your baby is now called a foetus. Fingers and toes are forming. The risk of miscarriage reduces significantly after week 12.
• Weeks 12 to 14: Dating scan. Your baby is now around 5 to 8 cm long. You will be offered combined screening for chromosomal conditions at this scan.
• Weeks 13 to 27, Second trimester: Many women feel their best during this period. Your bump becomes visible. Your baby's movements become noticeable between weeks 16 and 24.
• Week 20: Anomaly scan. A detailed examination of your baby's structure and development.
• Weeks 28 to 40, Third trimester: Your baby grows rapidly and gets into position for birth. The NHS will monitor your blood pressure and urine at each appointment.
• Week 37 onwards: Your pregnancy is classed as full term. Your baby can arrive safely at any point from now.
• Week 40: Your estimated due date. Most babies arrive within two weeks either side of this date.

What to Do After Getting a Positive Test : UK Checklist

Finding out you are pregnant is a big moment. Here is what to do in the first few days and weeks:

• Contact a midwife or GP. Book your antenatal care as soon as possible. Most NHS trusts allow self-referral online or by phone. Your booking appointment should happen before 10 weeks.
• Start taking folic acid. The NHS recommends 400 micrograms of folic acid daily from when you start trying to conceive until 12 weeks pregnant. Folic acid significantly reduces the risk of neural tube defects such as spina bifida.
• Take a vitamin D supplement. The NHS recommends 10 micrograms of vitamin D daily throughout pregnancy.
• Stop smoking and avoid alcohol. Both carry significant risks to your baby's development. Your midwife can refer you to the NHS Stop Smoking Service which is free and effective.
• Review any medications. If you take prescription or over-the-counter medicines, check with your GP whether they are safe to continue in pregnancy before your booking appointment.
• Tell your employer when you're ready. You are entitled to paid time off for all NHS antenatal appointments: your employer cannot refuse this.
• Register with a dentist. NHS dental treatment is free during pregnancy and for 12 months after your baby's birth. If you are not already registered, find an NHS dentist through nhs.uk.

Maternity Leave and Pay: When Does It Start?

Your estimated due date determines key maternity leave and pay dates. Here is a summary of your rights in England, Scotland and Wales.

You are entitled to up to 52 weeks of statutory maternity leave: 26 weeks of ordinary leave followed by 26 weeks of additional leave. The earliest you can start maternity leave is 11 weeks before your due date, which falls at week 29 of your pregnancy. If your baby is born before your leave start date, your leave automatically begins from the date of birth.

You will also receive a MATB1 certificate from your midwife or GP after 20 weeks of pregnancy. This is the official document your employer needs to process your Statutory Maternity Pay (SMP) or Maternity Allowance.

Statutory Maternity Pay in 2025/26 is:

• 90% of your average weekly earnings for the first 6 weeks
• £187.18 per week (or 90% of earnings if lower) for the remaining 33 weeks

If you are self-employed or do not qualify for SMP, you may be entitled to Maternity Allowance instead. Check your entitlement at gov.uk.

Overdue Babies : What Happens After 40 Weeks?

Most babies do not arrive on their exact due date. If you reach 40 weeks without going into labour, your NHS care continues with additional support.

At 40 weeks the NHS offers a membrane sweep to help encourage labour. If labour has not begun by 41 weeks, another sweep is offered and an induction appointment is booked.

A membrane sweep is an internal examination where your midwife or doctor gently separates the membranes of the amniotic sac from your cervix. It is not harmful and can bring on labour within 48 hours in some women.

Induction of labour is typically offered between 41 and 42 weeks. Going significantly post-term increases the risk of complications, which is why the NHS monitors overdue pregnancies closely and discusses the options with you. The decision to induce is yours, your midwife will explain the risks and benefits clearly so you can make an informed choice.

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