What Is a Pregnancy Due Date?
Your pregnancy due date — officially called the Estimated Due Date (EDD) — is the date on which your baby is expected to be born, calculated as 40 weeks from the first day of your last menstrual period (LMP). It is called an estimated date because no calculation can predict exactly when labour will begin.
Only around 4% of babies are born on their exact due date. The vast majority — approximately 80% — arrive within two weeks either side. Nevertheless, your EDD is a crucial clinical reference point that shapes your entire antenatal care plan, determining when your scans are booked, when you will be offered screening tests, and at what point the NHS may consider interventions such as induction of labour.
To calculate your own EDD instantly, use our free Pregnancy Due Date Calculator NHS — which supports both LMP and conception date methods and shows your full trimester breakdown and key pregnancy milestones.
🍼 Quick answer: Your pregnancy due date = first day of your last period + 280 days (40 weeks). This is Naegele's rule — the standard method used by NHS midwives and GPs in 2026.
How Does the NHS Calculate Your Due Date?
The NHS uses a method called Naegele's rule to calculate your estimated due date. Named after the 19th-century German obstetrician Franz Karl Naegele, this formula has been the international standard for pregnancy dating for over 150 years and remains in use by the NHS in 2026.
Or: Add 9 months + 7 days to the first day of your last period
Why 40 Weeks? Why Not 9 Months?
The 40-week calculation can be confusing because pregnancy is commonly described as lasting nine months. In reality, pregnancy lasts approximately 40 weeks (280 days) from the first day of your last menstrual period — which is approximately 38 weeks from the actual point of conception (since ovulation typically occurs around day 14 of a 28-day cycle). The 40-week standard was adopted because most women know their LMP more precisely than their exact ovulation or conception date.
If you do know your ovulation date — perhaps from using an ovulation test kit or monitoring your cycle with our Ovulation Calculator NHS — you can calculate your EDD by adding 266 days (38 weeks) to your confirmed ovulation date for a more precise estimate.
The NHS Dating Scan — Your Most Accurate Due Date
While Naegele's rule gives a good starting estimate, the most accurate way to determine your due date is through the NHS dating scan, also called the 12-week scan. This is offered to all pregnant women in England between 11 weeks and 13 weeks and 6 days of pregnancy.
During the dating scan, a sonographer measures the crown-rump length (CRL) of the baby — the distance from the top of the head to the bottom. This measurement is used to calculate gestational age very accurately, as fetal size in early pregnancy is consistent across different pregnancies regardless of genetics or maternal factors.
📅 NHS policy on due date adjustment: If your dating scan EDD differs from your LMP-based EDD by more than 7 days, the NHS will adjust your official EDD to the scan date. If the difference is 7 days or less, your LMP-based date is usually kept. After 14 weeks, gestational age assessment becomes less accurate.
The 12-week scan also screens for chromosomal conditions (including Down's syndrome when combined with blood tests), checks for a multiple pregnancy (twins or more), and assesses the position of the placenta. It is one of the most important antenatal appointments in your pregnancy.
How to Calculate Your Due Date — Step by Step
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Step 1: Note the first day of your last menstrual period (LMP) This is day 1 of the cycle in which you became pregnant — the first day of bleeding, not the last. If you have a regular 28-day cycle, this is the most important date for your EDD calculation.
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Step 2: Add 280 days (40 weeks) to your LMP date This gives you your EDD using Naegele's rule. Use our Pregnancy Due Date Calculator NHS to do this automatically — it also shows your trimesters, key milestones, and antenatal scan windows.
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Step 3: Adjust for irregular cycles (if needed) If your cycle is longer or shorter than 28 days, your EDD may need adjustment. For a 35-day cycle, add 7 extra days to the standard calculation. Your dating scan at 11–14 weeks will provide a more accurate EDD regardless of cycle length.
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Step 4: Confirm with your NHS dating scan Your midwife or GP will book your 12-week scan. The sonographer will measure your baby's CRL and provide an adjusted EDD if the scan date differs by more than 7 days from your calculated date.
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Step 5: Plan your antenatal care around your confirmed EDD Your confirmed EDD determines the timing of all subsequent appointments — 20-week anomaly scan, blood tests, and midwife check-ups. Your NHS maternity team will provide a schedule based on your confirmed dates.
Pregnancy Trimesters — What Happens When
Pregnancy is divided into three stages called trimesters, each lasting approximately 13 weeks. Understanding your trimester helps you anticipate symptoms, prepare for key appointments, and understand your baby's development.
Major organ development. Morning sickness, fatigue, and breast tenderness common. Dating scan at weeks 11–14. Risk of miscarriage highest in this period.
Baby grows rapidly. Movements begin (usually felt from week 18–20). 20-week anomaly scan offered. Most women feel significantly better in this trimester.
Final organ maturation and rapid weight gain. Braxton Hicks contractions may begin. Birth plan preparation. Induction may be offered at weeks 41–42.
Key NHS Pregnancy Milestones and Appointments 2026
The following table shows the key pregnancy milestones and NHS appointments scheduled around your EDD in 2026:
| Week | Milestone / Appointment | What Happens |
|---|---|---|
| Weeks 8–10 | Booking appointment | First midwife visit. Blood tests, blood pressure, urine check, EDD discussion |
| Weeks 11–13+6 | Dating scan (12-week scan) | Confirms EDD via CRL measurement. Combined screening for Down's syndrome |
| Week 16 | Midwife appointment | Review screening results. Blood pressure, urine. Growth check |
| Weeks 18–21 | 20-week anomaly scan | Detailed check of baby's structure and organs. Placenta position |
| Weeks 24–28 | Glucose tolerance test | Screening for gestational diabetes if risk factors present |
| Week 28 | Anti-D injection (if Rh-) | For women with Rh-negative blood group |
| Weeks 36–38 | Late pregnancy checks | Baby position (breech check), blood pressure, birth plan review |
| Week 40 | EDD — your due date | Baby considered full term. Majority of spontaneous labours occur 38–42 weeks |
| Week 41–42 | Post-dates review | NHS may offer induction of labour. Monitoring of baby's wellbeing |
Healthy Pregnancy Tips — NHS Guidance 2026
Alongside knowing your due date, maintaining good health throughout pregnancy is essential for both you and your baby. Here are the key NHS recommendations for 2026:
Take Folic Acid and Vitamin D Daily
The NHS recommends 400 micrograms of folic acid daily from before conception until 12 weeks of pregnancy — to reduce the risk of neural tube defects such as spina bifida. Vitamin D (10 micrograms daily) is recommended throughout pregnancy and breastfeeding. You may be prescribed higher-dose folic acid if you have certain medical conditions — discuss with your GP.
Stay Well Hydrated
Pregnant women need approximately 300 ml more fluid per day than usual — around 2.3 litres total. Good hydration reduces the risk of urinary tract infections (common in pregnancy), helps prevent constipation, and supports amniotic fluid levels. Use our Water Intake Calculator NHS for your personalised daily fluid target (be sure to select the pregnancy option).
Understand Healthy Pregnancy Weight Gain
Weight gain is normal and expected in pregnancy. The NHS recommends gaining 11–16 kg in total for women with a healthy pre-pregnancy BMI (18.5–24.9). Pre-pregnancy BMI directly influences recommended weight gain — women with obesity (BMI 30+) are advised to gain less (7–11 kg) to reduce risks of gestational diabetes and hypertension. Check your pre-pregnancy BMI with our Visual BMI Calculator and NHS Healthy Weight Calculator.
Monitor Your Blood Pressure
High blood pressure in pregnancy (gestational hypertension or pre-eclampsia) is one of the most serious complications, affecting approximately 6–8% of pregnancies. It is routinely checked at every antenatal appointment, but you can also monitor at home. Check what your reading means with our Blood Pressure Calculator NHS and blood pressure chart UK. Report persistent headaches, visual disturbances, or sudden swelling to your midwife immediately.
Eat a Balanced, Safe Diet
Follow NHS Eatwell Guide principles during pregnancy. Avoid: raw or undercooked meat and eggs; unpasteurised dairy; liver and liver products (excess vitamin A); shark, swordfish, and marlin (high mercury); and alcohol (the NHS advises abstinence throughout pregnancy). Oily fish (up to 2 portions per week) and lean protein are excellent choices. For managing calorie intake safely, see our Calorie Deficit Calculator NHS — note that pregnant women should not be in a significant calorie deficit.
After Your Baby Arrives — NHS Tools for New Parents
Once your baby is born, monitoring their growth and development becomes the focus. Our suite of NHS-aligned child health tools supports you through the early years:
- Baby Weight Percentile Calculator UK — track your baby's weight centile from birth to 2 years using NHS UK-WHO growth chart data
- Child Growth Chart Calculator UK — growth tracking for children aged 0–18 using NHS centile charts
- Percentile Calculator UK — height and weight centiles for children of all ages
- Child BMI Calculator NHS — BMI centile calculator for children aged 2–18
For your own health recovery after pregnancy, use our NHS Healthy Weight Calculator and Ideal Weight Calculator UK to set a safe, realistic post-pregnancy weight goal. The NHS recommends waiting until at least 6–8 weeks after birth before attempting weight loss, and following the safe rate of 0.5–1 kg per week. For understanding how calorie management works, read our guide on what is a calorie deficit and use our Calorie Deficit Calculator NHS.
💡 Complete pregnancy and family health toolkit: Pregnancy Due Date Calculator NHS · Ovulation Calculator NHS · Baby Weight Percentile UK · Child Growth Chart UK · Child BMI Calculator NHS · Percentile Calculator UK · Blood Pressure Calculator NHS · Water Intake Calculator NHS · QRISK Calculator NHS · BMI Formula Explained · How to Calculate BMI · BMI Equation vs Calculator · NHS vs CDC Guidelines · 0.5–1 kg Rule · How Much Per Week Safely.
Frequently Asked Questions
The NHS calculates your estimated due date (EDD) using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a regular 28-day cycle. Your EDD is then confirmed or adjusted by your 11–14 week dating scan, which measures the baby's crown-rump length (CRL). Use our free Pregnancy Due Date Calculator NHS to calculate your EDD instantly.
No due date is perfectly accurate. Only about 4% of babies are born on their exact due date. Around 80% arrive within two weeks either side. The dating scan at 11–14 weeks is more accurate than LMP-based calculation — particularly for women with irregular cycles. If the scan date differs by more than 7 days from the LMP date, the NHS adjusts your EDD to the scan date.
Pregnancy is divided into three trimesters: First trimester (weeks 1–12): major organ development, highest risk of miscarriage. Second trimester (weeks 13–27): rapid baby growth, movements begin, most women feel their best. Third trimester (weeks 28–40): final organ maturation, rapid weight gain, preparation for birth. Your dating scan and anomaly scan are both offered during the first two trimesters.
According to NHS 2026 guidelines, a baby is considered full term between 39 and 40 weeks. Early term is 37–38 weeks. Late term is 41 weeks. Post-term is 42+ weeks. Babies born before 37 weeks are considered premature (preterm). The NHS may offer induction of labour after 41–42 weeks to reduce the risk of stillbirth. Discuss your preferences with your midwife or obstetrician.
The NHS dating scan (12-week scan) is offered between 11 weeks and 13 weeks 6 days. It measures the baby's crown-rump length (CRL) to confirm or adjust your EDD, checks for multiple pregnancies, and assesses chromosomal risk when combined with blood tests. It is the most accurate method of pregnancy dating available. Book through your GP or self-refer to your local NHS maternity unit.
If you have irregular periods, the LMP-based Naegele's rule may not accurately predict your due date. The NHS places greater reliance on the dating scan in this case. You can also use your confirmed ovulation date (from ovulation tests or IVF records) — add 266 days for your EDD. Our Ovulation Calculator NHS can help you identify your fertile window and track your cycle going forward.