Pregnancy Due Date Calculator
Calculate when your baby is expected to arrive by entering the first day of your last menstrual period.
Your Estimated Due Date (EDD)
How to Use the Pregnancy Calculator
This calculator helps you estimate when your baby will arrive. Follow these steps for accurate results:
- Enter your LMP date: Select the first day of your last menstrual period from the calendar picker.
- Adjust cycle length: If your cycle is shorter or longer than 28 days, select the appropriate length from the dropdown menu.
- Calculate: Click the button to receive your estimated due date, current pregnancy week, and key milestones.
- Review milestones: Scroll through the important dates including trimester breakdowns, scans, and delivery window.
How the Calculation Works
The pregnancy due date is calculated using Naegele’s Rule, a standard method used by healthcare professionals worldwide.
The Formula
The calculation adds 280 days (40 weeks) to the first day of your last menstrual period, with adjustments made for cycle length:
- Standard 28-day cycle: LMP + 280 days = Due Date
- Shorter/longer cycles: The ovulation date is adjusted by the difference from 28 days, then 266 days (38 weeks) is added
Why 40 Weeks?
Pregnancy is measured from the first day of your last period, not from conception. This means:
- Week 1-2 count towards the 40 weeks, but conception hasn’t occurred yet
- Actual foetal development is approximately 38 weeks
- Ovulation typically occurs around day 14 of a 28-day cycle
- Conception happens shortly after ovulation
Cycle Length Adjustment
Women with irregular or non-standard cycles require adjustments:
- Shorter cycles (21-27 days): Ovulation occurs earlier, so the due date is calculated earlier
- Longer cycles (29-40 days): Ovulation occurs later, so the due date is calculated later
- The adjustment accounts for the shift in ovulation timing
Pregnancy Timeline Breakdown
| Trimester | Weeks | Key Developments |
|---|---|---|
| First Trimester | Weeks 1-13 | Major organs form, heartbeat begins, morning sickness common, booking appointment, 12-week dating scan |
| Second Trimester | Weeks 14-27 | Baby movement felt, anatomy scan at 20 weeks, rapid growth, gender may be visible, energy returns |
| Third Trimester | Weeks 28-40+ | Baby gains weight, lungs mature, gets into position for birth, antenatal classes, regular midwife checks |
Key Antenatal Appointments
- 8-10 weeks: Booking appointment with midwife – medical history, blood tests, screening options discussed
- 12 weeks: Dating scan – confirms due date, checks development, screening for Down’s syndrome offered
- 16 weeks: Midwife appointment – review test results, measure blood pressure, discuss concerns
- 20 weeks: Anomaly scan – detailed check of baby’s development and physical structure
- 25, 28, 31, 34, 36, 38, 40 weeks: Regular midwife appointments – monitor growth, position, and wellbeing
Frequently Asked Questions
How accurate is the due date calculator?
The calculator provides an estimate based on standard pregnancy length. Only about 5% of babies arrive exactly on their due date. The dating scan at 12 weeks provides the most accurate estimate, as it measures the baby’s actual size. Your due date may be adjusted following this scan.
What if I have irregular periods?
If your periods are irregular, you can still use the calculator by estimating your average cycle length. However, the result will be less accurate. Your 12-week dating scan will provide a more precise due date based on your baby’s measurements rather than menstrual dates.
What if I don’t know my last period date?
If you cannot remember your last period date, speak to your midwife or GP. They can arrange an early ultrasound scan to estimate how far along you are. This scan measures the baby’s size to calculate gestational age and due date.
When is my baby actually conceived?
Conception typically occurs around 2 weeks after the first day of your last period (during ovulation). The calculator estimates this date based on your cycle length. For a standard 28-day cycle, ovulation occurs around day 14.
What is considered full term?
A pregnancy is considered full term from 37 weeks onwards. Babies born between 37-42 weeks are classified as term births. Births before 37 weeks are preterm, whilst going beyond 42 weeks is post-term. Most women will be offered induction between 41-42 weeks.
Will my due date change?
Your due date may be adjusted following your 12-week dating scan if the measurements suggest you are more or less advanced than initially calculated. This is common and helps provide a more accurate estimate. After this scan, the date usually remains fixed.
What happens if I go past my due date?
Going past your due date is common. You’ll be offered additional monitoring from 41 weeks, including checks on your baby’s movement, heartbeat, and amniotic fluid. Induction of labour is typically offered between 41-42 weeks to reduce risks associated with post-term pregnancy.
Can I calculate my due date from conception date?
Yes, if you know your conception date (or the date of ovulation), add 266 days (38 weeks) to calculate the due date. However, most calculators use the last menstrual period as it’s easier to identify accurately.
Methods of Dating Pregnancy
Last Menstrual Period (LMP) Method
This is the most common method and the one used by this calculator. It assumes:
- A regular menstrual cycle
- Accurate recall of period dates
- Typical ovulation timing for your cycle length
Advantages: Simple, non-invasive, can be done immediately. Limitations: Less accurate for irregular cycles, requires accurate date recall.
Ultrasound Dating
Ultrasound scans measure the baby’s size to estimate gestational age:
- First trimester (before 14 weeks): Crown-rump length (CRL) measured – most accurate method
- Second trimester: Multiple measurements including head circumference, abdominal circumference, femur length
- Accuracy decreases as pregnancy progresses due to natural variation in baby size
Advantages: Very accurate in early pregnancy, doesn’t rely on LMP recall. Limitations: Requires scan appointment, accuracy decreases after first trimester.
Conception Date Method
If you know when conception occurred (from fertility tracking or assisted conception):
- Add 266 days (38 weeks) to the conception date
- Most accurate for IVF pregnancies where embryo transfer date is known
- Useful for those tracking ovulation with tests or fertility monitoring
Factors Affecting Due Date Accuracy
Biological Variables
- Natural variation: Pregnancy length naturally varies by up to 5 weeks between women
- First pregnancy: First-time mothers often deliver slightly later than average
- Previous pregnancy length: If previous pregnancies were longer/shorter, this one may follow a similar pattern
- Maternal age: Older mothers may have slightly longer pregnancies on average
- Baby’s genetics: Growth patterns are partly genetic and can affect dating by scan
Calculation Limitations
- Cycle irregularity: Irregular cycles make LMP-based calculations less reliable
- Recent contraceptive use: Coming off hormonal contraception can affect cycle regularity
- Breastfeeding: Conception whilst breastfeeding may occur without regular periods returning
- PCOS or other conditions: Conditions affecting ovulation reduce LMP accuracy
When Scans Are Essential
Ultrasound dating is particularly recommended if:
- You have irregular or infrequent periods
- You cannot recall your last period date
- You conceived whilst breastfeeding or using contraception
- There’s a significant discrepancy between LMP and initial scan
- You’re expecting multiples (twins, triplets, etc.)
What to Do After Calculating Your Due Date
Immediate Steps
- Contact your midwife or GP: Register your pregnancy to begin antenatal care
- Start folic acid: Take 400 micrograms daily (or 5mg if advised) until week 12
- Review medications: Check with your GP about any current medications or supplements
- Avoid alcohol: No amount of alcohol is safe during pregnancy
- Stop smoking: Seek support from your midwife or NHS smoking cessation services
- Check immunity: Confirm rubella immunity and discuss vaccinations with your GP
Planning Ahead
- Maternity leave: Start thinking about when you’ll begin leave (typically from 36 weeks)
- Antenatal classes: Book early as popular classes fill up quickly
- Birth preferences: Consider where and how you’d like to give birth
- Support network: Identify who will support you during pregnancy and after birth
- Financial planning: Research maternity pay, benefits, and childcare costs
References
- NHS. Pregnancy due date calculator. Available at: https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/ (Accessed December 2025)
- National Institute for Health and Care Excellence. Antenatal care for uncomplicated pregnancies. NICE guideline [NG201]. Published August 2021. Available at: https://www.nice.org.uk/guidance/ng201
- Tommy’s. Pregnancy due date calculator. Available at: https://www.tommys.org/pregnancy-information/calculators-tools-resources/pregnancy-due-date-calculator (Accessed December 2025)
- Royal College of Obstetricians and Gynaecologists. The Investigation and Management of the Small-for-Gestational-Age Fetus. Green-top Guideline No. 31. Published February 2013.
- Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates. American Journal of Obstetrics and Gynecology. 1996;174(1):278-281.
- Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ. Length of human pregnancy and contributors to its natural variation. Human Reproduction. 2013;28(10):2848-2855.
- NHS. You and your baby at 12 weeks pregnant. Available at: https://www.nhs.uk/pregnancy/week-by-week/1-to-12/12-weeks/ (Accessed December 2025)