Pregnancy Due Date Calculator
Calculate your estimated due date (EDD) based on the first day of your last menstrual period. This calculator uses the same method recommended by the NHS and healthcare professionals worldwide.
Your Estimated Due Date
How the Calculator Works
This pregnancy due date calculator uses Naegele’s Rule, the standard method employed by healthcare professionals worldwide. The calculation adds 280 days (40 weeks) to the first day of your last menstrual period, then adjusts for cycle length variations.
Naegele’s Rule Formula
LMP + 280 days = Estimated Due Date
Alternatively: LMP + 1 year – 3 months + 7 days
This method assumes ovulation occurs around day 14 of a 28-day cycle.
Cycle Length Adjustment
For cycles longer or shorter than 28 days, the calculator adjusts the due date accordingly.
Shorter cycles: Due date moves earlier
Longer cycles: Due date moves later
Pregnancy Timeline
First Trimester (Weeks 1-12)
Key developments include organ formation, heartbeat detection (around week 6), and the 12-week dating scan which provides the most accurate due date estimate.
Second Trimester (Weeks 13-27)
Often called the “golden period” of pregnancy. The 20-week anomaly scan occurs during this time, checking baby’s development and growth.
Third Trimester (Weeks 28-40+)
Final preparations for birth. Baby continues growing and developing, with full-term considered from 37 weeks onwards.
When Babies Actually Arrive
Full-Term Range
37-42 weeks is considered full-term. Most pregnancies naturally occur within this window without intervention.
Statistical Reality
Only 5% of babies arrive on their exact due date. About 90% are born within two weeks either side of the due date.
Factors Affecting Due Date Accuracy
Cycle Irregularity
Women with irregular cycles may find LMP-based calculations less accurate. Early ultrasound dating is more reliable in these cases.
Multiple Pregnancies
Twins and multiples typically arrive earlier, often between 34-37 weeks. Standard due date calculations may not apply.
First vs. Subsequent Pregnancies
First-time mothers often deliver slightly later than the estimated due date, whilst experienced mothers may deliver earlier.
Frequently Asked Questions
If you cannot remember your LMP date, speak to your midwife or GP. They can arrange an early dating scan, which is actually more accurate than LMP calculations, especially in the first trimester.
Dating scans performed between 8-13 weeks are considered the gold standard for due date prediction, accurate to within 3-5 days. LMP calculations can be accurate if cycles are regular but may vary by up to a week.
Yes, your healthcare provider may adjust your due date based on ultrasound measurements, particularly from early dating scans. Later scans are less reliable for dating as baby growth rates vary more significantly.
Going past your due date is normal and common. Your midwife will monitor you closely after 41 weeks. Induction may be offered around 41-42 weeks, depending on individual circumstances.
For IVF pregnancies, conception date is known precisely, making calculations more accurate. However, your fertility clinic will provide specific due date calculations based on embryo transfer dates.
When to Seek Medical Advice
Contact your midwife or GP if:
- You experience any unusual symptoms or concerns during pregnancy
- You have irregular periods and are unsure about your due date
- You need to discuss your birth plan or delivery options
- You have questions about pregnancy care and monitoring
- You’re approaching or have passed your due date
NHS Pregnancy Care
The NHS provides comprehensive antenatal care throughout pregnancy, including:
- Regular midwife appointments to monitor mother and baby
- Dating scan at 8-14 weeks for accurate due date assessment
- Anomaly scan at 18-21 weeks to check baby’s development
- Additional scans if medically indicated
- Birth planning and labour preparation