Quick answer: your estimated due date is typically 280 days (40 weeks) from the first day of your last menstrual period. Use our free Due Date Calculator for an instant estimate.
How the Estimate Works
Pregnancy is dated from the first day of your last period, not from conception, since ovulation date is often unknown. This means the "40 weeks" of pregnancy actually includes about 2 weeks before conception typically occurred.
Trimesters
Pregnancy is divided into three trimesters: first (weeks 1-13), second (weeks 14-27), and third (weeks 28-40+). Each carries distinct developmental milestones.
Frequently Asked Questions
How accurate is a due-date calculation?
Only about 5% of babies arrive exactly on their estimated due date — it's a helpful estimate, not a guarantee, with most births occurring within 2 weeks either side.
Can I calculate from conception date instead?
Yes, if you know your conception date, add 266 days for an alternative estimate.
Should I confirm this with a doctor?
Yes — an ultrasound-based estimate from a healthcare provider is more precise than a calendar-based calculation.
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Open the Tool →How the estimate is made
A common starting point is the first day of the last menstrual period. A simple estimate counts 280 days (40 weeks) from that date, assuming a typical 28-day cycle. If cycles are usually longer or shorter, the estimate may shift; uncertain dates and irregular cycles make a calendar calculation less dependable. A due date is an estimate, not a prediction of the exact day birth will happen.
The NHS notes that pregnancy normally lasts from 37 to 42 weeks from the first day of the last period. Its calculator asks for the last-period date and cycle length, and recommends speaking with a midwife or GP if the date is unknown or uncertain. An ultrasound and clinical assessment can provide a more informed estimate.
Use this calculator as an early guide
Enter the first day of the period, not the day it ended. If you are unsure of the date, have irregular cycles, or have questions about pregnancy care, contact a qualified maternity-care professional. Do not use an online date to make medical decisions or delay care. Read the NHS due-date guidance for its method and limitations.
What to do if dates are uncertain
If you cannot remember the first day of your last period, have unusually long or short cycles, or have bleeding that makes the date unclear, do not guess and treat the result as confirmed. A midwife or doctor can help estimate timing and advise what care is appropriate. The NHS describes its calculator as an estimate and says to speak with a midwife or GP when the date is unknown or uncertain. Use the due-date result for general planning only.