How the due date is worked out
The standard method (Naegele’s rule) counts a pregnancy as 280 days, or 40 weeks, from the first day of your last period, not from the day you conceived. It assumes a 28-day cycle with ovulation in the middle. If your cycle is longer or shorter than 28 days, the extra or missing days are added to or taken off the date, which is what this calculator does and what the NHS calculator’s cycle length box does too.
Example: the last period started on 10 January 2026 and the cycle is 28 days, so the due date is 10 January + 280 days = 17 October 2026. With a 32-day cycle it is 21 October, four days later. Counting from conception instead, the length is 266 days (280 minus 14): conceiving on 24 January 2026 also gives 17 October.
How to read the result
- Weeks of pregnancy are counted from the first day of the last period, so they run about two weeks ahead of the baby’s real age. That is why you can be “12 weeks pregnant” about 10 weeks after conception.
- Trimesters: the first runs to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks (the NICHD division).
- The due date is not a deadline: only about 4% of women give birth on day 280, and pregnancy normally lasts 37 to 42 weeks from the last period.
Common mistakes
- Using the last day of the period, or the date of intercourse, instead of the first day of bleeding.
- Ignoring cycle length. A 35-day cycle moves the date a week later than a 28-day one.
- Treating the calculator as the final word: studies show that an ultrasound measurement of the baby in the first trimester dates a pregnancy more accurately than counting from the last period. If your doctor gives a different date, follow it.
- Counting with irregular cycles or an uncertain date, where the estimate is least reliable.
This calculator gives general information, not a diagnosis or medical advice.
Sources
- NHS: due date calculator — adjusting for cycle length; pregnancy normally lasts 37 to 42 weeks.
- Jukic et al., Human Reproduction: length of human pregnancy — 280 days is the convention; only 4% deliver on day 280.
- NICHD: pregnancy — trimesters — how pregnancy is divided into trimesters.
- PMC8265063: accuracy of due-date estimation — early ultrasound dating is more accurate than counting from the last period.
Once you have a rough date, check-ups need booking. In MyClinic (Sehaty) you search for a doctor by speciality and location and book without a phone call, and your medical file holds your history.
How to book a doctor online in Egypt · See MyClinic · Request a demo
Questions
How do I calculate my due date from my last period?
Add 280 days to the first day of your last period (or, roughly, subtract three months and add 7 days). A period starting on 10 January gives 17 October. If your cycle is not 28 days, shift the date by the difference.
How do I work out how many weeks pregnant I am?
Count the days from the first day of your last period to today and divide by 7. The whole number is weeks and the remainder is days. After 100 days you are 14 weeks 2 days.
How accurate is a due date?
It is an estimate. Only about 4% of women give birth on the estimated day, and anywhere from 37 to 42 weeks is within the usual range. Your doctor’s ultrasound dating is the one that counts.
What if my periods are irregular?
The 280-day rule assumes a regular cycle, so it is less reliable if yours is irregular or you are unsure of the date. An early ultrasound scan can date the pregnancy instead.
When does the first trimester end?
At 13 weeks 6 days counted from the first day of your last period. The second trimester starts at 14 weeks and the third at 28 weeks.
Why did my due date change after a scan?
Counting from a period assumes ovulation on day 14, and for many women it comes earlier or later. studies show first-trimester measurements of the baby are more accurate, so if your doctor sets a different date, follow it.
Read the guide
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