How is the due date calculated?
The most common method is Naegele’s rule: you add 280 days, or 40 weeks, to the first day of your last period. By convention, pregnancy is counted from the first day of the last period rather than from conception, so at the moment of conception you are officially already about 3 weeks pregnant.
The rule assumes a 28-day cycle with ovulation around day 14. If your cycle is longer or shorter, ovulation happens later or earlier, so the calculator shifts the date by the difference: with a 32-day cycle the estimated date is 4 days later, with a 25-day cycle 3 days earlier.
Weeks and trimesters of pregnancy
Pregnancy is usually divided into three trimesters. The first lasts until the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. A birth between weeks 37 and 42 is considered full term, and before week 37 preterm.
The calculator shows how many full weeks and days have passed since the first day of your last period (adjusted for your cycle). A reading of 12 weeks and 3 days means you are in your 13th week of pregnancy.
| Trimester | Weeks |
|---|---|
| 1st trimester | Weeks 1–13 |
| 2nd trimester | Weeks 14–27 |
| 3rd trimester | Week 28 until birth |
How accurate is the calculation?
The date from the calculator is a good first estimate, but only an estimate. Studies show that only a small share of babies are born on the calculated day, while most arrive within two weeks before or after it. If you do not remember your last period exactly or your cycle is irregular, the difference can be larger.
The most reliable date is set by a gynaecologist using an ultrasound measurement of the embryo in the first trimester (crown-rump length). Once you have that date, go by it. If you suspect you are pregnant, book an appointment with a gynaecologist as soon as possible – they will guide you through check-ups, nutrition and any dietary supplements.
Frequently asked questions
How do I calculate my due date?
Add 280 days (40 weeks) to the first day of your last period. If your cycle is longer or shorter than 28 days, shift the date by the difference in days. The calculator does this for you.
Which week of pregnancy am I in?
Weeks are counted from the first day of your last period. If 8 weeks and 2 days have passed since then, you are in your 9th week of pregnancy, even though only about 6 weeks have passed since conception.
What if my cycles are irregular?
With irregular cycles, a calculation based on the last period is less reliable. In that case the most accurate date is the one your gynaecologist sets with an ultrasound in the first trimester.
Can the due date change?
Yes. After the first ultrasound, the gynaecologist often corrects the date by a few days. After that it normally stays the same, even if the baby grows faster or slower in later weeks.
When are babies most often born?
Most babies are born between weeks 38 and 42 of pregnancy. Only about 4% are born exactly on the calculated day, so the estimated date is more the middle of a period than an exact day.
Sources
- NHS – Pregnancy due date calculator
- ACOG – Methods for Estimating the Due Date (Committee Opinion 700)
- Jukic AM et al. – Length of human pregnancy and its natural variation (Human Reproduction, 2013)
Updated: October 2026
For information only, not a substitute for medical advice.