How is the due date calculated?
A pregnancy lasts on average 280 days (40 weeks), counted from the first day of your last menstrual period (LMP). That seems odd, because you are not yet pregnant on that day: with a 28-day cycle, conception happens around day 14. Midwives and doctors still count this way because the first day of a period is usually known while the date of conception is not. This is known as Naegele's rule.
- From your last period: due date = first day of last period + 280 days.
- Different cycle length: if your cycle is longer or shorter than 28 days, ovulation shifts and so does the due date. With a 32-day cycle, add 4 days.
- From conception: due date = date of conception + 266 days (38 weeks).
Example
If the first day of your last period was Saturday, August 1, 2026 and your cycle is 28 days:
- your estimated due date is Saturday, May 8, 2027;
- on October 6, 2026 you are 9 weeks and 3 days pregnant, in the first trimester;
- the first trimester (week 12) ends around October 24, 2026;
- week 20, when the anatomy scan usually takes place, falls around December 19, 2026.
With a 32-day cycle, the due date moves to Wednesday, May 12, 2027. If you know the date of conception, say August 15, 2026, adding 266 days also gives May 8, 2027.
Pregnancy weeks and trimesters
How far along you are is written as weeks plus days, for example 9+3: nine full weeks and three days. Pregnancy is divided into three trimesters:
| Trimester | Weeks | What happens |
|---|---|---|
| First | 0–13 | organs form; nausea and tiredness are common |
| Second | 14–27 | bump becomes visible; you feel the baby move |
| Third | 28–40+ | baby grows and drops; preparing for birth |
A birth between 37 and 42 weeks is called full term and is normal. Only a small share of babies (a few percent) arrive exactly on the due date.
The ultrasound sets the date
A calculation based on your period is an estimate. Most people have a dating scan in the first trimester, where the length of the embryo is measured. If the scan differs from the calculation, your midwife or doctor uses the date from the scan. This matters especially if your cycle is irregular, you recently stopped hormonal contraception or you are not sure of the date of your last period.
Milestones during pregnancy
Useful dates include the week of the dating scan, the 20-week anatomy scan and when you plan to start maternity leave. You can follow the days to your due date with the days until calculator, or work out the days between any two dates with days between dates.
Still trying to conceive? The ovulation calculator shows your fertile window.
This calculator is a guide and does not replace medical advice. If you have questions, contact your midwife, doctor or obstetrician.
Planning maternity leave
Maternity and parental leave rules differ widely between countries and employers. In many systems the start of leave and the amount of pay are linked to your estimated due date, so it helps to know it early and to tell your employer in good time. Check the official government website for your country, your contract and your HR department for the rules that apply to you.
Frequently asked questions
How do I calculate my due date?
Add 280 days (40 weeks) to the first day of your last period, or 266 days to the date of conception. The calculator does this for you and takes your cycle length into account.
How many weeks pregnant am I?
The number of weeks and days since the first day of your last period. The calculator shows it for today.
Why count from the period and not from conception?
Because the first day of the period is usually known and the date of conception is not. During the first two weeks of the "pregnancy" you are not actually pregnant yet.
How accurate is the due date?
It is an average. Most babies are born between 37 and 42 weeks; only a few percent arrive on the exact due date.
What if my cycle is irregular?
Then the calculation is less reliable. The dating scan gives a more accurate date.
Can I use this after IVF?
Yes, choose "Date of conception". After an embryo transfer, the clinic uses its own calculation; follow their date.
Last reviewed: 2026-10-06. Results are estimates for information only.