Due date calculator
Enter the first day of your last period, or the day you conceived, to see your estimated due date and how many weeks pregnant you're today.
Your due datePick a date and the estimate appears here straight away.
How the due date is worked out
Most calculators, and most first appointments, start from the first day of your last period and add 280 days, which is 40 weeks.[1] That rule assumes a 28-day cycle, where ovulation comes about 14 days before the next period.[2]
If your cycle is usually 32 days, you probably ovulated about 4 days later, because the gap from ovulation to the next period is the steadiest part of the cycle.[2] So the estimate moves 4 days later too, which is how the NHS calculator adjusts for cycle length.[3] From a known conception date, the count is 266 days, which is 38 weeks.[1]
| Starting point | What the calculator adds |
|---|---|
| First day of last period, 28-day cycle | 280 days |
| First day of last period, other cycle length | 280 days, plus or minus the difference from 28 |
| Conception date | 266 days |
Weeks and trimesters
Weeks are counted from the first day of your last period, so "6 weeks pregnant" means about 4 weeks since conception.[1] The result above shows the week you're in and the trimester:
- first trimester: up to the end of week 13
- second trimester: weeks 14 to 27
- third trimester: week 28 until birth[1]
A baby born from 37 weeks is called early term, and from 39 weeks full term.[4]
Keep track after the first scan
Once your care team gives you a date, the pregnancy mode in Her Good Days uses it for your week, appointments and symptoms.
Questions
How is a due date calculated?
The usual method counts 280 days (40 weeks) from the first day of your last period, assuming a 28-day cycle with ovulation around day 14. This calculator moves the date by the difference if your cycles are longer or shorter. From a conception date it counts 266 days.
How accurate is it?
It's an estimate. The NHS says pregnancy normally lasts 37 to 42 weeks, and that the 12-week scan dates it more accurately. If the two differ, go by the date your midwife or doctor gives you.
Why am I already two weeks pregnant at conception?
Pregnancy weeks are counted from the first day of your last period, about two weeks before conception. So at the moment of conception you're counted as roughly 2 weeks pregnant.
Do you store what I enter?
No. The calculation runs in your browser, and nothing you type is sent to us or saved.
Sources
- ACOG: How your fetus grows during pregnancy Checked October 30, 2026
- ACOG: Fertility awareness-based methods of family planning Checked October 30, 2026
- NHS: Due date calculator Checked October 30, 2026
- ACOG Committee Opinion 579 (PubMed): Definition of term pregnancy Checked October 30, 2026
Spotted an error? Write to [email protected]. How we check health information.