How to use the calculator
- Choose First day of last period or Conception date.
- Pick the date.
- If you use your last period and your cycle isn’t 28 days, enter your usual cycle length.
You’ll see the estimated due date in both calendars, how many weeks and days pregnant you are today, your trimester, the days left, and the dates of key milestones.
How the due date is worked out
- From your last period: due date = first day of last period + 280 days + (cycle length − 28)
- From conception: due date = conception date + 266 days
Pregnancy is counted from the first day of the last period, about two weeks before conception. That’s why a pregnancy is “40 weeks” even though the baby develops for about 38.
Example: if your last period started on 1 January 2026 and your cycle is 28 days, the estimated due date is 8 October 2026. With a 32-day cycle, it’s 12 October 2026.
Trimesters and milestones
| Stage | Weeks |
|---|---|
| First trimester | Weeks 1 to 13 |
| Second trimester | Weeks 14 to 27 |
| Third trimester | Week 28 until birth |
| Full term | From 37 weeks |
| Due date | 40 weeks |
A note on accuracy
The due date is a guide, not a promise. Your doctor may adjust it after an early ultrasound, which measures the baby directly. Follow your doctor’s advice about check-ups and any symptoms.