Pregnancy Due Date Calculator
Calculate your pregnancy due date, trimester, and week by week milestones from your last menstrual period.
About this calculator
Enter the first day of your last menstrual period and this calculator estimates your due date, then lays out the key milestones of your pregnancy: the end of the first trimester, the anatomy scan, viability at 24 weeks, the start of the third trimester and the point where you reach early term.
It uses Naegele's rule, which adds 280 days, or 40 weeks, to the first day of your last period. Only about 4% of babies arrive on the due date, and most arrive within two weeks either side, so treat it as the centre of a window. Your doctor may adjust it after an early ultrasound.
How to use it
- Enter the First Day of Your Last Period.
- Enter your Cycle Length if it differs from 28 days.
- Press Calculate.
- Read your due date, current week and milestone dates.
Naegele's rule
Due date = LMP + 280 days (adjusted by cycle length minus 28 days for cycles that are not 28 days). Gestational age counts from the first day of the last period, not from conception, so you are considered two weeks pregnant at the time of ovulation. The tool marks the first trimester as ending at week 12, the second at week 26 and the third as starting at week 28 (conventions vary by a week or two between sources).
Worked example
Last period began 1 January 2026, 28-day cycle
| Due date | 8 October 2026 |
| End of first trimester (week 12) | 26 March 2026 |
| Anatomy scan (about 20 weeks) | 21 May 2026 |
| Viability (24 weeks) | 18 June 2026 |
| Third trimester begins (28 weeks) | 16 July 2026 |
| Early term begins (37 weeks) | 17 September 2026 |
| Full term (39 weeks) | 1 October 2026 |
This tool gives general information, not medical advice. Talk to a doctor before making health decisions.
Tips and common mistakes
- Your first-trimester ultrasound gives the most accurate dating. Follow your doctor's revised date.
- Early term is 37 to 38 weeks, full term 39 to 40, and post-term starts after 42.
- Start folic acid before conception and during the first trimester.
- Keep a list of questions for each antenatal visit.
- Call your doctor at once for bleeding, severe pain, reduced baby movement or waters breaking.
Frequently asked questions
How is a due date calculated?
Add 280 days (40 weeks) to the first day of the last menstrual period.
How accurate is the due date?
Only about 4% of babies are born on it. Most arrive between 37 and 42 weeks.
When does the first trimester end?
Around week 12 to 13, depending on the convention. This tool marks week 12.
What if my cycle is irregular?
An early ultrasound is a better way to date the pregnancy.
When is a baby considered full term?
From 39 weeks to 40 weeks and 6 days, according to ACOG.
Is the first day of the last period when I conceived?
No. Conception happens about two weeks later, around ovulation.