Due Date Calculator
Your estimated due date by Naegele’s rule, with current gestational age and the milestones ahead.
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Milestones
| Milestone | Week | Date |
|---|---|---|
| End of the first trimester | 13 | 15/01/2027 |
| End of the second trimester | 27 | 23/04/2027 |
| Full term (37 weeks) | 37 | 02/07/2027 |
| Due date (40 weeks) | 40 | 23/07/2027 |
How it works
A due date is counted from the first day of the last menstrual period, not from conception. The reason is practical: almost nobody knows the date of conception, while most people know when their last period started.
Naegele’s rule adds 280 days — forty weeks — to that date. Those 280 days already include roughly two weeks before conception actually happened, which is why "eight weeks pregnant" describes an embryo about six weeks old. That is the obstetric convention, not an error.
The correction most calculators skip is cycle length. Naegele’s rule assumes a 28-day cycle with ovulation on day 14. A longer cycle means later ovulation, so the due date moves back by the difference. For a 35-day cycle that is a full week — enough to turn a normal pregnancy into a "post-term" one on paper.
This applies that shift to both the due date and the estimated conception date, so the two stay consistent with each other.
Trimesters follow the standard boundaries: the first runs to the end of week 13, the second to the end of week 27, the third from week 28 until birth. From 37 weeks a pregnancy is considered full term.
Finally, a due date is an estimate rather than an appointment. Only about 4% of babies arrive on it, and most are born within two weeks either side. A first-trimester ultrasound dates a pregnancy more accurately than Naegele’s rule, and is what a clinician will go by if the two disagree.
Formula
Due date = last period + 280 days + (cycle length − 28) • Estimated conception = last period + 14 + (cycle length − 28)
Gestational age is the days elapsed since the last period, expressed in weeks and days. It therefore runs about two weeks ahead of embryonic age.
Worked example
Last period on 1 January, comparing a 28-day and a 35-day cycle:
- 28-day cycle — due date
- 8 October (280 days)
- 28-day cycle — estimated conception
- 15 January
- 35-day cycle — shift
- +7 days
- 35-day cycle — due date
- 15 October
- 35-day cycle — estimated conception
- 22 January
The same last period gives due dates a week apart. At 41 weeks, that week is the difference between continued monitoring and a conversation about induction.
Frequently asked questions
Why count from the last period rather than conception?
Because conception dates are rarely known and period dates usually are. The whole of obstetrics uses this convention, so the gestational age quoted at a scan is measured the same way.
Why is an eight-week pregnancy a six-week embryo?
Gestational age starts about two weeks before conception. It is a convention rather than a mistake — every antenatal milestone and growth chart is expressed in gestational weeks.
What if my cycles are irregular?
Enter your approximate average cycle length for a starting estimate, then rely on a first-trimester ultrasound. That is the most accurate method with irregular cycles, and what a clinician will use to fix the dates.
How many babies arrive on the due date?
About 4%. Most are born within two weeks either side. Treat it as the centre of a window rather than a date in the diary.
When is a pregnancy considered full term?
From 37 weeks. Before 37 weeks is preterm; from 42 weeks is post-term and usually monitored more closely.
My scan gives a different date. Which should I trust?
The scan, particularly a first-trimester one. Where the two differ materially, clinicians re-date the pregnancy by ultrasound because a direct measurement beats an inference from a remembered date.
Does this replace medical advice?
No. It applies a standard formula and is useful for picturing the timeline. Decisions about a pregnancy need actual clinical assessment.