👶 Pregnancy Due Date Calculator
Calculate your estimated baby due date, current gestational age in weeks and days, and pregnancy trimester timeline using Naegele’s rule. 100% free.
What Pregnancy Due Date Calculator Does
A due date is an estimate, and the way it is calculated is older than almost anything else still in routine clinical use. Naegele’s rule takes the first day of your last menstrual period and adds 280 days — forty weeks. It assumes a 28-day cycle with ovulation on day 14, which is a reasonable average and is not most people.
That assumption is the main thing this calculator fixes. If your cycles run to 32 days you probably ovulated later, so the estimate shifts later by the difference; a 25-day cycle shifts it earlier. The adjustment is simply the gap between your cycle length and 28, added to the standard date.
The more important point is what the number is for. The NHS states that pregnancy normally lasts from 37 to 42 weeks from the first day of your last period — a five-week window. The due date is the midpoint of a range, not an appointment, and a dating scan at around 12 weeks estimates gestational age more accurately than any calculation from a remembered date. When the scan and the calculation disagree, the scan is the one that counts.
So this gives you the estimate, the window either side of it, and the milestones on the way — while being clear that it is arithmetic on a date you supplied, not a clinical assessment.
How to Use Pregnancy Due Date Calculator
- Select calculation method: Last Menstrual Period (LMP) or Conception Date
- Enter the date and your average menstrual cycle length
- View your estimated due date, current weeks/days pregnant, and trimester progress bar
Formula Used by Pregnancy Due Date Calculator
Naegele’s rule, adjusted for cycle length
EDD = first day of last period + 280 days + (cycle length − 28)
- 280 days
- 40 weeks — the convention, counted from the last period rather than from conception
- cycle length − 28
- the adjustment for ovulating earlier or later than the rule assumes
Worked example
Last period began 1 March, cycles of 32 days.
- Standard: 1 March + 280 days = 6 December
- Cycle adjustment: 32 − 28 = +4 days
- Adjusted: 10 December
Result: 10 December — with a normal range of roughly 37 to 42 weeks, meaning arrival any time from about 12 November to 17 December.
What the weeks mean
Counted from the first day of the last period, which is the convention — so you are counted as "2 weeks pregnant" around conception.
| Milestone | Weeks from last period |
|---|---|
| Conception (approximately) | 2 |
| End of the first trimester | 13 |
| Dating scan, typically offered | 11 to 14 |
| End of the second trimester | 27 |
| Considered full term | 39 to 40 |
| Normal range for birth | 37 to 42 |
How to Read Your Result
The scan overrides the calculation
A dating scan measures the baby directly and estimates gestational age more accurately than a date recalled weeks later. If the scan gives a different due date from this one, the scan is the one your care will be based on. This calculation is most useful before that scan, for knowing roughly where you are.
A date is really a five-week window
Very few babies arrive on the estimated date, and normal covers 37 to 42 weeks. Treating the number as a deadline causes a lot of unnecessary anxiety in the last fortnight; treating it as the middle of a range is both more accurate and more useful for planning.
Irregular cycles make the estimate weaker
The whole method rests on ovulation happening a predictable number of days after the period began. If your cycles vary substantially, the cycle-length adjustment is averaging something that is not stable, and the estimate carries more uncertainty than the arithmetic suggests. Conception-date dating, where you know it, is firmer.
Limitations & Accuracy Notes
- This is arithmetic on the dates you enter. It is not a clinical assessment and nothing here is medical advice — your midwife or doctor is the right source.
- Naegele’s rule assumes regular ovulation about 14 days after the period begins; irregular cycles weaken it.
- It does not handle IVF dating, which is calculated from the transfer date and embryo age rather than from a period.
- Twins and higher multiples typically arrive earlier, and the standard 40-week estimate does not reflect that.
- A dating scan is more accurate and supersedes this calculation.
Frequently Asked Questions
How is a pregnancy due date calculated?
What percentage of babies are born on their exact due date?
How is the due date calculated?
How likely is the baby to arrive on that date?
What if my cycle is not 28 days?
Why did my clinic give a different date?
What do the trimesters mean?
Is any of this medical advice?
References & Further Reading
- NHS — Pregnancy due date calculator — Source of the 37-to-42-week normal range, and of the point that the 12-week scan dates a pregnancy more accurately
- NHS — You and your baby at 12 weeks pregnant — What the dating scan does and when it is offered