User Guide
Step 1 — Enter your dates
- Add the requested dates and details.
Step 2 — See your results
- Key dates and a visual timeline update instantly.
Step 3 — Explore
- Review the breakdown, schedule and guidance.
Step 4 — Export
- Download a PDF to share with your provider.
About the Pregnancy Due Date Calculator
This tool estimates when your baby is due and turns that into a clear picture of your pregnancy: how many weeks and days along you are, which trimester you are in, how far you have progressed through the 40-week journey, your approximate conception date, what is developing this week, and a typical antenatal appointment schedule — all downloadable to share with your midwife.
How the estimate works
It uses Naegele’s rule, the long-standing clinical method: about 280 days from the first day of your last menstrual period. Because that assumes a 28-day cycle with ovulation around day 14, the tool adjusts the date if your cycle is longer or shorter, since conception shifts with ovulation. The result is a best estimate, not a deadline — only around 5% of babies arrive exactly on the due date, and births anywhere between 37 and 42 weeks are considered full term.
More than a date
Knowing the date is only part of it. The tool places you on a 40-week timeline split into trimesters, counts down the days remaining, and shows a short, accurate note on what is developing in your baby this week. It also lays out the typical schedule of antenatal checks — booking appointment, dating and anomaly scans, and the later growth and wellbeing visits — so you know roughly what to expect and when.
Use it alongside your care, not instead of it
An ultrasound gives more accurate dating than the last period, especially if your cycles are irregular, so your provider’s date takes precedence over any calculator. This tool gives general information for education, not medical advice; always follow your midwife, doctor or other qualified healthcare professional. Pair this with the Ovulation Calculator if you are still trying to conceive, or the Pregnancy Weight Gain Calculator once your pregnancy is confirmed.
The formula, and a worked example
The standard method is Naegele’s rule, which assumes a 28-day cycle with ovulation on day 14:
Due date = first day of last menstrual period + 280 days (40 weeks)
If your last period began 1 March 2026, the estimated due date is 6 December 2026 — 280 days, or 40 weeks, later. (You may also see the shorthand “add 7 days, subtract 3 months, add a year”, which gives 8 December here; it assumes 30-day months, so it can land a day or two off the exact 280-day count. This tool uses the exact count.) Note what this means: pregnancy is dated from the last period, not from conception, so at the moment of conception you are already counted as about 2 weeks pregnant.
Longer or shorter cycles shift the date. On a 32-day cycle you ovulate around day 18 rather than 14, so the due date moves roughly four days later — which is why the tool lets you adjust cycle length instead of assuming 28 days for everyone.
Sources & further reading
- American College of Obstetricians and Gynecologists (ACOG) — guidance on estimating the due date and why early ultrasound dating takes precedence over LMP.
- NHS — Due date calculator and pregnancy dating — the UK reference for the same 40-week convention.
- ACOG — How your fetus grows during pregnancy — week-by-week development behind the trimester milestones.
Only about 4% of babies arrive exactly on the estimated date; a normal term birth is anywhere from 37 to 42 weeks. A dating scan in the first trimester is more accurate than any calculator, particularly with irregular cycles.
Frequently Asked Questions
How is my due date calculated?
By Naegele’s rule: roughly 280 days (40 weeks) from the first day of your last menstrual period, adjusted if your cycle is longer or shorter than 28 days.
How accurate is the due date?
It is an estimate. Only about 1 in 20 babies arrive on the exact date, and a normal full-term birth is anywhere from 37 to 42 weeks. An early ultrasound dates a pregnancy more accurately than the last period.
What if my cycle isn’t 28 days?
Enter your average cycle length and the tool shifts the estimate accordingly, because ovulation (and so conception) happens later in longer cycles and earlier in shorter ones.
What are the trimesters?
The first trimester is weeks 1–13, the second 14–27, and the third 28 to birth. The tool shows which one you are in and how far along you are in weeks and days.
Can I use this if I know my conception or IVF date?
This version works from the last period. If you conceived via IVF or know your exact conception date, your clinic’s dating will be more precise — use theirs. This tool gives general information for education, not medical advice; always follow your midwife, doctor or other qualified healthcare professional.