How a due date is actually calculated
Your estimated due date is the first day of your last menstrual period plus 280 days — 40 weeks. That method is called Naegele's rule, and it is the standard used in antenatal care worldwide. It is what this calculator does, and what the Hamli app does.
The one thing most calculators skip is cycle length. Naegele's rule quietly assumes a 28-day cycle with ovulation on day 14. If your cycle runs to 35 days, you ovulate about a week later, so your due date sits about a week later too. This calculator shifts the date by that difference instead of pretending everyone is the same.
Why pregnancy is dated from your last period, not conception
The day you actually conceived is almost impossible to pin down. The day your last period started usually is not. So clinicians use it as a shared starting line. One consequence surprises a lot of people: gestational age runs roughly two weeks ahead of the embryo's real age. At "6 weeks pregnant" the embryo has been developing for about four. That is expected, and it is not a mistake in the maths.
Pregnancy weeks, months and trimesters
"I'm 20 weeks — what month is that?" is one of the most common questions in pregnancy, because weeks and calendar months do not line up neatly. Here is the mapping used in antenatal care:
| Month | Weeks | Trimester |
|---|---|---|
| 1 | 1 – 4 | First |
| 2 | 5 – 8 | First |
| 3 | 9 – 13 | First |
| 4 | 14 – 17 | Second |
| 5 | 18 – 22 | Second |
| 6 | 23 – 26 | Second |
| 7 | 27 – 30 | Third |
| 8 | 31 – 35 | Third |
| 9 | 36 – 40 | Third |
The first trimester runs from week 1 to 13, the second from 14 to 27, and the third from 28 until birth. A full-term birth is anywhere from week 37 to week 42 — so arriving a fortnight either side of your date is completely normal.
How accurate is this?
The arithmetic is exact. The date is an estimate. Fewer than 5% of people give birth on their due date, and most births land within a two-week window around it. Treat it as the centre of a range, not a deadline.
- An early scan beats the calendar. Measuring the baby in the first trimester is the most accurate way to date a pregnancy.
- If your doctor revises your date, use theirs. Switch to the "From due date" tab above and the calculator will work your week out from the revised date.
- Irregular cycles reduce accuracy. If your cycle length varies a lot, an early ultrasound is not a luxury — it is the reference.
You know your week. Now what?
A week number on its own does not tell you much. Each week has its own development, its own body changes, its own common symptoms, its own warning signs and its own scheduled checks. That is what the Hamli app is for: all 42 weeks in detail, the daily tools, and a red-flag guide that stays free and ad-free — all working offline, with your health data never leaving your phone.
More free tools
Medical note: this calculator and the information around it are educational only. They do not replace your doctor or midwife, and they are not a substitute for antenatal care. Any decision about your health or your baby's health belongs with your healthcare provider.
Frequently asked questions
How do I calculate my due date from my last period?
Add 280 days (40 weeks) to the first day of your last period — Naegele's rule. The calculator above does it instantly, and adjusts for your cycle length if it is not 28 days.
How many weeks pregnant am I?
Enter the first day of your last period and you will get your current week, the day within that week, your month and your trimester.
How accurate is a due date calculator?
The arithmetic is exact but the date is an estimate. A first-trimester scan is more accurate, and if your doctor revises your date, use theirs.
My cycle is irregular — can I still use this?
Yes. Pick the closest average from the list and treat the result as provisional until you have a scan. With irregular cycles the scan is the reference, not the calendar.
Does this calculator store my data?
No. Everything is calculated inside your browser. Nothing is sent to any server — not even your period date.
Can I calculate backwards from my due date?
Yes. Use the "From due date" tab and enter the date your doctor gave you; the calculator works out your current week from it.
Sources and review
- American College of Obstetricians and Gynecologists (ACOG) — estimating due date and gestational age
- World Health Organization (WHO) — antenatal care guidelines
- Naegele's rule — the standard method for estimating a due date
Content last reviewed: · By the NasrTech team · Contact: support@nasrtech.dev