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Due date calculator: how many weeks pregnant am I?

Calculate from
Enter a date to get your estimated due date.
An estimate, not a certain date

This gives an approximate date. Only the due date set by the midwife or doctor caring for you counts, usually after the dating scan.

No data is sent: the calculation runs in your browser. Unless the box is ticked, nothing is kept once you close the page.

Find your estimated due date

This due date calculator gives your estimated due date, how many weeks pregnant you are today, your current trimester and a calendar of key dates. Everything happens in your browser: nothing is sent anywhere.

How to use it

  1. Under "Calculate from", choose what you know: "Last period", "Conception date", "Dating scan" or "IVF embryo transfer".
  2. For your last period, enter its first day and your "Usual cycle length (days)", from 21 to 45. Leave 28 if you are not sure.
  3. For a scan, enter its date and the gestational age written on the report, such as 12 weeks + 3 days. For IVF, pick a day 3 or day 5 embryo.
  4. The results appear straight away, with the calculation spelled out. "Download calendar (.ics)" creates a file on your device that you can open in your calendar app.

How the due date is worked out

The calculator uses Naegele's rule: the estimated due date is 280 days, or 40 weeks, after the first day of your last menstrual period. For example, a last period that started on January 10, 2026 gives a due date of October 17, 2026.

The rule assumes a 28-day cycle, with ovulation around day 14. If your cycles are longer, ovulation tends to come later, so the date moves by the same number of days. With a 35-day cycle, the same January 10 start gives October 24, 2026, one week later. From a conception date, the tool adds 266 days. For IVF, it counts 261 days after a day 5 transfer and 263 days after a day 3 transfer: a day 5 transfer on February 3, 2026 points to October 22, 2026.

Weeks pregnant: two ways of counting

Midwives and doctors count pregnancy weeks from the first day of the last period. This is called gestational age, and it is the figure behind "how many weeks pregnant am I". Conception usually happens about two weeks after that date, so the age counted from conception is two weeks lower. On the same day, a pregnancy can be 36 weeks + 6 days by gestational age and 34 weeks + 6 days since conception. The calendar uses gestational age, as scan reports do.

Trimesters, scans and term

The calculator splits pregnancy into three trimesters: the first runs to 13 weeks + 6 days, the second to 27 weeks + 6 days, and the third lasts until birth. The calendar also marks the usual international windows for the first scan (11 to 14 weeks) and the mid-pregnancy scan (18 to 22 weeks). The scans actually offered depend on where you live and on your care, so treat these as reference points. The term period runs from 37 weeks to 41 weeks + 6 days.

The first scan often doubles as a dating scan. Measuring the baby can confirm or adjust the date based on your last period, which helps when cycles are irregular or the date is uncertain. If your due date was changed after a scan, choose "Dating scan" and copy the age from the report: a scan on March 23, 2026 at 12 weeks + 0 days gives a due date of October 5, 2026.

An estimate, not a promise

Only a small share of babies arrive on their exact due date; most are born in the weeks around it. The date is a guide for planning your care. The due date set by your midwife, doctor or the professional caring for you is the one that counts.

Frequently asked questions

I don't know the date of my last period. What can I do?

Use the date of your dating scan and the age written on the report. If you have not had a scan yet, talk to your midwife or doctor: the scan is what will date the pregnancy.

My cycles are irregular. Is the result reliable?

The cycle length adjustment assumes regular cycles. With irregular cycles, a date based on your period is less precise, and the date from your scan will usually be the one used.

Why are there 40 weeks if pregnancy lasts nine months?

The 40 weeks start at the last period, about two weeks before conception. Counted from conception, the pregnancy lasts about 38 weeks, which is close to nine calendar months.

Is my information saved?

Not unless you tick "Remember my entry on this device". Your entry then stays on this device only. Unticking the box deletes it.