Pregnancy Calculator
Find your due date and how far along you are from your last period, a conception date, an IVF transfer or an ultrasound scan, with the trimester, days to go and a timeline of key dates.
How to Use
- Choose what you know: Last period, Conception, IVF transfer, Ultrasound or a Due date you have already been given.
- Enter the date. For a last period, change the cycle length if your cycles are usually longer or shorter than 28 days; for IVF, pick a day-3, day-5 or day-6 embryo.
- For a scan, enter the date and the weeks and days it measured. Add your last period too to see whether the two due dates agree.
- Read the due date, how far along you are, the trimester and the days to go. The On date starts at today; change it to see any other day.
- The key-dates table lists the end of each trimester and the early-, full- and late-term dates, with a tick on the ones already passed.
Worked Example
From a last period. The last period started on Monday 2 March 2026 and cycles are 28 days. The due date is 2 March + 280 days = Monday 7 December 2026. On Sunday 4 October 2026 that is 216 days since the start, and 216 = 30 × 7 + 6, so you are 30 weeks 6 days: in the third trimester, with 64 days to go.
The common mistake: ignoring a long cycle. With a 32-day cycle, ovulation is usually about 4 days later than in a 28-day cycle, so the due date moves to 2 March + 284 days = Friday 11 December 2026, and on 4 October you are 30w 2d, not 30w 6d. The same kind of slip happens with IVF: counting a day-5 transfer on 20 March 2026 as if it were conception (+ 266 days) gives 11 December, but the day-5 rule (+ 261 days) gives Sunday 6 December 2026.
Scan and last period together. A scan on Sunday 20 September 2026 measured 8 weeks 2 days (58 days), so the due date is 20 September + (280 − 58) days = Friday 30 April 2027. A last period on 21 July 2026 would give Tuesday 27 April 2027, 3 days earlier. Before 9 weeks ACOG Committee Opinion 700 uses a cut-off of more than 5 days, so these two agree closely enough that the last-period date would usually stand; a 6-day gap would usually mean dating by the scan.
Show Work
Formulas
From Naegele’s Rule to Ultrasound Dating
The 280-day rule carries the name of Franz Karl Naegele (1778–1851), a German obstetrician at Heidelberg. It is often taught as a calendar shortcut: add a year, take away three months and add seven days. Because months are not all the same length, the shortcut can miss 280 days: for a last period on 2 March 2026 it gives 9 December, while counting 280 days gives 7 December. This calculator counts days.
The rule assumes a 28-day cycle with ovulation 14 days after the period starts, and that the date of the period is remembered correctly. ACOG Committee Opinion 700 (2017, reaffirmed 2025, with the American Institute of Ultrasound in Medicine and the Society for Maternal–Fetal Medicine) notes that only about half of women recall their last period accurately, and that a first-trimester ultrasound measuring the crown–rump length, up to 13 weeks 6 days, is the most accurate way to set or confirm the due date, to within about 5 to 7 days. For IVF pregnancies it says to date from the transfer and the embryo’s age.
In 2013 ACOG Committee Opinion 579 (reaffirmed 2025) replaced the old five-week “term” window, three weeks before to two weeks after the due date, with early, full, late and postterm, because outcomes for babies were found to differ across those weeks.
About This Calculator
This calculator works out a due date from a last period (adjusted for cycle length), a conception date, an IVF transfer, an ultrasound scan or a due date you already have, then shows how far along you are on any date, the trimester, the days to go and a timeline of the key dates. It counts whole calendar days, so time zones and clock changes never move the answer. It includes what the separate Due Date Calculator used to do.
Every date stays in your browser; nothing is sent anywhere.
For general information only — not medical advice. Talk to a doctor or other qualified professional about your own health.
Related tools: hCG Doubling Time Calculator, Corrected Age Calculator, and Period Calculator.
Frequently Asked Questions
How is the due date worked out?
By convention the due date is 280 days (40 weeks) after the first day of the last menstrual period (Naegele’s rule, the method in ACOG Committee Opinion 700). A last period starting Monday 2 March 2026 gives a due date of Monday 7 December 2026. With a 32-day cycle ovulation is usually about 4 days later, so the date moves 4 days to Friday 11 December 2026.
Why are you already two weeks pregnant at conception?
Pregnancy is counted from the first day of the last period, about two weeks before ovulation and conception in a 28-day cycle. So conception falls at 2 weeks 0 days, and the due date is 266 days after conception: conception on 16 March 2026 gives the same due date, 7 December 2026, as a last period on 2 March.
When does each trimester start?
ACOG Committee Opinion 700 uses up to 13 weeks 6 days for the first trimester, 14 weeks 0 days to 27 weeks 6 days for the second, and 28 weeks 0 days onwards for the third. For a due date of 7 December 2026 the first trimester ends on Sunday 7 June 2026 and the third starts on Monday 14 September 2026.
What do early term, full term and late term mean?
ACOG Committee Opinion 579 replaced the single word “term” with four labels: early term is 37w 0d to 38w 6d, full term 39w 0d to 40w 6d, late term 41w 0d to 41w 6d, and postterm 42w 0d or later. For a due date of 7 December 2026, full term runs from Monday 30 November to Sunday 13 December 2026.
How is an IVF due date worked out?
From the transfer date and the age of the embryo. ACOG Committee Opinion 700 gives 261 days after the transfer for a day-5 embryo and 263 days for a day-3 embryo. A day-5 transfer on Friday 20 March 2026 gives Sunday 6 December 2026; treating the transfer as conception (266 days) would put it 5 days late, on 11 December.
How do I use the Pregnancy Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Does it cost anything or need an account?
No. The tool is completely free, there is no account to create, and it keeps working offline after the page first loads.
Is anything I type uploaded?
No. The tool works entirely on your device, so the values you enter never leave your browser.
Common Use Cases
A new pregnancy
A last period that started 10 weeks ago means you are 10w 0d today, in the first trimester, with 210 days to the due date.
IVF and frozen transfers
A day-5 transfer 3 weeks ago counts as 5w 5d today, because the embryo was already 5 days old and conception is counted as 2w 0d.
Checking a scan
A scan on 20 September 2026 measuring 8w 2d gives a due date of 30 April 2027; a last period of 21 July 2026 gives 27 April, only 3 days apart.
Planning leave
With a due date of 7 December 2026, early term starts on 16 November and full term on 30 November.
Any other date
Change the On date to see how far along you will be on a holiday or an appointment, for example 25w 5d on 4 October with a due date of 12 January 2027.
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