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How to use the due date calculator
This due date calculator gives you an estimated due date, how many weeks pregnant you are today, your trimester and a list of the usual milestone dates. Pick the starting point you actually have.
- Last period: enter the first day of your last period and your usual cycle length. Leave it at 28 if you are not sure.
- Conception: use this only if you know the date, for example from ovulation tests or a single instance of intercourse.
- IVF transfer: enter the transfer date and choose a day 3, day 5 or day 6 embryo.
- Ultrasound: enter the scan date and the gestational age on the report, for example 10 weeks 0 days.
The “Calculate as of” box defaults to today. Change it to see how far along you will be at a holiday, a wedding or a work deadline. Press Save as PDF for a printable copy, or download the calendar file to put the main dates in your phone.
How is a due date calculated?
Doctors count pregnancy from the first day of the last period, not from conception. A full-term pregnancy is therefore 40 weeks, or 280 days, even though the baby has only been growing for about 38 of them. This is Naegele's rule:
Cycle adjustment = + (cycle length − 28) days
From conception: date + 266 days
Example: a last period starting on 1 January 2026 gives 8 October 2026. The shortcut you may see online (add 7 days, subtract 3 months, add 1 year) usually lands on the same day, but it can differ by a day or two depending on month lengths. Counting 280 days is the more consistent way, and it is what this page does.
What if my cycle is not 28 days?
The 280-day rule assumes ovulation on day 14. If your cycle is longer, ovulation comes later and so does the due date. If it is shorter, the date moves earlier. For a last period on 1 January 2026:
| Cycle length | Adjustment | Due date |
|---|---|---|
| 21 days | −7 days | 1 October 2026 |
| 24 days | −4 days | 4 October 2026 |
| 28 days | 0 | 8 October 2026 |
| 30 days | +2 days | 10 October 2026 |
| 35 days | +7 days | 15 October 2026 |
Irregular cycles are the case where a last-period date is least reliable. If yours vary a lot, an early ultrasound will give a better date than any formula.
How is the due date worked out after IVF?
With IVF the date of fertilization is known, so the calculation starts from the transfer. Add 14 days for the two weeks that are counted before conception, then the age of the embryo, and count 280 days from that imaginary start.
| Embryo | Add to transfer date | Transfer on 1 Feb 2026 |
|---|---|---|
| Day 3 | 263 days | 22 October 2026 |
| Day 5 | 261 days | 20 October 2026 |
| Day 6 | 260 days | 19 October 2026 |
Clinics sometimes use their own conventions, so if the clinic gives you a date, use theirs.
Key weeks in a 40-week pregnancy
Here are the common timings for the 1 January 2026 example:
| Week | What happens | Date |
|---|---|---|
| 4 | Period is due; a home test is usually positive | 29 January |
| 11 to 13 | First-trimester screening window | 19 March to 8 April |
| 14 | Second trimester starts | 9 April |
| 18 to 22 | Mid-pregnancy ultrasound window | 7 May to 10 June |
| 28 | Third trimester starts | 16 July |
| 37 | Early term begins | 17 September |
| 39 | Full term begins | 1 October |
| 40 | Due date | 8 October |
These are typical timings. Your doctor or midwife decides what you actually need and when.
When is the fertile window?
Sperm can survive up to five days, and an egg lasts about a day. The fertile window is the five days before ovulation, ovulation day and the day after. In a 28-day cycle starting 1 January, ovulation falls around 15 January, so the window runs roughly 10 to 16 January. The calculator shows this for the cycle that led to your pregnancy. It is not a contraception method, and cycles shift from month to month.
How accurate is a due date?
Treat it as the middle of a range. Only about 4 in 100 babies are born on the day itself, and a normal pregnancy can end any time from 37 to 42 weeks. A first-trimester ultrasound is generally the most reliable way to date a pregnancy, and if it differs from your last period by more than a week or so, your doctor may change the date. This page gives estimates only. Talk to your doctor or midwife about your own due date and care.
Frequently asked questions
How is my due date calculated?
The usual method is Naegele's rule: take the first day of your last period and add 280 days (40 weeks). If your last period began on 1 January 2026, the due date is 8 October 2026. This assumes a 28-day cycle and ovulation around day 14.
How do I calculate my due date if my cycle is not 28 days?
Add or subtract the difference from 28 days. With a 32-day cycle you add 4 days, so a last period on 1 January 2026 gives 12 October 2026. With a 25-day cycle you subtract 3 days. The calculator does this when you change the cycle length.
How do I calculate a due date from conception?
Add 266 days (38 weeks) to the conception date. A conception on 15 January 2026 gives a due date of 8 October 2026. This works best when the date is known, for example from ovulation tests, because most people do not know exactly when conception happened.
How is the due date worked out after IVF?
From the embryo transfer date: add 263 days for a day 3 embryo, 261 days for a day 5 embryo and 260 days for a day 6 embryo. A day 5 transfer on 1 February 2026 gives 20 October 2026. Your clinic may give you its own date, so follow theirs.
How many weeks pregnant am I?
Count the days from the first day of your last period (or the equivalent start date for other methods) and divide by 7. 87 days is 12 weeks and 3 days. Pregnancy weeks are counted from the last period, so you are already counted as 2 weeks pregnant at the time of conception.
How accurate is a due date?
It is an estimate. Only about 4 in 100 babies are born on the due date, and a normal pregnancy can end anywhere from 37 to 42 weeks. A first-trimester ultrasound is generally the most reliable way to date a pregnancy, and your doctor may change the date based on it.
When does the second and third trimester start?
The first trimester runs to 13 weeks and 6 days, the second from 14 weeks to 27 weeks and 6 days, and the third from 28 weeks until birth. With a due date of 8 October 2026, the second trimester begins on 9 April 2026 and the third on 16 July 2026.
What is the fertile window?
It is the five days before ovulation, plus ovulation day and the day after, because sperm can survive for up to five days. In a 28-day cycle, ovulation is around day 14, so the window is roughly days 9 to 15. Cycles vary, and this page is not a contraception tool.