Your due date is 280 days (40 weeks) after the first day of your last menstrual period (LMP), if your cycle is 28 days. For a last period that started on 1 January 2026, the due date is 8 October 2026. A longer or shorter cycle moves the due date later or earlier by the same number of days.
Pick how you want to work it out, enter the date and the calculator shows your estimated due date, how far along you are today in weeks and days, your trimester and month, and the dates of the main scans and tests.
How the due date is calculated
Doctors count pregnancy from the first day of the last period, not from conception. That is why a pregnancy lasts 40 weeks on paper even though the baby has been growing for about 38.
- From the last period (Naegele's rule): LMP + 280 days. If your cycle is not 28 days, add or take away the difference. A 32-day cycle adds 4 days; a 25-day cycle takes away 3.
- From the conception date: conception + 266 days (38 weeks).
- From an IVF transfer: the American College of Obstetricians and Gynecologists (ACOG) dates IVF pregnancies from the embryo's age. For a day-5 blastocyst, the due date is the transfer date + 261 days. For a day-3 embryo it is + 263 days.
- From a known due date: if your doctor has already given you a due date from a scan, enter it to see your current week and key dates.
Worked example
Last period started on 1 January 2026, cycle 28 days.
| Step | Result |
|---|---|
| 1 January 2026 + 280 days | 8 October 2026 |
| Likely conception (LMP + 14 days) | 15 January 2026 |
| On 1 April 2026: 90 days since LMP | 12 weeks 6 days |
| Trimester | First (it ends at 13 weeks 6 days) |
| Anomaly scan window, 18+0 to 20+6 weeks | 7 to 27 May 2026 |
A day-5 embryo transferred on 20 January 2026 gives the same due date: 20 January + 261 days is 8 October 2026.
Cycle length changes the due date
Due dates for a last period starting on 1 January 2026.
| Cycle length | Adjustment | Due date |
|---|---|---|
| 24 days | −4 days | 4 October 2026 |
| 26 days | −2 days | 6 October 2026 |
| 28 days | none | 8 October 2026 |
| 30 days | +2 days | 10 October 2026 |
| 32 days | +4 days | 12 October 2026 |
| 35 days | +7 days | 15 October 2026 |
How to calculate EDD from LMP by hand
The quick hand method is add 7 days to the first day of your LMP, subtract 3 months, and add 1 year. For an LMP of 1 January 2026: 8 January, back 3 months to 8 October, which falls in 2026.
This shortcut can be a day or two off from exactly 280 days, because months have different lengths. For an LMP of 1 March 2027 it gives 8 December 2027, while 280 days gives 6 December 2027. This calculator and dating scans use the exact 280 days.
Pregnancy weeks to months
There is no official rule for turning weeks into months. This common grouping keeps three months in each trimester and is the one the calculator uses.
| Month of pregnancy | Pregnancy weeks | Trimester |
|---|---|---|
| Month 1 | Week 1 to 4 | First trimester |
| Month 2 | Week 5 to 8 | First trimester |
| Month 3 | Week 9 to 13 | First trimester |
| Month 4 | Week 14 to 17 | First trimester |
| Month 5 | Week 18 to 22 | Second trimester |
| Month 6 | Week 23 to 27 | Second trimester |
| Month 7 | Week 28 to 31 | Second trimester |
| Month 8 | Week 32 to 35 | Third trimester |
| Month 9 | Week 36 to 40 | Third trimester |
Forty weeks is a little over nine calendar months from the first day of your last period, which is why some families count it as nine months and some as ten.
Trimesters and key dates
ACOG counts the first trimester up to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 to birth. The calculator lists these dates for your pregnancy:
- Dating and nuchal translucency scan, 11 weeks 2 days to 14 weeks 1 day (the NHS screening window).
- Anomaly scan, 18 weeks to 20 weeks 6 days.
- Glucose tolerance test, 24 to 28 weeks, for women at higher risk of gestational diabetes. NICE lists a family origin with a high rate of diabetes, including Pakistani, Indian and Bangladeshi origin, as a risk factor, so many Pakistani women are offered this test.
- Term: early term 37+0 to 38+6 weeks, full term 39+0 to 40+6, late term 41+0 to 41+6, and post term from 42 weeks.
Your own doctor or hospital may use slightly different windows. Follow the schedule they give you.
When a scan changes your due date
An early ultrasound is more accurate than the date of your last period, especially if your cycles are irregular or you are unsure of the date. ACOG advises changing the due date when a first trimester scan differs from the LMP date by more than 5 days before 9 weeks, or more than 7 days from 9 weeks to 13 weeks 6 days. Once your due date has been set from a scan, choose "Due date" above and enter it.
This calculator estimates dates from the information you enter. It cannot confirm a pregnancy, check that it is healthy, or predict the baby's sex. See a doctor, lady health visitor or midwife as soon as you think you are pregnant, and seek care straight away for bleeding, severe pain, severe headache or reduced movements.
Pregnancy calculator questions
How many weeks pregnant am I?
Count the days from the first day of your last period to today, then divide by 7. The whole number is completed weeks and the remainder is days. If your last period started on 1 January 2026, on 1 April 2026 you are 90 days, which is 12 weeks 6 days pregnant. The calculator does this for you.
How do I calculate my due date with irregular periods?
If you know your usual cycle length, enter it and the calculator adjusts the date. If your cycles vary a lot, the LMP date is not reliable, and your due date should come from an early ultrasound. Once you have a scan date, choose the due date option to follow your weeks.
How is the due date worked out after IVF?
IVF dates are exact, so they are used instead of the last period. For a day-5 blastocyst, add 261 days to the transfer date. For a day-3 embryo, add 263 days. Both match a pregnancy that started 266 days before the due date, as with natural conception.
How accurate is the due date?
It is an estimate. Most babies are born between 37 and 42 weeks, and only a small share arrive on the due date itself. An early scan narrows the estimate. First babies and later babies follow the same dating rules.
Can a pregnancy calculator predict a boy or a girl?
No. Dates, cycle length and conception day cannot predict the baby's sex, whatever some websites claim. A study of timing and conception in The New England Journal of Medicine in 1995 found that when intercourse happens in the fertile window does not change the sex of the baby. Only medical tests or scans can show it.
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Last reviewed 30 September 2026. Results are estimates; official notices always take precedence.