Due Date Calculator
Estimate a pregnancy due date and current gestational age from the last menstrual period, with an honest account of how few babies arrive on the date.
Forty weeks pregnant is thirty-eight weeks of pregnancy
Gestational age is counted from the first day of the last period, which on a textbook cycle is about two weeks before ovulation and therefore about two weeks before conception. So the number everyone uses is a fortnight ahead of the thing it appears to describe.
| Weeks pregnant | Weeks since conception |
|---|---|
| 8 | about 6 |
| 12 | about 10 |
| 20 | about 18 |
| 28 | about 26 |
| 40 | about 38 |
The offset is exactly the assumed ovulation day — 14 days, or 2 weeks. It is counted that way because the last period is a date somebody can actually name and the moment of conception is not. The convention is sound, and the arithmetic surprises almost everyone the first time.
And the cycle length moves the date by a fortnight, end to end
The 280 days assume a 28-day cycle with ovulation on day 14. A longer cycle ovulates later, so conception is later, so the date is later — by exactly the difference between the cycle and 28.
| Cycle | Days from the last period | Against the default | Ovulation on day |
|---|---|---|---|
| 21 days | 273 | -7 | 7 |
| 24 days | 276 | -4 | 10 |
| 26 days | 278 | -2 | 12 |
| 28 days | 280 | the default | 14 |
| 30 days | 282 | +2 | 16 |
| 32 days | 284 | +4 | 18 |
| 35 days | 287 | +7 | 21 |
| 40 days | 292 | +12 | 26 |
A 21-day cycle and a 35-day cycle are two weeks apart on the same last period, and the whole sampled range spans 19 days. Anyone whose cycle is not 28 days and who uses the uncorrected figure is working from a date that is knowably wrong — which is the entire reason the cycle box is on this page.
All of which sits underneath the larger point: a due date is a middle, not a deadline. It is the day by which about half of births have happened, and a normal term stretches for weeks either side of it.
The three trimesters are 13, 14 and 13 weeks
Splitting forty weeks into three equal parts would give 13.38 weeks each. The conventional boundaries — the end of week 13 and the end of week 27 — give something else.
| Trimester | Days | Length | Weeks |
|---|---|---|---|
| 1st | 0–90 | 91 days | 13.00 |
| 2nd | 91–188 | 98 days | 14.00 |
| 3rd | 189–280 | 92 days | 13.14 |
The second is the long one, by a whole week. It happens because the first two boundaries were put on whole weeks rather than on equal shares — 13 and 14 exactly, with whatever is left over making up the third. Nothing depends on it; it is simply worth knowing that the thirds are not thirds.
How to use
- Enter the first day of your last menstrual period.
- Adjust the cycle length if yours is not 28 days.
- Read the estimated due date and current gestation.
- Treat any dating scan as more accurate than this.
Frequently asked questions
How is a due date calculated?
By Naegele's rule: 280 days, or 40 weeks, from the first day of the last menstrual period. That counts from before conception occurred, so gestational age is roughly two weeks more than the actual age of the pregnancy — a convention that confuses almost everyone at first.
How many babies arrive on their due date?
Very few — around 4 per cent. Term spans several weeks, and spontaneous labour most commonly begins somewhat after the estimated date for first pregnancies. The due date is the midpoint of a wide distribution, not a prediction.
Does cycle length matter?
Yes. The standard calculation assumes a 28-day cycle with ovulation on day 14. A consistently longer or shorter cycle shifts the date accordingly, and using the standard rule without adjusting can be off by several days.
Is an ultrasound more accurate?
Yes, particularly in the first trimester, where measurements date a pregnancy within a few days. Where a scan and the menstrual date disagree by more than a set margin, clinical practice is to use the scan. Later scans are progressively less reliable for dating, because growth varies more between babies.
What do the trimesters cover?
Roughly: the first through week 13, the second from 14 to 27, and the third from 28 to birth. The boundaries vary slightly by source, since they mark general phases of development rather than sharp transitions.
Can I rely on this calculator?
For a rough estimate, yes; for anything clinical, no. Pregnancy care depends on accurate dating, and that is a matter for a midwife or doctor with access to scans and your history. Nothing here is medical advice.
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