How a due date is worked out, and how much it can move
What Naegele's rule assumes, why a long cycle shifts the date by a week, and how rarely babies arrive on the day itself.
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In short
- The standard rule adds 280 days to the first day of the last period, which assumes a 28-day cycle and ovulation on day 14.
- A 35-day cycle moves the due date a week later. The calculator adjusts for cycle length; many do not.
- Gestational age is counted from the last period, so you are already about two weeks pregnant at conception.
- Only about 4% of babies arrive on the due date. Roughly 80% arrive within the two weeks either side of it.
- A first-trimester ultrasound dates a pregnancy more accurately than any calculation, and the scan date is what clinicians use.
A due date is an estimate with a wide distribution around it, and treating it as a deadline is the single most common misunderstanding about it.
Naegele's rule
The standard calculation adds 280 days — forty weeks — to the first day of the last menstrual period.
due date = LMP + 280 days + (cycle length − 28). The adjustment matters because the rule assumes a 28-day cycle: on a 32-day cycle the date moves four days later, and on a 25-day cycle three days earlier.
The shorthand version, from Franz Naegele in the early nineteenth century, is to add one year, subtract three months and add seven days. From 1 January 2026 that gives 8 October 2026, and the direct addition of 280 days gives the same.
Two assumptions are built in, and both are frequently wrong:
A 28-day cycle. Real cycles vary from about 21 to 35 days, and many women's vary between months.
Ovulation on day 14. Ovulation actually occurs about 14 days before the next period, which is only day 14 in a 28-day cycle.
Adjusting for cycle length
Because ovulation is fixed relative to the next period rather than the last one, a longer cycle delays conception and therefore the due date.
From an LMP of 1 January 2026:
| Cycle length | Estimated conception | Due date |
|---|---|---|
| 21 days | 8 January | 1 October 2026 |
| 28 days | 15 January | 8 October 2026 |
| 35 days | 22 January | 15 October 2026 |
Two weeks between the extremes — which, at the end of a pregnancy, is the difference between being told you are overdue and being told you are on time. This calculator makes the adjustment; a great many do not, and the unadjusted date is what causes women with long cycles to be offered induction earlier than they should be.
Why you are two weeks pregnant at conception
Gestational age is counted from the first day of the last period, not from conception, because that date is the one reliably known.
The consequence is that gestational age runs about two weeks ahead of the actual age of the embryo. At "six weeks pregnant" the embryo is about four weeks old. A positive test typically appears at four to five weeks gestational, which is two to three weeks after conception.
Fetal age, counted from conception, is used in embryology and almost never in clinical practice. Where two sources disagree by two weeks, this is usually why.
How rarely the date is right
Only about 4% of babies are born on their due date. The distribution around it:
| Timing | Approximate share |
|---|---|
| Before 37 weeks (preterm) | 10% |
| 37–38 weeks (early term) | 25% |
| 39–40 weeks (full term) | 40% |
| 41 weeks (late term) | 20% |
| 42 weeks or later (post-term) | 5% |
Roughly 80% of births fall in the two weeks either side of the due date, and first pregnancies run slightly longer on average than subsequent ones.
Some clinicians and midwives now prefer a "due month" or "due window" for exactly this reason — a date invites the expectation of a deadline, and the last two weeks of a first pregnancy are difficult enough without one.
What the scan does
A first-trimester dating scan, between 8 and 13 weeks, measures crown-rump length and dates the pregnancy to within about five days.
It is more accurate than any calculation, because early embryonic growth is remarkably consistent between pregnancies — far more so than cycle length or the recollection of a last period.
Standard practice is to redate the pregnancy if the scan differs from the LMP estimate by more than five to seven days in the first trimester. After that, the original date usually stands, because growth becomes more variable and a small baby at 30 weeks may be small rather than younger.
If you have had a dating scan, that date is the one to use. Everything on this page is what to do before you have.
Milestones from the due date
| Gestational age | Significance |
|---|---|
| 12 weeks | End of first trimester; risk of miscarriage falls sharply |
| 20 weeks | Anomaly scan; halfway point |
| 24 weeks | Viability — survival outside the womb becomes possible with intensive care |
| 28 weeks | Third trimester begins |
| 34 weeks | Lungs usually mature enough that outcomes are generally good |
| 37 weeks | Full term begins |
| 40 weeks | The due date |
| 42 weeks | Post-term; induction is normally offered before this |
For an LMP of 1 January 2026 with a 28-day cycle, viability falls on 18 June 2026 and full term begins on 17 September 2026.
IVF and known conception dates
Where conception is known precisely, the LMP calculation is unnecessary and less accurate.
IVF with a day-5 blastocyst transfer: due date is 261 days from transfer.
IVF with a day-3 embryo transfer: 263 days from transfer.
Known ovulation or insemination date: 266 days from conception.
These are considerably more reliable than any LMP-based estimate, because the uncertainty about when conception occurred is removed entirely.
What actually varies
Length of gestation genuinely differs between women and between pregnancies, and not only because of dating error.
A study measuring hormonal markers to identify the exact day of ovulation found natural variation of five weeks in the length of pregnancy even when the conception date was known precisely. Later implantation of the embryo was associated with a longer pregnancy.
Contributing factors include first pregnancy, which runs slightly longer; the mother's own birth weight; ethnicity, with some populations averaging a few days shorter; and a previous pregnancy's length, which is one of the better predictors of the next.
None of this makes the estimate useless. It makes it an estimate, which is what it was always meant to be.
Trimesters, and what happens in each
The three trimesters are a convention rather than a biological boundary, and the divisions vary slightly between sources. The common split:
First trimester, weeks 1 to 13. All major organs form. Miscarriage risk is highest and falls sharply after a heartbeat is seen. Nausea, fatigue and breast tenderness are common and usually ease towards the end. Folic acid matters most here, and ideally before conception.
Second trimester, weeks 14 to 27. Usually the most comfortable stretch. Movement is first felt between 16 and 22 weeks, later in a first pregnancy. The anomaly scan falls at 18 to 22 weeks, and gestational diabetes screening at 24 to 28.
Third trimester, weeks 28 to birth. Rapid growth, and increasing physical discomfort. Appointments become more frequent, fetal movement is monitored, and the baby usually settles head down between 32 and 36 weeks.
Where a source places the boundaries a week either way, nothing clinical turns on it.
Preparing without fixating on the date
Two practical habits reduce the strain of the last few weeks.
Have the bag and the plan ready by 36 weeks, not by 40. A tenth of babies arrive before 37 weeks and being ready early costs nothing.
Tell people a due month rather than a due date. The last two weeks of a first pregnancy are made considerably harder by a phone that rings every day asking whether anything has happened. A window absorbs that; a date invites it.
Induction is normally offered between 41 and 42 weeks, so a pregnancy continuing past the due date is being watched rather than ignored. Reduced fetal movement, bleeding, severe headache or visual disturbance are reasons to call immediately at any stage, regardless of what week the calculator says.
What this calculator assumes
- 280 days from the first day of the last menstrual period, adjusted for the cycle length you enter.
- A regular cycle. Where cycles are irregular, the LMP method is unreliable and a scan is the answer.
- Gestational age measured from LMP, so it runs about two weeks ahead of the age of the embryo.
- A single pregnancy. Twins and higher multiples deliver earlier on average and are managed on a different schedule.
- This is an estimate and not medical advice. A dating scan supersedes it.