Pregnancy Due Date
Estimate due date from last menstrual period.
About the Pregnancy Due Date
A due date is calculated as 280 days from the first day of your last period, not from conception. It is an estimate: most babies arrive in the two weeks either side.
How to use it
- Enter the first day of your last menstrual period.
- Adjust the cycle length if yours is not 28 days.
- Read the estimated due date.
The formula
Due dates are calculated with Naegele's rule, which counts forward from the first day of the last menstrual period rather than from conception.
Due date = First day of LMP + 280 days
Equivalently: LMP − 3 months + 7 days + 1 year
280 days is 40 weeks. The convention counts from the LMP because it is a date most people can identify, while conception typically occurs about two weeks later — which means you are considered two weeks pregnant at the moment of conception. That quirk confuses nearly everyone, and it is a bookkeeping convention rather than a biological claim.
The rule assumes a 28-day cycle with ovulation on day 14. For longer or shorter cycles the estimate shifts accordingly, which is one reason an early ultrasound is considered more reliable than the arithmetic.
Worked examples
| Milestone | Weeks from LMP | What it marks |
|---|---|---|
| Conception | ~2 | roughly two weeks after LMP begins |
| End of first trimester | 13 | risk of miscarriage falls markedly |
| End of second trimester | 27 | viability improves substantially |
| Full term | 39–40 | the target range |
| Estimated due date | 40 | 280 days from LMP |
The single most useful fact about a due date is how rarely it lands. Only about 4% of babies arrive on the estimated day, and roughly 90% arrive within two weeks either side. It is the centre of a distribution, not an appointment — which is why clinicians increasingly describe a due window rather than a date.
Common mistakes
- Counting from conception instead of the LMP. The standard method counts from the first day of the last period, about two weeks before conception. Counting from conception makes the pregnancy appear two weeks less advanced than clinicians will describe it.
- Applying the rule to an irregular cycle. Naegele's rule assumes 28 days with ovulation on day 14. Longer or shorter cycles shift the date, and irregular ones make the arithmetic unreliable.
- Treating the date as fixed. It is an estimate with a wide distribution. Only about 4% of births occur on the estimated day.
- Preferring the calculation to an early scan. A first-trimester ultrasound dates a pregnancy more accurately than any calculation from the LMP, and clinicians will revise the date if the two disagree materially.
Terms explained
- LMP
- Last menstrual period — specifically its first day, the starting point for the standard calculation.
- Gestational age
- Time elapsed since the LMP. The measure used clinically, and about two weeks ahead of fetal age.
- Fetal age
- Time since conception, roughly two weeks less than gestational age. Rarely used in clinical conversation.
- Naegele's rule
- The 280-day calculation, named for the nineteenth-century obstetrician who popularised it.
- Trimester
- One of three roughly three-month phases, conventionally ending at weeks 13 and 27.
- Full term
- 39 weeks 0 days to 40 weeks 6 days. Earlier is 'early term', later is 'late term' — distinctions introduced because outcomes differ across them.
Common questions
- How is a due date calculated?
- By adding 280 days — 40 weeks — to the first day of your last menstrual period. The shortcut is to subtract three months, add seven days, and add a year.
- Why does it count from my last period rather than conception?
- Because the LMP is a date most people can identify, while the moment of conception usually is not. It is a practical convention, and it means gestational age runs about two weeks ahead of fetal age.
- How accurate is a due date?
- About 4% of babies arrive on the estimated day, and roughly 90% within two weeks either side. It marks the centre of a range rather than a scheduled event.
- What if my cycle is not 28 days?
- The estimate shifts. A consistently longer cycle generally means a later due date. An early ultrasound handles this far better than any adjustment to the formula.
- Is an ultrasound more reliable?
- Yes, particularly in the first trimester, where measurements date a pregnancy within a few days. Clinicians will usually revise an LMP-based date if a scan disagrees substantially.
- What does full term actually mean?
- 39 weeks 0 days to 40 weeks 6 days. The category exists because outcomes at 37 weeks differ measurably from those at 39, which earlier definitions obscured.
- How are the trimesters divided?
- Conventionally the first ends around week 13 and the second around week 27, with the third running to birth. The boundaries are approximate and vary slightly between sources.
- Should I make plans around the due date?
- Plan around the fortnight either side of it. The date is genuinely useful for tracking progress and scheduling care, and it is not a prediction of any particular day.