🤰 Due Date Calculator
Estimate your expected due date from your last menstrual period (LMP) or conception date using the standard Naegele's rule (LMP + 280 days).
The two methods, and which to pick
| Method | Counts | Use it when |
|---|---|---|
| Last menstrual period (LMP) | 280 days — 40 weeks | You know the first day of your last period. This is the standard clinical method. |
| Conception date | 266 days — 38 weeks | You know the date of conception, for example with IVF or tracked ovulation. |
The 14-day gap between them is not a discrepancy. Pregnancy is conventionally counted from the first day of the last period, roughly two weeks before conception actually happens — so at the moment of conception you are already described as two weeks pregnant.
Where the 280 days comes from
The LMP method is Naegele's rule, in use since the early nineteenth century: add a year, subtract three months, add seven days. It assumes a regular 28-day cycle with ovulation around day 14.
That assumption is the main source of error. If your cycles run consistently longer or shorter than 28 days, ovulation moves with them and the estimate shifts by the same amount — a regular 35-day cycle puts the real date about a week later than the rule suggests.
How much weight the date deserves
Very little as a specific day. Only around 4% of babies arrive on their estimated due date. Roughly 90% arrive within the two weeks either side of it, which is why a full-term pregnancy is defined as a range — 37 to 42 weeks — rather than a date.
The date is genuinely useful for what it was built for: sizing the trimesters, timing screening tests, and deciding when a pregnancy is post-term. It is a planning anchor, not a prediction.
Reading the rest of the output
- Days remaining — plain subtraction between today and the estimated date. Negative once the date has passed, which is normal and not a cause for alarm on its own.
- Weeks pregnant — shown for the LMP method only, because it is measured from that date by definition. Choosing the conception method leaves it blank rather than guessing backwards.
Frequently asked questions
How is a due date calculated?
By adding 280 days to the first day of your last menstrual period, or 266 days to the date of conception. Both come to 40 weeks and 38 weeks respectively, and both assume a regular cycle.
My cycle is not 28 days. Is the estimate wrong?
It will be off by roughly the difference. A regular 35-day cycle ovulates about a week later than a 28-day one, so the real date tends to fall about a week after the calculated one. An early ultrasound corrects for this and is what clinicians rely on.
How likely is the baby to arrive on the due date?
About 4%. Roughly nine in ten births fall within two weeks either side, and anything from 37 to 42 weeks is considered full term.
Which method should I use for IVF?
The conception-date method, using the transfer or retrieval date your clinic gives you, since that date is known rather than inferred. Clinics usually adjust further for the embryo's age at transfer, so use their figure if you have it.
Why does the tool say I am already several weeks pregnant?
Because pregnancy is counted from the first day of your last period, not from conception. That convention adds about two weeks at the very start, and every clinical week count uses it.