Counts forty weeks from the first day of your last period to give an estimated due date, and shows the window either side of it that most births actually fall into. The window is the part worth remembering.
Key terms
- LMP — the first day of your last menstrual period. Gestational age is counted from here, which is why pregnancy is described as 40 weeks even though conception happens about two weeks into it.
- Estimated due date — 280 days after that first day.
- Full term — from 37 weeks. Past 42 weeks is called post-term.
How it works
MedlinePlus describes gestational age as measured in weeks from the first day of the last menstrual period, and puts most pregnancies at 37 to 42 weeks. The date here is the 40-week mark; the window is those two ends.
Estimated due date
due date = first day of last period + 280 days
37 weeks is 259 days and 42 weeks is 294 days from the same starting point. The rule assumes a 28-day cycle, so a different cycle length shifts all three dates by the difference.
Why the window matters more than the date
A single day is easy to remember and it is the least reliable part of this answer. MedlinePlus lists the reasons plainly: many people do not know the exact date of their last period, cycles vary in length, and not everyone has an early ultrasound. Births scatter across five weeks around the estimate, and arriving before or after the date is ordinary rather than a problem.
An early ultrasound dates a pregnancy more accurately than arithmetic from a remembered date. Where a scan date and this calculation disagree, the scan is the better number, and your provider will normally use it.
If your cycle is not 28 days
The 280-day rule assumes ovulation around day 14 of a 28-day cycle. A 35-day cycle ovulates about a week later, so the due date moves about a week later too — the option is there for that reason. It is the same rule adjusted, not a different method, and it does not make the estimate more precise than the window suggests.
How to use this calculator
- Enter the first day of your last period — the day bleeding started, not when it ended.
- If your cycle is not usually 28 days, open the cycle option and enter its length.
- Read the window alongside the date, and note how far along today’s reading says you are.
- Take the date to your first appointment as a starting point. A dating scan may move it.
Frequently asked questions
Why is pregnancy counted from my last period rather than conception?
Because the first day of a period is a date most people can identify, while the day of conception usually is not. It adds roughly two weeks at the start, which is why you are considered four weeks pregnant about two weeks after conception.
I do not know the date of my last period. What now?
Then this calculation cannot help much, and an early ultrasound is the reliable way to date the pregnancy. Your provider can arrange one and will use it to set the due date.
How likely am I to give birth on the due date?
Not very — it is the middle of a range rather than a prediction of a day. Most births happen somewhere across the 37 to 42 week window, which is exactly why the window is shown next to the date here.
What happens if I go past 42 weeks?
That is called post-term, and it is something to manage with your provider rather than wait out alone. They will discuss monitoring or induction. This page flags it; it does not advise on it.
Does this work for twins or IVF?
No. Multiple pregnancies commonly arrive earlier, and an IVF due date is calculated from the transfer date rather than a period, so both need dates your clinic will give you.
