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Your due date — the estimated date of delivery, or EDD — is the anchor every prenatal appointment, scan and milestone is measured against. It’s calculated from the first day of your last menstrual period (LMP), not from conception, because the LMP is the one date most people can pin down reliably.
Enter the first day of your last period and your usual cycle length, and this tool gives your estimated due date plus the dates each trimester begins. It’s an estimate: only about 1 in 20 babies actually arrives on the due date itself.
How is it calculated?
Naegele’s rule
The standard method adds 280 days (40 weeks) to the first day of your last period:
| Step | Calculation |
|---|---|
| Start | First day of last menstrual period (LMP) |
| Add | 280 days (40 weeks) |
| Result | Estimated due date (EDD) |
A common shortcut is the same thing stated differently: take the LMP, subtract 3 months, add 7 days and a year.
The cycle-length adjustment
The 280-day rule assumes a textbook 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, ovulation shifts and so does your true due date. This tool adjusts for the cycle length you enter: a 32-day cycle pushes the estimate roughly 4 days later, a 24-day cycle about 4 days earlier.
Trimesters
Pregnancy is split into three trimesters, dated from the LMP:
| Trimester | Weeks |
|---|---|
| First | 0 – 13 |
| Second | 14 – 27 |
| Third | 28 – birth |
Why it’s only an estimate
An LMP-based date assumes regular cycles and a known period date. A first-trimester ultrasound measures the baby directly and is more accurate — if your scan gives a different date, clinicians use that. A normal, full-term birth can happen any time from 37 to 42 weeks.
Worked example
Suppose your last period began on 1 March 2026 and you have a regular 28-day cycle. Adding 280 days gives an estimated due date of 6 December 2026. The second trimester starts 14 weeks after the LMP (around 7 June), and the third trimester at 28 weeks (around 13 September). If your cycles instead run 32 days, ovulation is later, so the estimate moves about four days out to roughly 10 December.
FAQ
Is the due date counted from my last period or from conception?+
From the first day of your last menstrual period (LMP). Because conception typically happens about two weeks after the LMP, “40 weeks pregnant” actually means about 38 weeks since conception. The LMP is used because it’s the date most people can identify precisely.
How accurate is a due date?+
It’s an estimate. Only around 4–5% of babies are born on their exact due date; most arrive within the two weeks either side. A first-trimester ultrasound is more accurate than an LMP calculation and takes precedence if the two disagree.
What if my cycle isn’t 28 days?+
Enter your real average cycle length. The classic rule assumes a 28-day cycle with ovulation on day 14; longer or shorter cycles shift ovulation and the due date accordingly, and this tool adjusts for it.
What counts as full term?+
A birth between 37 and 42 completed weeks is considered term. Before 37 weeks is preterm; after 42 is post-term. The due date marks the middle of the normal range, not a deadline.
Can I rely on this instead of seeing a doctor?+
No. This is an informational estimate, not medical advice. Your due date should be confirmed by a healthcare provider, usually with an early ultrasound, and prenatal care should follow their guidance.