Pregnancy Due Date Calculator

What an estimated due date means

The estimated due date (EDD) is the day around which your baby is most likely to arrive — 40 weeks from the start of your last period. It's a helpful planning anchor for prenatal appointments, tests and time off work, but it's genuinely an estimate, not a deadline. Only about 1 in 20 babies (around 4–5%) arrive on the exact due date; the great majority are born in the two weeks before or after. Thinking of it as a "due window" rather than a single day tends to be far less stressful as the date approaches.

This calculator estimates your EDD from the first day of your last period, adjusts for your cycle length if you provide it, and shows how many weeks along you are, your current trimester, and an estimated conception date.

How to use this calculator

Enter the first day of your last menstrual period (LMP) — the first day of bleeding, not when it ended — and, if you know it, your average cycle length. The default assumes a 28-day cycle; if yours is longer or shorter, providing it lets the tool adjust for when you likely ovulated. It then returns your estimated due date along with how far along you are today.

How it's calculated

The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last period. A common shorthand is to take that date, subtract three months and add seven days and a year. The rule assumes a regular 28-day cycle with ovulation around day 14, so if your cycle differs the calculator shifts the date accordingly. Note that this counts pregnancy from your last period — about two weeks before conception actually happens — which is why "weeks pregnant" is always roughly two weeks ahead of the embryo's true age.

When cycles are irregular or the LMP is uncertain, dating from an early ultrasound scan is more accurate. A first-trimester scan measures the baby's size to estimate age and is considered the most reliable way to set or confirm a due date; if it differs significantly from the LMP estimate, providers usually go with the scan.

Worked example

If the first day of your last period was 1 March, Naegele's rule adds 280 days to give an estimated due date of about 6 December the same year. By the rule-of-thumb shortcut: 1 March − 3 months = 1 December, + 7 days = 8 December (the small difference reflects month lengths). Either way, the actual birth could reasonably land any time from late November to mid-December.

Limitations & tips

A "full-term" pregnancy spans 37 to 42 weeks, so a birth a few weeks before or after the due date is entirely normal. This LMP-based method is least reliable when periods are irregular, when the last-period date is unknown, or after recent hormonal contraception — in those cases an ultrasound dating scan is the better source. Most importantly, this tool gives general information only and is not medical advice or a diagnosis. It cannot replace prenatal care: please confirm your due date and look after your pregnancy with a midwife, obstetrician or other qualified healthcare provider, and contact them promptly about any concerns or warning signs.

Sources & references

• American College of Obstetricians and Gynecologists — Committee Opinion No. 700, "Methods for Estimating the Due Date" — official guidance on dating pregnancies and when ultrasound should take precedence over LMP-based estimates.
• "Naegele's rule and the length of pregnancy – a review" — history and accuracy of the 280-day (40-week) rule this calculator uses.