How Due Dates Are Estimated
This calculator uses Naegele's Rule, the standard clinical method: adding 280 days (40 weeks) to the first day of your last menstrual period, with a small adjustment if your average cycle is longer or shorter than 28 days.
Due Date = LMP Date + 280 days + (Cycle Length − 28)
For example, imagine your last menstrual period started on January 1st, and your average cycle is a typical 28 days. Adding 280 days lands the estimated due date around October 8th. Now let's say your cycle actually runs 32 days instead of 28 — the calculator adds those extra 4 days too, since a longer cycle generally means ovulation happens later, shifting conception (and the eventual due date) later along with it.
Why 280 Days, Specifically?
This number comes from a widely used clinical convention developed by German obstetrician Franz Naegele in the 1800s, based on the observation that a typical pregnancy lasts about 40 weeks from the last menstrual period — which works out to roughly 38 weeks from actual conception, since ovulation and conception usually happen around two weeks into a standard 28-day cycle, not on day one. This is a common point of confusion: pregnancy "weeks" as doctors count them start from your LMP, not from the day you actually conceived, so you're technically counted as being a couple of weeks pregnant before conception has even occurred.
Understanding Trimesters
Pregnancy is typically divided into three trimesters: the first spans weeks 1-13, the second weeks 14-27, and the third weeks 28-40. Each stage brings different developmental milestones for the baby and physical changes for the mother. A typical mistake we often see expecting parents make is assuming the trimesters split evenly into three roughly equal chunks — in practice, the boundaries are set by developmental milestones rather than a perfectly even three-way split of the full 40 weeks.
Why Cycle Length Matters for Accuracy
Naegele's Rule was originally built around the assumption of a standard 28-day cycle with ovulation occurring around day 14. Many people don't have exactly a 28-day cycle, which is exactly why this calculator lets you adjust for your own average cycle length. Imagine you're someone with a consistently longer 35-day cycle — using the standard 280-day calculation without adjusting for your actual cycle would systematically estimate your due date too early, since ovulation in a longer cycle typically happens later than day 14.
Important Note on Accuracy
Only about 5% of babies are actually born on their exact estimated due date — it's a target window, not a guarantee. Most babies arrive somewhere within about two weeks before or after the estimated date, and that's considered entirely normal. Your doctor may adjust this estimate based on an ultrasound, which is often more precise than LMP-based dating, especially with irregular cycles, since ultrasound measurements of the developing baby in early pregnancy tend to correlate very closely with actual gestational age regardless of how irregular a person's cycle history is.
What This Calculator Is, and Isn't, Useful For
This tool is genuinely useful for early planning purposes — getting a rough sense of your trimester timeline, when to expect key prenatal appointments, or simply satisfying natural curiosity about timing. What it isn't is a substitute for the dating your doctor or midwife provides based on clinical exam and ultrasound, particularly if your cycle history is uncertain or irregular, since those clinical methods are considered more reliable for actual medical planning and care decisions.