Estimate your due date from your last menstrual period (LMP).
Estimated due date = first day of last menstrual period + 280 days (40 weeks). Actual delivery dates vary — this is a standard estimate, not a diagnosis.
For general wellness information only — not medical advice. Consult a healthcare professional for personal guidance.
The standard due-date method, Naegele's Rule, calculates an estimated date by adding 280 days (40 weeks) to the first day of your last menstrual period. It's a genuinely useful planning reference, but it rests on one assumption that doesn't hold for everyone: a textbook 28-day menstrual cycle with ovulation happening exactly on day 14. In practice, cycle length varies significantly between individuals, and factors including ethnicity, height, parity (whether it's a first pregnancy or not) and maternal weight have all been shown to influence typical pregnancy length — which is exactly why only around 4-5% of babies are actually born on their calculated due date.
A due date is far more accurately understood as the middle of a normal window than a specific deadline: about 80% of babies are born within two weeks before or after the estimated date, and a full-term pregnancy is medically defined as anywhere from 37 to 42 completed weeks of gestation — a five-week range, not a single day. If your cycles are irregular or you know your ovulation date more precisely (for example, from fertility tracking or IVF), an early dating ultrasound typically gives a more accurate estimate than the LMP-based calculation alone, since it measures the embryo or fetus directly rather than assuming a standard cycle.
The standard method (Naegele's Rule) adds 280 days to the first day of your last menstrual period, assuming a 28-day cycle with ovulation on day 14. It's a solid planning estimate, but only about 4-5% of babies are actually born on that exact date — it's best understood as the centre of a normal range, not a precise prediction.
Yes, very normal. About 80% of babies are born within two weeks before or after the calculated due date, and full-term pregnancy is medically defined as a five-week window — 37 to 42 completed weeks of gestation — not a single day. Going a week or two past the date, or delivering a bit early, both fall well within normal medical ranges.
An early dating ultrasound measures the embryo or fetus directly, which can be more precise than a calculation based purely on your last menstrual period, especially if your cycle isn't a standard 28 days or you're unsure of your exact cycle timing. It's common and expected for a healthcare provider to adjust the estimated date based on ultrasound measurements.
Research has linked several factors to natural variation in pregnancy length, including ethnicity, maternal height and weight, and parity (whether you've been pregnant before). This is part of why Naegele's Rule, which assumes one standard cycle pattern for everyone, produces only a rough estimate rather than a precise date for any individual pregnancy.