When is your due date?
Based on the first day of your last period, using the standard 280-day (40-week) estimate.
This is a general estimate based on a standard 40-week pregnancy and doesn't account for individual variation. Your healthcare provider's estimate, especially one based on an ultrasound, is more accurate.
How the pregnancy due date estimate works
This calculator estimates a due date by adding 280 days, or 40 weeks, to the first day of the last menstrual period. It is a standard calendar estimate and can also give a general indication of pregnancy progress.
Pregnancy timing varies, and an estimated due date is not a diagnosis or a substitute for prenatal care. A healthcare professional may adjust the estimate using cycle information, clinical assessment, or ultrasound findings.
Where the 280-day estimate comes from
This method is known as Naegele's rule, named after the 19th-century German obstetrician Franz Naegele, who formalized it. It assumes a 28-day menstrual cycle with ovulation around day 14, and counts 40 weeks from the first day of the last period — which works out to roughly 38 weeks from actual conception, since conception typically happens about two weeks after a period starts. It's a widely used starting estimate precisely because the last menstrual period date is something most people can pinpoint more easily than the exact date of conception.
Why only about 1 in 20 babies arrive exactly on the due date
A due date is a statistical estimate of the midpoint of a normal range, not a prediction of the actual birth day. Full-term birth is generally considered to span a window of about five weeks (37 to 42 weeks), and a wide range of individual factors — cycle length, ovulation timing, first pregnancy versus subsequent ones — shift where within that window any specific birth is likely to fall. This is exactly why healthcare providers treat a due date as a planning reference rather than an appointment to circle on the calendar.
Trimester breakdown
A full-term pregnancy is typically divided into three trimesters: the first trimester runs through week 13, the second trimester spans weeks 14 through 27, and the third trimester covers week 28 through birth. Each trimester is associated with different typical developmental stages and different common experiences, which is why healthcare providers often discuss pregnancy progress in trimester terms rather than week numbers alone.
Related tools: Create a personal countdown · Calculate days between dates
Frequently asked questions
How is the due date calculated?
We add 280 days (40 weeks) to the first day of your last menstrual period — the same method (Naegele's rule) most standard pregnancy wheels and calculators use.
Is this accurate for everyone?
It's a general estimate. Cycle length and ovulation timing vary, so an ultrasound-based estimate from a healthcare provider is generally more precise, especially for anyone with irregular cycles.
Why might my doctor give a different due date?
Ultrasound measurements, especially in the first trimester, can more precisely date a pregnancy than a last-period-based calculation, particularly if your cycle isn't a standard 28 days or if the exact date of your last period isn't certain.
General information only — not medical advice. For guidance about your pregnancy, speak with a qualified healthcare provider.
Gestational dating is a clinical convention
Most pregnancy wheels count from the first day of the last menstrual period, not from conception. That is why “week 4” can appear before a test is positive. Ultrasound dating can disagree by days. This page is an educational due-date sketch, not prenatal care.
Contact a clinician for bleeding, pain, or any decision about health. Do not use a website date to replace an appointment.