Estimate your pregnancy due date based on your last menstrual period.
A due date calculator estimates when your baby is likely to be born, typically 280 days (40 weeks) from the first day of your last menstrual period.
This tool provides a general estimate for informational purposes only and is not a substitute for professional medical advice. Please consult your doctor for accurate pregnancy tracking.
The standard method, known as Naegele's rule, estimates a due date as 280 days (40 weeks) from the first day of the last menstrual period, assuming a typical 28-day cycle with ovulation on day 14. This method has been used clinically since the 19th century and remains the standard starting estimate, even though it doesn't account for individual cycle length variation, which is exactly why an early ultrasound (typically in the first trimester) is generally considered more accurate for dating a pregnancy, since it directly measures fetal size rather than relying on cycle assumptions.
Only a small percentage of babies are actually born on their calculated due date - most pregnancies deliver within a window of about two weeks before or after it, which is why healthcare providers consistently describe a due date as an estimate rather than a precise prediction.
People with irregular cycles, or who aren't certain of their last period date, often get a more accurate estimate from an early ultrasound than from a calculation based on last menstrual period alone, since the ultrasound method doesn't depend on assumptions about cycle regularity.
"Due date" calculations appear across many unrelated contexts, each with its own conventions. In pregnancy, a due date is calculated as 40 weeks from the last menstrual period. In invoicing, a due date is typically calculated as a fixed number of days from an invoice date (like "net 30"). In library systems, a due date is calculated from a checkout date based on loan period policy. In project management, a due date is calculated backward from a deadline to determine when a task must start.
Getting these calculations wrong has different consequences depending on context — a miscalculated invoice due date can affect cash flow and trigger late fees, while a miscalculated project due date can cascade into missed downstream deadlines across an entire team's schedule.
Many due date calculations in practice include an implicit or explicit grace period beyond the calculated date — a "net 30" invoice might not actually incur late fees until day 35 or 40, and library due dates often have a grace period before fines begin accruing. Understanding whether a stated due date is a hard deadline or has built-in flexibility is important context that a raw calculation alone doesn't capture.
For businesses managing many invoices or contracts simultaneously, manually tracking individual due dates becomes error-prone past a handful of active items, which is why most invoicing and project management software automates due date calculation and sends reminders as deadlines approach. Understanding the underlying calculation, though, remains useful for spot-checking automated systems and catching configuration errors before they cause a missed deadline.
This is an estimate based on a standard 280-day pregnancy. Actual delivery dates vary and should be confirmed with your healthcare provider.
It's a standard estimate used clinically, but only a small percentage of babies arrive exactly on their calculated due date - most deliveries fall within about two weeks of it.
Yes, particularly for people with irregular cycles, an early first-trimester ultrasound is generally considered more accurate than a calculation based on last period date alone.
The standard calculation assumes a typical 28-day cycle, so results are less precise for people with irregular cycles - a healthcare provider can adjust for this individually.
