Pregnancy Due Date Calculator
Estimate your due date and current week of pregnancy based on the first day of your last menstrual period.
How this calculator works
This uses Naegele's rule, the standard clinical method of estimating a due date by adding 280 days (40 weeks) to the first day of your last menstrual period.
Assumes a typical 28-day cycle; your provider may adjust this based on ultrasound dating.
How accurate is this estimate?
Naegele's rule gives a standard estimate, but only about 5% of babies arrive exactly on the calculated due date — a range of 1-2 weeks before or after is completely normal. An early ultrasound typically provides a more precise estimate.
Factors that can shift your actual due date
Naegele's rule assumes a standard 28-day cycle with ovulation on day 14 — but cycle length varies significantly between individuals, and someone with a longer or shorter typical cycle may have a due date that differs from this calculation by several days or more.
Only about 1 in 20 babies are actually born exactly on their calculated due date — most arrive within a 1-2 week window before or after, which is considered entirely normal and expected.
A worked example
If the first day of your last period was March 10, adding 280 days lands on approximately December 15 as the estimated due date.
By late June (about 15-16 weeks after the March 10 start), this pregnancy would be entering the second trimester, with roughly 24-25 weeks remaining until the December due date.
Pregnancy trimesters and key milestones
Pregnancy is typically divided into three trimesters: weeks 1-13, weeks 14-27, and weeks 28-40. The estimated due date marks the end of week 40, though full-term delivery is generally considered to span weeks 37-42.
Early ultrasounds, typically performed in the first trimester, are considered the most accurate method of dating a pregnancy and may adjust the estimated due date from the LMP-based calculation shown here, particularly for those with irregular cycles.
Frequently asked questions
How is a due date actually calculated?
The standard method adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a 28-day cycle.
Is this as accurate as an ultrasound estimate?
An early ultrasound is generally considered more precise than LMP-based dating alone, especially for people with irregular cycles.
Why did my doctor give me a different due date?
Providers often adjust the estimate based on early ultrasound measurements, which can be more precise than LMP-based dating alone, especially for those with irregular cycles.
What week is considered full term?
Full term is generally considered 39-40 weeks, with 37-38 weeks considered early term and pregnancies continuing past 41 weeks considered late term — ranges that reflect typical, healthy variation.
What counts as 'full term' for a pregnancy?
Full term is generally considered weeks 37-42, meaning delivery any time in that range is within the typical range, not just on the exact calculated due date.