Updated June 17, 2026
Find your due date
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A due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP), a formula called Naegele's rule. If your cycles are regular and 28 days long, this gives a reliable estimate. For irregular cycles or uncertain LMP dates, an early ultrasound is the clinical standard for dating a pregnancy.
What is Naegele's rule?
Naegele's rule, developed in the 19th century, takes the first day of your LMP, adds one year, subtracts three months, and adds seven days. The result is the same as adding 40 weeks. Most online calculators (including the Due Date Calculator) do this arithmetic instantly. The 40-week count includes roughly two weeks before conception actually occurs, because LMP is an easier reference date than the day of ovulation.
Why 40 weeks if conception is at 38 weeks?
Pregnancy is dated from the LMP, not from conception. Ovulation and fertilization typically happen around day 14 of a 28-day cycle, so the embryo is about two weeks old when the pregnancy is already counted as two weeks along. That is why a full-term pregnancy is described as 40 weeks from LMP but about 38 weeks from conception.
When does an ultrasound change the due date?
If an early ultrasound (ideally between 8 and 12 weeks) shows a fetal size that differs from the LMP-based date by more than about 5 to 7 days, clinicians will often revise the due date to match the scan measurement. Ultrasound dating is most accurate in the first trimester, before normal variation in fetal size widens. Later scans are less precise for dating and are rarely used to revise a well-established date.
What if my cycle is not 28 days?
Naegele's rule assumes a standard 28-day cycle with ovulation on day 14. If your cycles run longer or shorter, conception likely happened earlier or later than the formula assumes, and your LMP-based date may be off by several days. An early ultrasound accounts for this by measuring the embryo directly rather than relying on cycle assumptions.
Is the due date a firm date or a range?
It is a statistical midpoint, not a firm arrival date. Only about 4 to 5 percent of babies are born on their exact calculated due date. Most arrive within a window of about two weeks on either side. A full-term birth is defined as 39 to 40 weeks, but 37 to 42 weeks is considered the normal range. See how accurate due dates really are for more on what the window means in practice.
These are general estimates. Your own due date and any concerns about your pregnancy should always be confirmed with your doctor or midwife, who can factor in your full clinical picture.
Find your due date
Get your estimated due date and current week in seconds.