Pregnancy Due Date Calculator: How Accurate Are They?

A due date is a calendar estimate, not a prediction of the day your baby will arrive. Our calculator gives you the date that falls 280 days after the first day of your last period, and only a small minority of babies are born on that exact day: our calculator says about 5 percent, and an encyclopedia summary of the data says about 4 percent. The date is best read as the middle of a window of several weeks.

How the date is calculated

The method is Naegele's rule: due date = first day of last menstrual period (LMP) + 280 days, or 40 weeks. It assumes a 28-day cycle with ovulation on day 14. The classic shortcut, add one year, subtract three months and add seven days, is the same idea, but it can differ from the 280-day count by a few days because months have different lengths. For an LMP of May 31, 2027, counting 280 days gives March 6, 2028, while the month shortcut gives March 9, 2028. The calculator uses the 280-day count. For an LMP of January 11, 2027 both methods agree on October 18, 2027.

Why your cycle length changes the answer

If your cycle is not 28 days, ovulation probably did not fall on day 14. The calculator handles this by shifting the date by the difference between your cycle length and 28 days, which assumes the time from ovulation to the next period stays constant. Here is the effect for the same LMP of January 11, 2027:

The shift applies only to the LMP method. If you enter a conception date or an IVF transfer date, the cycle-length field is ignored, because those dates already tell the calculator when the pregnancy began. Cycle length is also an average. If your cycles vary by a week or more, no adjustment fixes the uncertainty, and an early ultrasound is the better way to set the date.

A due date is a window, not a day

The US National Institute of Child Health and Human Development describes a typical pregnancy as about 40 weeks, full term as 39 weeks 0 days through 40 weeks 6 days, early term as 37 to 38 weeks and preterm as before 37 weeks. The calculator's reference list adds late term (41 weeks 0 days to 41 weeks 6 days) and post-term (42 weeks and beyond). For an estimated due date of October 18, 2027, those labels fall on these dates:

Notice that the due date itself, October 18, 2027, is the 40-week mark, in the middle of the full-term stretch. A birth on October 4, 2027 falls in the early-term stretch and one on October 28, 2027 in the late-term stretch; neither is on the date, which is why providers talk about windows.

What ultrasound dating changes

The calculator's references, which cite the American College of Obstetricians and Gynecologists, say that crown-rump length measured by ultrasound at 8 to 13 weeks dates a pregnancy to within about 5 to 7 days, and that a provider usually revises the LMP-based date when the scan differs from it by 7 days or more. In our example, 8 weeks falls on March 8, 2027 and 13 weeks 6 days on April 18, 2027. Suppose a scan in that window suggests a date 9 days earlier than the LMP estimate. Because that is more than 7 days, the due date would typically move from October 18, 2027 to October 9, 2027. A 4-day difference, by contrast, would normally leave the LMP date alone. That example is an illustration of the rule, not a prediction about any real scan, and the decision belongs to your provider.

Common mistakes

• Treating the date as a deadline. The window above spans five weeks, from the start of early term to the start of post-term.
• Entering a conception date as if it were the LMP. The calculator subtracts 14 days from a conception date to find the equivalent LMP, so enter the date you believe ovulation and conception happened.
• Using the wrong IVF offset. For a day-5 blastocyst transfer the calculator counts back 19 days to an LMP equivalent, which means the due date is the transfer date plus 261 days.
• Confusing gestational age with fetal age. Gestational age counts from the LMP and is what providers use; fetal age counts from conception and is about two weeks less.
• Comparing results from tools that ignore cycle length. They can differ from ours by a week or more for long or short cycles.