Due Date Calculator

How the Due Date Is Estimated

The standard method is Naegele's rule, which estimates the due date as 280 days (40 weeks) from the first day of your last menstrual period. That 280-day figure assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter than 28 days, the calculator adjusts the estimate accordingly, because ovulation shifts with cycle length.

When the date of conception is known, the estimate is 266 days from conception. This is more precise than the last-period method because it does not depend on assumptions about cycle timing. For IVF, the transfer date gives an even more exact anchor, since the age of the embryo at transfer is known.

How Accurate Is a Due Date?

A due date is a statistical estimate, not an appointment. Research suggests that only around 4 percent of babies are born on their estimated due date, and the majority arrive within two weeks either side. Because of this, clinicians often describe a due window rather than a single day. An early ultrasound, typically between 8 and 13 weeks, is generally the most accurate way to establish gestational age, and it may revise a date calculated from your last period.

Trimester Boundaries

Pregnancy is commonly divided into three trimesters. The first runs from week 1 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. These boundaries are conventions used for describing stages of development and scheduling care; they are not sharp physiological transitions.

What This Calculator Does Not Do

It does not diagnose pregnancy, assess fetal development, or replace any clinical measurement. It also cannot account for individual variation in ovulation timing, which is why an early scan may give a different date. Use the result as a planning estimate and confirmation of the date your provider records.