Due Date Calculator

A due date is estimated from the first day of your last period, adjusted for how long your cycle usually is. This shows the estimated date, how many weeks along you are today, and the trimester you are in.

How it works

The standard estimate assumes a 40-week pregnancy counted from the first day of the last menstrual period rather than from conception, because the period date is usually known. That is 280 days. If your cycle is longer or shorter than 28 days, ovulation happens later or earlier, so the estimate is shifted by the same number of days.

The calculator also estimates conception as 14 days before the next expected period, and provides an estimated full-term date range for context. Only a small proportion of babies are born on the estimated due date itself.

Formula

Naegele's rule, with a cycle-length correction.

• Due date = first day of last period + 280 days + (cycle length − 28)
• Estimated conception = first day of last period + cycle length − 14
• Weeks pregnant = days since last period ÷ 7
• Full term = 39 weeks 0 days through 40 weeks 6 days of gestational age (due date − 7 days through due date + 6 days)

Worked example

First day of last period 2 March 2026 with a 28-day cycle: adding 280 days gives 7 December 2026. The same date with a 32-day cycle would be 11 December 2026, four days later, because ovulation is assumed to be four days later.

Assumptions

The estimate assumes ovulation occurred 14 days before the next period would have started, which is typical but not universal, and that the cycle length entered is your usual one rather than an average of irregular cycles. If you know the date of conception or have had a dating scan, those are more accurate. Assisted conception is dated from the transfer or insemination date, not from the period.

Reading the result

Use the due date as an estimate, not an appointment. Clinical terminology distinguishes early term (37 weeks 0 days through 38 weeks 6 days), full term (39 weeks 0 days through 40 weeks 6 days), late term (41 weeks 0 days through 41 weeks 6 days), and postterm (42 weeks or later), as described by the American College of Obstetricians and Gynecologists. Decisions about monitoring, induction or delivery timing depend on individual clinical circumstances and must be discussed with your maternity-care professional. Read ACOG's definition of term pregnancy. A dating scan may revise the estimate, particularly when cycles are irregular or dates are uncertain.

For general information only, not medical advice. Always confirm dates with your midwife or doctor.

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