How Pregnancy Due Dates Are Calculated

Count from the first day of the last period

Pregnancy dating commonly starts with the first day of the last menstrual period, abbreviated LMP. That date is not the same as the date of conception. In the usual 28-day-cycle model, the estimated due date is 40 weeks, or 280 days, after the LMP. ^[1]

Estimated due date = first day of LMP + 280 days

Use the first day bleeding began for the relevant menstrual period, not its last day. Uncertain dates or bleeding that was not a normal period can make an LMP estimate less reliable.

Understand the cycle-length assumption

A simple adjustment adds the difference between the usual cycle length and 28 days. For a 32-day cycle, that model adds four days to the basic estimate. It assumes the timing of ovulation shifts in a way that may not describe an individual cycle.

The NHS due-date tool allows cycle-length information and emphasizes that an ultrasound scan can estimate the pregnancy more accurately. Irregular cycles, uncertain ovulation, and an uncertain LMP are reasons not to treat an adjusted calendar estimate as definitive. ^[1]

Tracking apps and remembered intercourse dates do not necessarily identify the exact fertilization date.

A calendar example

If the first day of the LMP was January 1, 2026, adding 280 days gives October 8, 2026 under the 28-day model. A simple 32-day-cycle adjustment would move that estimate to October 12.

This example demonstrates date arithmetic only. It does not establish a clinically confirmed due date or the likely day of delivery. Calendar calculations should use actual dates rather than assuming every month has four weeks.

Gestational age is typically expressed in completed weeks and additional days. For example, 10 weeks and 3 days is not the same as 10.3 weeks in decimal arithmetic.

Ultrasound and fertility treatment can change the basis

A clinician may use ultrasound findings and the full clinical history to establish or confirm the pregnancy’s estimated due date. Once a clinical date is established, do not repeatedly replace it with whichever online calculator produces a preferred result.

For IVF or other fertility treatment, the clinic has information about procedures and embryo timing that a generic LMP form does not capture. Use the treatment team’s dating rather than inventing a period date to force a calculator to match.

A difference between calendar and clinical estimates is a question for the care team, not evidence that the pregnancy is developing abnormally.

Do not turn the due date into a deadline

The date is a reference point for prenatal care and gestational timing. Birth does not necessarily occur on that exact day. Planning decisions and assessment near or beyond the estimated date depend on the pregnancy and clinical guidance.

A due-date calculator cannot assess fetal growth, pregnancy location, symptoms, or whether urgent care is needed. Bleeding, severe pain, or other concerning symptoms should be discussed promptly with an appropriate healthcare professional rather than interpreted through a calendar result.

Use the estimate carefully

Record the LMP date, usual cycle length, and whether the date is preliminary or confirmed by the care team. Bring uncertainty about dates to a prenatal appointment. An honest estimate is more useful than a falsely precise date built on uncertain inputs.

The basic LMP method uses a stated cycle adjustment. It is not an IVF dating method or a replacement for ultrasound and clinical assessment.

Use this guide thoughtfully. Educational information, not a diagnosis or individualized medical advice. A screening result or formula cannot establish what is safe or appropriate for a particular person. Consult a qualified healthcare professional for clinical decisions.