Pregnancy Due Date Calculator

How This Due Date Calculator Works

This calculator uses two widely used methods to estimate a pregnancy due date. Both are based on the length of a full-term pregnancy, which is generally counted as 280 days (40 weeks) from the first day of the last menstrual period.

Due Date Based on LMP (Naegele's Rule)

The most common method is Naegele's rule: take the first day of your last menstrual period and add 280 days (40 weeks). This assumes ovulation occurs around day 14 of a 28-day cycle. If your cycle is longer or shorter, the calculator adjusts the conception window estimate accordingly, though the standard 280-day rule still applies from LMP.

Due Date Based on Conception Date

If you know your conception date, the calculator adds 266 days (38 weeks) to arrive at the estimated due date. Conception is counted from actual fertilisation, which is approximately two weeks after the LMP in a typical cycle. This is why conception-based dates and LMP-based dates often produce similar results.

Why Due Dates May Change

A due date calculated from LMP or conception date is a starting estimate. Your doctor or midwife may revise it based on an early ultrasound scan, which measures the size of the baby and can date the pregnancy more precisely. It is normal for due dates to be adjusted, and it does not mean anything is wrong.

What Happens in Each Trimester

• First trimester (weeks 1–13): The embryo develops all major organs. Early symptoms such as nausea and fatigue are common.
• Second trimester (weeks 14–26): Often described as the more comfortable stage. The baby grows significantly and movements may become noticeable.
• Third trimester (weeks 27–40+): Rapid growth continues. Preparation for birth typically begins in this period.

When to Speak with a Healthcare Professional

If you have just found out you are pregnant, contact your GP, midwife or maternity unit as soon as possible. Early antenatal appointments help confirm the pregnancy, check your health, and plan your care. Do not rely solely on online calculators for pregnancy planning — professional guidance is essential.