How the due date estimate works
The most common way to estimate a due date is Naegele's rule, which counts about 280 days — 40 weeks — from the first day of your last menstrual period (LMP). It works because pregnancy is conventionally dated from the LMP rather than from conception, which usually happens around two weeks later. This calculator also adjusts for your cycle length: if you ovulate later than the textbook day 14, your due date shifts accordingly.
Why pregnancy is counted from your last period
It might seem odd to count from before conception, but the LMP is the one date most people can pin down, whereas the exact day of ovulation and fertilisation usually isn't known. That's why a "12-week" pregnancy is measured 12 weeks from the LMP — and why you're considered roughly two weeks pregnant at the moment of conception. Clinicians use the same convention, so the weeks shown here line up with what you'll hear at your appointments.
Understanding the trimesters
- First trimester — conception through week 13. Early development and, for many, the toughest symptoms.
- Second trimester — weeks 14 to 27. Often the most comfortable stretch, when movement is first felt.
- Third trimester — week 28 to birth. Rapid growth and preparation for delivery.
How accurate is it?
An LMP-based date is a solid starting point but only an estimate. Just one baby in twenty actually arrives on the predicted day; the large majority are born within two weeks either side, anywhere from 37 to 42 weeks. If your cycles are irregular or you're unsure of your LMP, an early "dating" ultrasound — usually between 8 and 14 weeks — gives a more accurate due date and will often replace the LMP estimate in your notes.
Use this as a guide, not a diagnosis
This tool is for planning and curiosity, not medical decisions. Book antenatal care as soon as you can, and let your midwife or doctor confirm your dates and monitor your health and your baby's. For other everyday date maths, the date calculator and age calculator use the same underlying engine.