Pregnancy Due Date & Week Calculator

How this calculator works

Most due dates are estimated using Naegele's rule: pregnancy is counted as 280 days (40 weeks) from the first day of your last menstrual period (LMP), assuming a average 28-day cycle. If you know your LMP, we count forward 280 days to estimate your due date. If you already know your due date — perhaps from an earlier scan — we work backwards to estimate your LMP and current week instead.

For a full week-by-week look at what's happening at each stage, browse ourweek-by-week pregnancy guide, which covers baby's development, common symptoms, and what to expect at every week from 1 to 40.

Why this estimate might differ from your doctor's

This calculator gives a good general estimate, but it can't account for individual variation in your cycle. If your periods are irregular, or if you're not sure of your exact LMP, tracking ovulationgoing forward can help you date future cycles more precisely. Your midwife or doctor will usually confirm your due date with an early dating ultrasound, which is more accurate than LMP-based calculations alone — especially if your cycle isn't a textbook 28 days.

Whichever due date you end up working with, it's worth reviewing what to eat (and what to avoid) as early as possible — our guide tofoods to avoid during pregnancyis a good place to start.

Frequently asked questions

How accurate is a due date calculator?

A due date calculated from your last menstrual period is an estimate, not a guarantee — only around 1 in 20 babies are born on their exact due date. An early dating ultrasound (usually done between 8 and 13 weeks) is generally considered the most accurate way to confirm gestational age, because it measures the baby directly rather than relying on cycle assumptions.

What if I have an irregular menstrual cycle?

This calculator assumes a standard 28-day cycle with ovulation around day 14, which is how the 280-day (40-week) rule works. If your cycles are longer, shorter, or irregular, the estimate may be off by several days or more — your midwife or doctor can give you a more accurate date using an early ultrasound.

What's the difference between gestational age and my baby's actual age?

Gestational age is counted from the first day of your last period, which is roughly two weeks before conception actually happens. So your baby's actual developmental age is usually about two weeks younger than the 'week of pregnancy' shown here — this is standard medical convention, not an error.

Can this replace what my midwife or doctor tells me?

No. This tool is for general information only and isn't a substitute for professional medical advice. Always confirm your due date and gestational age with your midwife or OB-GYN, especially if it's based on a dating ultrasound rather than your last period.