Pregnancy Week Calculator

Gestational age — the count doctors and this pregnancy week calculator use — starts on the first day of your last menstrual period (LMP), not on the day you conceived. The reason is practical: almost everyone knows when their last period started, but very few know the exact day of ovulation or fertilisation. Counting from the LMP gives clinicians one consistent yardstick for every patient. The quirk is that you are already counted as “2 weeks pregnant” around the time you actually conceive, so gestational age runs roughly two weeks ahead of the embryo’s true age. That is normal and expected — every chart, scan and milestone in pregnancy uses the same LMP-based count, so your weeks here line up with what your clinic tells you.

The standard 280-day rule assumes a 28-day cycle with ovulation on day 14. If your cycle is reliably longer or shorter, you ovulate later or earlier, so this calculator shifts the dating by the difference — your cycle length minus 28 — which is the same correction a clinician would make by hand. That helps when your cycles are consistent. If they are genuinely irregular, swing widely from month to month, or you recently came off hormonal birth control, then the LMP is a weak anchor and any calculation built on it, including this one, is only a rough estimate of how many weeks pregnant you are. In that case an early ultrasound, which measures the baby directly, is the only reliable way to date the pregnancy — and it is what your doctor will go by.

Your estimated due date is the first day of your last period plus 280 days — 40 weeks. That is the classic Naegele’s rule, usually stated as: take the first day of your last period, subtract 3 months, add 7 days, then add a year. Both routes land on the same date. This due date calculator adds one refinement most simple tools skip: if your average cycle is not 28 days, it adds the difference, so a 32-day cycle pushes the due date about 4 days later and a 25-day cycle pulls it about 3 days earlier. The result is an estimate of a 40-week pregnancy — a signpost for your care, not a booking for a delivery date.

About 5% — only around 1 baby in 20 arrives on its estimated due date. The due date is the midpoint of a range, not a deadline. Most babies are born between 37 and 42 weeks of gestational age, with 38 to 42 weeks treated as the normal window for delivery, and first babies tend to arrive a little later than the estimate. So read the date this due date calculator gives you as the centre of a month-wide landing zone, and use the week count to follow along with your pregnancy rather than to count down to one fixed day.

No. This pregnancy week calculator runs entirely in your browser — your last period date, cycle length and calculation date are never uploaded, tracked or shared. Your last entry is kept only in your own browser’s localStorage, so the form is already filled in when you come back, and you can wipe it at any time by clearing your browser data. There is no account, no server call and no analytics attached to the calculation: nothing about your pregnancy leaves your device.