Estimates only, from the standard Naegele’s rule — confirm your dates with your midwife or doctor, and expect an early scan to be the final word.
Trimesters
| Trimester | Weeks | Dates |
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Milestones
| Milestone | Typical timing | Date |
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An estimated due date is the middle of a window, not an appointment: only about 1 in 20 babies arrive on it, and healthy births commonly fall between 37 and 42 weeks. This calculator is general information, not medical advice.
From the first day of your last period: your estimated due date, likely conception date, how many weeks pregnant you are today, and every trimester and milestone date in between.
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Estimate a pregnancy due date from the first day of the last period, adjusted for cycle length: get the estimated conception date, how many weeks and days pregnant you are today, and the start dates of each trimester — the same Naegele’s rule maths used in clinics, computed in your browser.
With an LMP of 1 January 2026 and a 28-day cycle, Naegele’s rule adds one year, subtracts three months and adds seven days: the estimated due date is 8 October 2026. Conception most likely happened around 15 January, the second trimester begins around 9 April, and the third around 16 July. A 32-day cycle would move every one of those dates four days later.
The calculator applies Naegele’s rule — due date = LMP + 280 days for a 28-day cycle — then adjusts by the difference between your stated cycle and 28 days, because ovulation is assumed to occur 14 days before the next period rather than on a fixed cycle day. Weeks pregnant are counted from the LMP, the convention every clinician and scan uses, which is why “weeks pregnant” is about two weeks ahead of the age of the embryo itself. All maths runs locally in your browser.
It reproduces the standard clinical estimate faithfully, but the estimate itself is approximate: most babies arrive within about a week either side, and only around 5% on the exact day. An early ultrasound dating scan is considered more accurate and is what most providers rely on if the two disagree.
The rule assumes ovulation — and therefore conception — happens about 14 days before the next period starts. A 35-day cycle implies ovulation near day 21 rather than day 14, so conception and the due date sit about a week later than the unadjusted rule suggests.
Add 266 days to a known conception date (from IVF transfer dates or certain tracking) to get the equivalent due date, and count weeks as conception + 14 days. This version of the calculator is LMP-based; an IVF or conception-date mode is the first thing we would add if readers ask for it.
The first trimester runs to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks until birth. Providers vary by a few days on the boundaries; the dates here follow the common convention.
You can, with a wider margin: use your best average cycle length and treat the result as a rough window until a dating scan pins it down. If your LMP date itself is uncertain, tell your provider — they will rely on the scan instead.