Zerdly

Pregnancy Due Date Calculator

Estimate your baby's due date and current week of pregnancy from your last period or conception date with this free calculator in your browser.

100% FreeRuns in your browserBasic
How to use this calculator

Today unless you change it.

These dates stay in this browser tab. Nothing is saved, put in the page address or sent anywhere.

Result

Choose a dating method and enter its date.

Estimate only — not medical advice. Cycles, ovulation and due dates vary between people and from cycle to cycle. Confirm with a healthcare professional.

How the method works

Three source dates, one due-date arithmetic
Illustrative method
Three source dates, one due-date arithmeticA day axis from the last menstrual period (day 0) to the estimated due date (day 280). Arrows show LMP plus 280 days, fertilisation (day 14) plus 266 days, and a day-5 embryo transfer (day 19) plus 261 days all reaching the same due date.Day 0 · LMPDay 280 · due date10 wk20 wk30 wkLMP + 280 daysfrom the last period start (day 0)Fertilisation + 266 daysfrom the conception estimate (day 14)Day-5 transfer + 261 days (266 − 5)from the IVF transfer (day 19)IVF: EDD = transfer + 266 − embryo age
Estimated due date = LMP + 280 days = fertilisation + 266 days = IVF transfer + 266 − embryo age in days (day 5 → + 261). Example: a day-5 transfer on 2026-03-31 gives 2026-12-17. Weeks are counted from the LMP, so fertilisation sits at about 2 weeks 0 days.

A clinician's dating scan or established due date takes precedence; this tool never re-dates it. Fixed example values for illustration — not your result.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Choose the dating method: last period, known conception, IVF transfer, a dating scan, or a due date your clinician has already set.
  2. Enter the date (and the embryo age, scan gestational age or cycle adjustment if asked).
  3. Change the reference date if you want the gestational age on a day other than today.
  4. Read the estimated due date, gestational age, time remaining, trimester and milestones; download them as an .ics file if you wish.

How it works

Last period: due date = LMP + 280 days; the optional cycle adjustment adds (cycle length − 28) days and is labelled a heuristic. Conception: + 266 days. IVF: transfer + 266 − embryo age (3, 5 or 6 days). Scan: scan date + (280 − gestational age at the scan).

Gestational age on a date = 280 − (due date − that date), shown in whole weeks and days. Before the dating start it says how many days before, rather than showing 0.

Trimesters are taken as: first to 13 weeks 6 days, second 14w0d to 27w6d, third from 28 weeks.

All arithmetic uses whole calendar days, so daylight-saving changes cannot shift a date. A due date your clinician has set is used as given and never re-dated.

Worked example

LMP 1 Jan 2026. Estimated due date 8 Oct 2026 (a conception date of 15 Jan gives the same). A day-5 embryo transferred on 31 Mar is due 17 Dec; a day-6 embryo, 16 Dec. On 1 Jan itself the gestational age is 0 weeks 0 days.

Common mistakes

  • Expecting birth on the due date — it is the 40-week point, and most babies arrive within a couple of weeks either side.
  • Using the LMP when an early dating scan is available; clinical dating usually takes priority.
  • Counting from the conception date with the LMP formula (that adds two weeks too many).

Tips

  • Estimate only — not medical advice. Dates and fertile windows vary between people and cycles; confirm with a healthcare professional.

Frequently asked questions

How accurate is a due date?

Only about 1 in 20 babies arrive on the exact date — it's an estimate; a scan gives a better one.

Is my data uploaded?

Runs entirely in your browser. Your inputs are not saved or transmitted.

Related tools

Explore more in Family, Parenting & Custody.

Cite this tool

Zerdly. (2026). Pregnancy Due Date Calculator. Retrieved from https://www.zerdly.com/tools/pregnancy-due-date-calculator/

Tip: Enter any known values to calculate the remaining results.

Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.