Reverse due date calculator
A due date of 15 October 2026 points to an estimated conception date of 22 January 2026, 266 days earlier. The tool also shows the matching last-period estimate. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Due date
- 2026-10-15
- Conception date
- 2026-01-22
- Days subtracted
- 266
- Pregnancy length days
- 280
- Lmp to conception days
- 14
This reverse due date calculator estimates the conception date from a given estimated due date by subtracting 266 calendar days. That offset is the classic mean interval from fertilization to a 280-day last-menstrual-period pregnancy clock: forty weeks from a notional LMP minus about fourteen days from LMP to ovulation and conception. Treat the result as an educational and engineering estimate, not a clinical diagnosis or a substitute for professional obstetric judgment.
What counting back from the due date tells you
Obstetric due dates are usually quoted from a last menstrual period using a 280-day, forty-week clock, not from the day of fertilization. Mean conception sits about two weeks after that notional LMP, so the typical interval from fertilization to the estimated due date is 266 days. This reverse due date calculator inverts that teaching clock: given a calendar EDD, it subtracts 266 whole civil days and returns the estimated conception date on the proleptic Gregorian calendar.
A formula string, a short method name, a one-line message, and ordered steps make the arithmetic inspectable for homework keys and automated tests. Optional fields that do not apply are omitted rather than filled with null. There is no ultrasound biometry, no cycle-length adjustment, no twin correction, and no clinical risk score in this contract—those belong to other tools such as last-menstrual-period due-date calculators or CRL dating.
The 266-day constant is a mean teaching figure, not a guarantee that fertilization occurred on that exact calendar day for any individual pregnancy.
Where a reverse due date calculator earns its keep in education and software
Midwifery, nursing, and health-education coursework introduce the 266-day fertilization-to-EDD interval when textbooks contrast gestational age from last menstrual period with gestational age from conception. Learners are often given a printed due date and asked to recover an estimated conception date, or to check that a mobile app agrees with the teaching clock. Clinical-engineering demos sometimes show this reverse clock beside an LMP-based due date purely as a teaching contrast, while reminding readers that real dating policy depends on cycle length, ultrasound, and professional interpretation.
Treat the output as an educational and engineering estimate: if clinical accuracy is required, prefer your institution’s dating protocol and a qualified clinician.
What you can do with it
Midwifery and nursing coursework
Recover an estimated conception date from a printed EDD for labs, homework keys, and exam checks that teach the 266-day fertilization-to-due-date clock.
Due-date form demos and input validation
Normalize YYYY-MM-DD due dates, reject impossible civil days, and return a conception date plus educational LMP estimate for prototype prenatal UIs.
FAQ
What does the reverse due date calculator cost?
It is free in your browser on this page.
Why subtract 266 days instead of 280?
A standard obstetric pregnancy is counted as 280 days from last menstrual period. Mean conception is about 14 days after LMP, so the mean fertilization-to-EDD interval is 266 days. This tool inverts that clock.
Is this medical advice or a diagnostic tool?
No. The result is an educational and engineering estimate from a fixed 266-day mean interval. Clinical dating decisions need professional obstetric interpretation, cycle history, and often ultrasound.
Can it also tell me the date of my last period?
Yes, as a teaching estimate: it subtracts 280 days from the due date. For a due date of 15 October 2026 that gives 8 January 2026, two weeks before the estimated conception.
Does it adjust for cycle length, ultrasound or twins?
No. It always uses the fixed 266-day average, with no correction for cycle length, ultrasound measurements or multiple pregnancies. Real dating depends on those factors and on a qualified clinician's judgement.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/health/reverse-due-date \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"due_date":"2026-10-15"}'const res = await fetch("https://api.kit.forhosting.com/health/reverse-due-date", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"due_date": "2026-10-15"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/health/reverse-due-date",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"due_date": "2026-10-15"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/health/reverse-due-date", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"due_date":"2026-10-15"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"due_date":"2026-10-15"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/health/reverse-due-date", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"due_date": "2026-10-15"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "health.reverse_due_date",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
min_year | 1 |
max_year | 9999 |
days_subtracted | 266 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Failed validation is not billed; each successful API estimate costs $0.002.
What reverse due dating means and what the response contains
The JSON also reports the due date as normalized YYYY-MM-DD, the fixed days_subtracted value of 266, the 280-day pregnancy_length_days used as context, a 14-day lmp_to_conception_days offset, and an educational lmp_estimate equal to the due date minus 280 days. A missing due date, a token that is not YYYY-MM-DD, or an impossible civil day such as the thirtieth of February fails with invalid_input and a field-named message so a bad form cell never becomes a wrong conception date.
How the due date is read and how results stay reproducible
The value must be a real Gregorian calendar day: years from 0001 to 9999, months 01 through 12, and a day that exists in that month, including 29 February only in leap years. Tokens with slashes, month names, times of day, or extra characters are rejected; so are empty strings and missing fields. The same walk minus 280 days produces the educational LMP estimate so scripts can store both conception and LMP without a second request. Failed validation is not charged. The preferred field is due_date as a string in ISO-8601 extended form YYYY-MM-DD, with aliases date, edd, estimated_due_date, and due for form builders that prefer shorter or clinical names. Because the path is a pure function of the civil date, two runs with 2026-10-15 always return the same conception_date, and a leap-year due date such as the day after 29 February lands on a stable ISO string instead of drifting with a host clock. Pair the free browser run on this page with the priced API when you need the identical mathematical result inside an automated pipeline, LMS grader, or prenatal worksheet demo that must not re-derive calendar subtraction by hand. Through the API each successful estimate costs $0.002, and nothing is stored after the response returns.
Where a reverse due date calculator earns its keep in education and software
On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split—free interactive exploration and a priced, identical API—matches the rest of the KIT tier-A catalog and lets you move from a classroom demo to an automated pipeline without changing the mathematical result.
Integration use cases
Deterministic fixtures for prenatal software
Generate stable conception_date and lmp_estimate fields for unit tests and CI without re-coding Gregorian subtraction or depending on a host clock.
Technical questions
What date format does due_date accept?
A real Gregorian calendar day in YYYY-MM-DD form, for example 2026-10-15. Month names, slashes, times of day, and impossible days such as 2024-02-30 are rejected as invalid_input.
Does the result include an LMP estimate as well?
Yes. Besides conception_date the response includes lmp_estimate equal to the due date minus 280 days, plus the fixed offsets days_subtracted (266) and pregnancy_length_days (280).