Pregnancy Conception CalculatorFree online fitness & health calculator — instant results, no sign-up
Likely conception window from due date.
For general information only. This is not medical advice, diagnosis, dosing guidance or a treatment recommendation. Talk to a doctor or other qualified health professional about your health, pregnancy, or diet.
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About the Pregnancy Conception Calculator
The Pregnancy Conception Calculator estimates the probable date and window of conception based on an estimated pregnancy due date. It is designed for expectant parents and individuals seeking to identify roughly when fertilization occurred during their cycle.
- Due date
- The estimated delivery date (EDD) calculated by an ultrasound or estimated from the first day of the last menstrual period, formatted as a calendar date.
Formula and method
Probable Conception = Due Date − 266 days; Window Start = Due Date − 271 days; Window End = Due Date − 261 days
- Due DateEstimated delivery date (EDD) of the pregnancy
- Probable ConceptionThe estimated day of fertilization, based on an average 266-day human gestation period from conception
- Window StartThe earliest estimated conception boundary (271 days before due date)
- Window EndThe latest estimated conception boundary (261 days before due date)
The calculation assumes an average gestational length of 266 days (38 weeks) from fertilization to delivery. It determines the most probable conception date by subtracting 266 days from the entered due date, and defines a ten-day plausible range by taking 5 days before and 5 days after that target date (from 271 to 261 days prior to the due date).
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Due date1970-07-20
Results
- Probable conceptionMon, October 27, 1969
- Possible windowWed, October 22, 1969 – Sat, November 1, 1969
How to use the Pregnancy Conception Calculator
- 1Enter the estimated pregnancy due date.
- 2Subtract 266 calendar days from the due date to establish the single most probable conception date.
- 3Subtract 271 days from the due date to establish the beginning of the conception window.
- 4Subtract 261 days from the due date to establish the end of the conception window.
Reading your result
The 'Probable conception' date represents the theoretical midpoint of fertilization based on a typical 38-week post-conception timeline. The 'Possible window' provides a 10-day span covering five days before and after this midpoint, accounting for standard variations in ovulation timing and sperm viability.
Limitations
This calculator relies entirely on the accuracy of the entered due date, which can shift depending on ultrasound measurements or irregular menstrual cycles. It does not track actual coitus dates, basal body temperature, or the exact lifespan of sperm (up to 5 days) versus the ovum (12 to 24 hours).
Common questions
Why is conception estimated at 266 days before the due date instead of 280 days?+
A typical full-term pregnancy is measured as 280 days (40 weeks) from the first day of the last menstrual period (LMP). Because ovulation and conception generally happen about 14 days after LMP in an average 28-day cycle, the actual post-conception gestational age is roughly 266 days (38 weeks).
Does the conception window indicate the exact date intercourse occurred?+
Not necessarily. Sperm can survive inside the female reproductive tract for up to 5 days before ovulation, meaning intercourse could have taken place several days prior to the biological moment of conception.
How does cycle length affect this estimation?+
If your due date was calculated assuming a standard 28-day cycle but your natural cycle is significantly longer or shorter, the actual ovulation date relative to your period may differ. An early ultrasound estimate provides a more accurate due date for back-calculating conception.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
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Results are estimates for informational purposes and are not financial, medical or legal advice.