Conception Date Calculator — Estimate from Due Date
Enter an estimated due date and average cycle length to estimate conception and last-period dates. A calendar estimate, not an exact date derived from a birthday.
Formula and sources updated · Data sources · 1View algorithm, constants and default inputs (JSON)
Results
About this calculator
This calculator works backward from an estimated due date using the conventional 280-day model and adjusts the estimated last-period date by the cycle length. It cannot determine ovulation or conception precisely; cycle variation and clinical dating, including ultrasound, can change estimates. This is not a pregnancy test or medical advice.
Worked example
calculated with the default values below
Estimated due date 2027-07-01 · Average cycle length 28 days Estimated conception date 2026-10-08 · First day of last period 2026-09-24 · Days since last period 37 days
Method and assumptions
LMP = D − (280 + c − 28); C_d = D − 266 LMP = D − (280 + c − 28); conception = D − 266. The entered cycle length shifts the estimated LMP, while conception remains 266 days before the fixed due date under this model. This calculator works backward from an estimated due date using the conventional 280-day model and adjusts the estimated last-period date by the cycle length. It cannot determine ovulation or conception precisely; cycle variation and clinical dating, including ultrasound, can change estimates. This is not a pregnancy test or medical advice. The references provide context and limitations. They do not validate this page’s illustrative multipliers, lookup values or predictions.
Data sources
The references provide context and limitations. They do not validate this page’s illustrative multipliers, lookup values or predictions.
Disclaimer
These calculators give general date results. Public holidays, working-day conventions and time-zone rules are set locally and change, so confirm against the official calendar for anything legally or contractually binding.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
LMP = D − (280 + c − 28); C_d = D − 266
- D
- due date
- c
- cycle length in days
- LMP
- last menstrual period
- C_d
- conception date
LMP = D − (280 + c − 28); conception = D − 266. The entered cycle length shifts the estimated LMP, while conception remains 266 days before the fixed due date under this model.
Frequently asked questions
Can I use a birthday instead of an estimated due date?
This mode requires the estimated due date, not the actual birthday. Birth may occur before or after the due date, so a birthday alone cannot establish the conception date. The implemented estimate is due date minus 266 days; cycle length adjusts the estimated last period, not that fixed conception offset.
Which inputs and limits apply to Conception Date Calculator — Estimate from Due Date?
Use the units shown beside each field. These are form limits, not recommended settings. Average cycle length: 20–45 days LMP = D − (280 + c − 28); C_d = D − 266 LMP = D − (280 + c − 28); conception = D − 266. The entered cycle length shifts the estimated LMP, while conception remains 266 days before the fixed due date under this model. This calculator works backward from an estimated due date using the conventional 280-day model and adjusts the estimated last-period date by the cycle length. It cannot determine ovulation or conception precisely; cycle variation and clinical dating, including ultrasound, can change estimates. This is not a pregnancy test or medical advice. The references provide context and limitations. They do not validate this page’s illustrative multipliers, lookup values or predictions.
What is a complete worked example for Estimated due date = 2027-06-12; Average cycle length = 28 days?
Inputs: Estimated due date = 2027-06-12; Average cycle length = 28 days. Calculation: LMP = D − (280 + c − 28); C_d = D − 266. Results: Estimated conception date = 2026-09-19; First day of last period = 2026-09-05; Days since last period = 37 days. Reference date: 2026-10-12 (UTC).