Measure consistently
Use the established clinical due date rather than a date generated by another casual estimate whenever possible.
Conception-window back-calculation
Work backward from a due date or gestational-age date.
Estimate conception and LMP-equivalent dates from an established due date, then show a deliberately broad conception window rather than a falsely exact day.
The calculation runs locally in this browser. Educational outputs do not replace professional assessment, diagnosis, treatment or emergency care.
Method and interpretation guide
Use the conception-window result as a transparent estimate
Use the established clinical due date rather than a date generated by another casual estimate whenever possible. The worksheet exposes its assumptions so the result can be checked instead of accepted as a black box.
Read the assumptions
Pregnancy is dated from LMP convention, while conception usually occurs later. Ovulation, implantation and clinical dating each vary, so back-calculation is approximate.
Escalate important decisions
A calculator cannot examine symptoms, medical history, medication effects or measurement quality. Use qualified professional guidance when the decision has health consequences.
Local calculation
Inputs are processed in this browser. No account or external calculator is required to produce the result.
False precision
Extra decimal places do not remove biological variation, input error or limits in the underlying population equation.
Scope
Pregnancy is dated from LMP convention, while conception usually occurs later. Ovulation, implantation and clinical dating each vary, so back-calculation is approximate.
Simple by design
Three steps to a clearer number.
Enter your details
Use realistic values in each field. You can change them anytime.
Calculate instantly
The formula runs locally, so there is no account or waiting time.
Use the result
Treat the answer as a practical estimate for your next decision.