Work backward from a known pregnancy to an estimated conception date, fertile window, and due date, right in your browser.
Pregnancy Conception Calculator
Work backward from a known pregnancy to an estimated conception date, fertile window, and due date, right in your browser.
The Pregnancy Conception Calculator runs entirely in your browser — the dates you enter never leave your device. This is a general estimate using a standard formula, not medical advice; talk to a healthcare provider for guidance specific to your pregnancy.
The Pregnancy Conception Calculator works backward from an already-confirmed pregnancy to estimate when conception likely happened. Choose the milestone you actually know — the first day of your last menstrual period, a due date given by a provider, or an ultrasound dating scan with its measured gestational age — and it back-calculates the likely conception date, a three-day-wide fertile window around it, and an estimated due date. Each mode uses a standard obstetric formula: a fixed 14-day luteal phase for last-period and due-date estimates, and gestational age math for ultrasound estimates. This is a general estimate using a standard formula, not medical advice — an early ultrasound remains the most accurate single dating method. All calculations run locally in your browser; nothing you enter is uploaded.
Features
Three input modes: last menstrual period, due date, or ultrasound dating scan
Back-calculates a best-estimate conception date for an already-known pregnancy
Shows a three-day likely conception window around that estimate
Also returns an estimated due date consistent with the chosen input
Ultrasound mode accepts the scan date plus its measured gestational age in weeks
Uses the standard 14-day luteal-phase and 280-day gestation formulas
One-click copy of the full conception, window, and due-date summary
Everything computes on your device, with nothing sent anywhere
How to use the Pregnancy Conception Calculator
Pick the milestone you know: last period, due date, or ultrasound
Enter that date (and, for ultrasound, the gestational age in weeks at the scan)
Read the estimated conception date, fertile window, and due date
Copy the summary or switch modes to compare with another known date
Example
Input
ultrasound 8w on 2026-03-01
Output
Conception: ~Jan 18, 2026
An 8-week gestational age at the scan back-calculates to conception about 42 days earlier.
Common errors & troubleshooting
Ultrasound mode shows no result even though a date was entered. — Also fill in the gestational age in weeks measured at that scan (0–42) — both fields are required for this mode.
The estimated conception date looks a week or two off from what a provider said. — This calculator assumes a standard 28-day cycle and 14-day luteal phase; a longer or shorter cycle shifts actual ovulation, and an ultrasound dating is more precise for an individual pregnancy.
Due-date mode and last-period mode give slightly different due dates for the same pregnancy. — That is expected if the entered due date was itself adjusted for cycle length or an ultrasound — feed the same source date into a single mode for a consistent estimate.
Gestational age entered as days instead of weeks. — The ultrasound field expects completed weeks (e.g. 8, not 56) — convert days to weeks first.
Frequently asked questions
What is the Pregnancy Conception Calculator?
It is a tool that estimates when conception likely happened for a pregnancy you already know about, using the first day of your last period, a due date, or an ultrasound dating scan as the starting point.
How does the Pregnancy Conception Calculator estimate conception from an ultrasound?
It subtracts the gestational age measured at the scan (converted to days), then adds back the standard 14-day luteal phase, to back-calculate the likely conception date and a three-day window around it.
Is the conception date from this calculator exact?
No — it is a general estimate using a standard formula, not medical advice. Actual ovulation and conception timing vary by cycle length, and an early ultrasound is the most accurate single dating method for an individual pregnancy.
Can the Pregnancy Conception Calculator also give me a due date?
Yes. Alongside the conception date and fertile window, it returns an estimated due date consistent with whichever milestone (last period, due date, or ultrasound) you entered.
Does the Pregnancy Conception Calculator upload my dates anywhere?
No. Every date you enter and every result shown is computed locally in your browser and never leaves your device.
Why does the fertile window span several days instead of one exact date?
Conception timing naturally varies by a few days even within a single cycle, so the calculator shows a three-day window around the best-estimate conception date rather than a single fixed day.
Related tools
Conception Calculator — Estimate the likely conception date from a due date or last period.
Due Date Calculator — Estimate a pregnancy due date from the last menstrual period (Naegele’s rule).
Pregnancy Calculator — Track pregnancy progress, trimester and milestones from the due date or LMP.
Ovulation Calculator — Estimate your fertile window and ovulation day from your cycle.
Period Calculator — Predict your next several periods and fertile windows from your cycle.