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.
Open the Conception Calculator
About Pregnancy Conception Calculator
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
All ArrayKit tools