Pregnancy Conception Calculator
Already know your due date, or your baby's actual birth date? Work backward to estimate when conception likely happened, plus a realistic date range and estimated LMP date.
266
DAYS FROM CONCEPTION
±2
DAY WINDOW
280
DAYS FROM LMP
Your Conception Timeline
Estimated Conception Date
February 27, 2026
Based on the from due date method
- Conception Window
- Feb 25, 2026 – Mar 1, 2026
- Estimated Due Date
- Nov 20, 2026
Feb 25, 2026
Window Start
Mar 1, 2026
Window End
Feb 13, 2026
Estimated LMP Date
Nov 20, 2026
Estimated Due Date
🗓️ Reverse Conception Timeline
⚠️ This is an estimate based on average pregnancy timelines and is not suitable for legal or paternity purposes — those determinations require medical or genetic testing.
Working Backward to Estimate a Conception Date
Most pregnancy tools project forward — from your last period toward a due date. This one runs in reverse: it starts from a date you already know (a due date or a birth date) and works backward to estimate when conception most likely occurred. It's the tool to reach for when you're reconstructing a timeline after the fact — for baby-milestone curiosity, journaling, or general record-keeping — rather than trying to conceive. For forward planning from your last period, see our conception calculator or ovulation calculator.
🧮 The Key Formulas
From Due Date: Conception ≈ Due Date − 266 days | Estimated LMP ≈ Due Date − 280 daysFrom Last Period (LMP): Conception ≈ LMP + (Cycle Length − 14) days | Due Date ≈ LMP + 280 daysConception Window: Estimated Conception Date ± 2 days📌 An Estimate, Not a Verified Record
Every figure this tool returns is a statistical estimate derived from average gestation lengths — real pregnancies vary in actual gestation by roughly two weeks in either direction, and due dates are frequently revised after an early ultrasound. This calculator is not suitable for legal, custody, or paternity determinations, which require medical dating or genetic/DNA testing to establish with certainty.
📏
Why There's a Range, Not One Day
Sperm can survive up to 5 days before ovulation, and fertilization can occur anywhere within that window — so a single exact conception date can't be pinpointed after the fact, only estimated within a range.
🔁
Conception vs LMP Date
LMP is the clinical starting point doctors count pregnancy from, but actual conception happens about two weeks later, around ovulation — the two dates describe the same pregnancy but are never the same day.
🩺
Early Ultrasound Dating
A first-trimester ultrasound measures fetal size to estimate gestational age far more precisely than LMP-based math, which is why due dates are often adjusted after that scan.
🌱
Implantation Timing
A fertilized egg typically implants in the uterine lining 6-10 days after conception — this is what triggers the hormone (hCG) that pregnancy tests detect, roughly two weeks after the estimated conception date.
— FAQ