Work backward from your due date or last period to estimate the most likely day you conceived — and the 6-day window when it could have happened.
Written and reviewed by the babybumpkit editorial team.
How a conception date is calculated
Conception almost always happens at ovulation — when an egg is released and meets sperm in the fallopian tube. The math works in two directions:
From a due date: conception = due date − 266 days. Pregnancy averages 266 days from conception to birth.
From a last menstrual period (LMP): conception = LMP + (cycle length − 14). Ovulation lands ~14 days before the next expected period, regardless of cycle length.
Both give you the same answer when the inputs are accurate — they just start from different anchors. Use the due-date mode when an ultrasound has confirmed your dating; use the LMP mode when you're working from your period.
Why the calculator shows a window
Conception happens on a single day, but the sex that led to it could have happened over a 6-day window. Sperm can survive in the reproductive tract for up to five days waiting for ovulation. So fertilization can result from intercourse anywhere from 5 days before ovulation through ovulation day itself.
This matters most for people trying to work out timing — whether for paternity questions, planning, or just understanding what happened. The calculator gives you both the most likely conception day and the wider 6-day window when the fertilizing intercourse could have occurred.
How accurate is the estimate?
Accuracy depends on which input you use:
Ultrasound-confirmed due date: very accurate — an early dating scan measures the embryo directly, and the calculator just reverses that with simple subtraction.
LMP + regular cycles: usually within 1–3 days of actual conception.
LMP + irregular cycles: can be off by a week or more, because ovulation timing varies.
For higher precision, an early dating ultrasound (8–12 weeks) is the most accurate way to confirm conception timing. ACOG guidance is that ultrasound dating supersedes LMP-based dating when the two differ by more than 5–7 days in the first trimester.
When this calculator can and can't help
The conception calculator is useful for:
Figuring out roughly when you conceived (a common early-pregnancy curiosity)
Understanding the gap between LMP-based gestational age and actual fetal age
Narrowing down the timing window for personal or relationship clarity
It cannot reliably determine paternity. The 6-day fertile window is genuinely a window — if you had different partners within that range, a calculator can't tell you whose sperm fertilized the egg. The only definitive answer is DNA testing (prenatal NIPT-based paternity from 8+ weeks, or postnatal). Anyone selling certainty from dates alone is selling something they can't deliver.
Frequently asked questions
Conception almost always happens at ovulation, when sperm fertilizes a released egg. Working from a known due date, conception is roughly 266 days earlier (the average length of pregnancy from conception). Working from your last menstrual period (LMP), conception is roughly cycle length minus 14 days after the start of that period — that's when ovulation typically lands.
The estimate is most reliable when you have a precise due date confirmed by an early dating ultrasound. From an LMP entry on regular cycles, accuracy is within 1–3 days. From an LMP entry on irregular cycles, the estimate can be off by a week or more. Conception itself is a window, not a single day — sperm can survive for up to 5 days, so the actual fertilizing intercourse may have been a few days before conception happened.
Because conception is almost always preceded by 1–5 days of sperm waiting in the fallopian tube for an egg. The day of fertilization (conception) is usually the day of ovulation, but the sex that led to it can have happened anywhere in the 6-day fertile window before. If you're trying to pinpoint when intercourse led to conception, the window is the honest answer.
Not reliably. The conception window is 6 days long, and sperm can survive in the body for up to 5 days. If you had different partners within that window, a calculator can't determine paternity from dates alone. The only reliable answer is DNA testing — either prenatal (non-invasive paternity testing from a blood draw at 8+ weeks) or postnatal. A calculator can narrow the window, but it cannot rule a specific person in or out.
Gestational age is measured from the first day of your last menstrual period (LMP), which is roughly 2 weeks before conception. So if you're 8 weeks gestational, conception was about 6 weeks ago. Doctors, prenatal apps, and ultrasounds use gestational age. Conception age (fetal age) is what this calculator gives you.
Yes — that's actually the most common scenario. Sperm can live in the reproductive tract for up to 5 days, waiting for ovulation. So sex on days 1–5 before ovulation can result in conception on the day of ovulation. The day of sex and the day of conception are usually different. This is why conception calculators report a window rather than a single intercourse date.
Not precisely. IVF pregnancies are dated from the embryo transfer day, not from a natural conception event — and the embryo's age at transfer (3-day or 5-day) shifts the math. If you conceived through IVF, our conception-date due date calculator includes an IVF mode that uses the transfer date directly. The conception calculator on this page assumes natural conception.
Use the ultrasound-revised due date. An early dating ultrasound (typically 8–12 weeks) measures the embryo directly and is the most accurate dating method available. ACOG guidance is that ultrasound dating replaces LMP-based dating when they differ by more than 5–7 days in the first trimester. Plug the revised due date into the calculator for the most accurate conception estimate.
Stress, illness, travel, and sudden sleep or weight changes can shift the timing of ovulation in any given cycle — which means conception can happen earlier or later than calendar math predicts. That's part of why the calculator gives a window. If you suspect ovulation was significantly off in the cycle you conceived in, an early ultrasound is the only way to confirm.
A due date calculator works forward in time — given LMP or conception date, predict the due date. This calculator works backward — given a due date or LMP, predict when conception likely happened. Both use the same math (266 days from conception to birth, ~14 days from LMP to ovulation); they just start the calculation from different points.
Sources and medical references
The conception-timing math on this page comes from published guidance and peer-reviewed research.