Conception Date Estimator

About this tool

Likely conception date, fertile window + due date

Conception date estimator. Work out the likely conception date from your last menstrual period and cycle length, or from a known due date, with the fertile window, estimated due date, and current gestational age.

RT-HLT-057 · Health & Fitness · Reviewed May 2026

⚠ Disclaimer: For general information only. Not medical advice. RECATOOLS is not a medical device under FDA 21 CFR 820 or EU MDR 2017/745. These calculations are educational and must not replace consultation with a qualified healthcare professional. If you are pregnant, ill, or considering a change to medication or routine, speak with your doctor.

Estimate the most likely conception date from the first day of your last menstrual period and your usual cycle length — or work backwards from a known due date. You'll also get the fertile window, estimated due date, and current gestational age.

First day of last menstrual period (LMP)

Average cycle length

days

📅 Research current as of 30 May 2026 · Sources: Conception ≈ LMP + (cycle − 14) days; due date = LMP + 280 days (Naegele); from EDD, conception ≈ EDD − 266 days.

Rates, regulations, and lender practices change frequently — verify current figures with your provider or licensed advisor before acting.

Estimated conception date

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Enter your last period date to begin.

Conception (est.)

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Fertile window

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Estimated due date

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Gestational age today

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How to Use the Conception Date Estimator

Choose your starting point

If you know the first day of your last period, use "From last period." If a scan or earlier calculation already gave you a due date, switch to "From due date" and work backwards instead.

Enter the date

Pick the first day of your last menstrual period (not the day it ended), or your estimated due date. Use the most accurate date you have — the whole estimate hinges on it.

Set your cycle length

The default is 28 days. If your cycles are typically shorter or longer, adjust it — ovulation, and therefore conception, happens about 14 days before your next period, so cycle length shifts the conception date.

Read the estimate

You'll see the most likely conception date, the fertile window around it, the estimated due date, and how many weeks pregnant you are today. Treat all of these as estimates and confirm with an ultrasound.

How Conception Dates Are Estimated

Counting From the Last Period

Pregnancy dating has a quirk that surprises many people: the clock usually starts about two weeks before conception actually happens. By long-standing medical convention, gestational age is counted from the first day of the last menstrual period (LMP), not from the day the egg was fertilised. That's because the LMP is a date most people can identify, whereas the exact moment of conception is invisible. In a textbook 28-day cycle, ovulation — and therefore conception — occurs roughly 14 days after the LMP, halfway through the cycle. So the simplest estimate of the conception date is the LMP plus 14 days. If your cycles are longer or shorter than 28 days, the timing shifts: ovulation happens about 14 days before your next period, so for a 30-day cycle conception lands closer to day 16, and for a 26-day cycle closer to day 12. This calculator uses LMP plus (cycle length minus 14) to account for that.

The same arithmetic gives the estimated due date. The classic rule, Naegele's rule, adds 280 days (40 weeks) to the LMP — which is the same as 266 days (38 weeks) after conception, since conception is about two weeks into that count. That's why, if you already have a due date from a scan, you can run the logic in reverse: the estimated conception date is roughly the due date minus 266 days. Around the single best-guess conception date there's also a fertile window, because sperm can survive for several days in the body while the egg lives for about a day. The realistic window stretches from a few days before ovulation to a day or so after, which is why a precise conception date can never be pinned down from dates alone — only estimated.

"Pregnancy is counted from the last period, not from conception — so 'how many weeks' already includes the two weeks before the egg was ever fertilised. Conception sits about 14 days after the LMP."

Why It's an Estimate, Not a Fact

A conception date worked out from dates is genuinely useful — for satisfying curiosity, for understanding the fertile window, or for a rough sense of how far along a pregnancy is — but it is always an estimate, and several things blur it. Cycle length varies from person to person and even month to month, ovulation doesn't always happen exactly mid-cycle, and the fertile window means the egg could have been fertilised on any of a few days. Spotting that gets mistaken for a period, or an uncertain LMP, can throw the starting date off entirely. This is why clinicians treat an early ultrasound as the gold standard: a first-trimester scan measures the embryo directly and can date a pregnancy to within a few days, often overriding the LMP-based estimate. If your cycles are irregular, if you were on hormonal contraception recently, or if you simply aren't sure of your last period, the dating from this tool will be looser still. Use the result as a helpful approximation and a way to understand the biology, but rely on your doctor or midwife and an ultrasound for the dates that guide care. Nothing you enter here is stored — the calculation runs entirely in your browser.

10 Facts About Conception Dating

Pregnancy is dated from the last period, not from conception.

Conception is about 14 days after the LMP in a 28-day cycle.

Ovulation happens roughly 14 days before the next period.

Longer or shorter cycles shift the conception date.

Due date = LMP + 280 days (Naegele's rule).

That's the same as 266 days after conception.

Sperm survive ~5 days; the egg lives about a day.

So there's a fertile window, not a single day.

An early ultrasound dates a pregnancy most accurately.

From a due date, conception ≈ due date − 266 days.

Frequently Asked Questions

The most common method counts from the first day of your last menstrual period (LMP). Conception happens around ovulation, which is roughly 14 days before your next period — so conception ≈ LMP + (cycle length − 14) days. For a standard 28-day cycle that's about LMP + 14 days. If you already know your due date, you can work backwards instead: conception ≈ due date − 266 days. Both are estimates because ovulation timing varies.
Because the first day of the last period is a date most people can identify, while the exact moment of conception is invisible. By convention, gestational age starts from the LMP, which means the count already includes the roughly two weeks before the egg is fertilised. So when someone is "6 weeks pregnant," conception was only about 4 weeks earlier. It's a standard medical convention that keeps dating consistent across pregnancies.
It's a reasonable estimate but not exact. Cycle length varies, ovulation doesn't always happen exactly mid-cycle, and the fertile window spans several days, so the true conception date could be a few days either side of the estimate. An early-pregnancy ultrasound measures the embryo directly and dates the pregnancy to within a few days, which is why clinicians treat the scan, not the LMP, as the gold standard when the two disagree.
It's the span of days during which intercourse can lead to pregnancy. Because sperm can survive in the reproductive tract for up to about five days and the released egg lives for roughly 24 hours, the window runs from about five days before ovulation to a day or so after. The calculator shows this band around the estimated conception date. It's the reason a precise conception date can't be pinned down from dates — fertilisation could have happened on any day in that window.
Yes. Ovulation occurs about 14 days before your next period, regardless of total cycle length — so the first half of the cycle is what varies. For a 28-day cycle, conception is around day 14; for a 32-day cycle it's around day 18; for a 24-day cycle around day 10. That's why entering your real average cycle length gives a better estimate than assuming 28 days. If your cycles are irregular, the estimate is naturally looser.
The classic method is Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. That's equivalent to 266 days (38 weeks) after conception. The calculator shows this estimated due date alongside the conception date. Only about 1 in 20 babies actually arrive on the exact due date — it marks the middle of a normal range, with full-term birth anywhere from 37 to 42 weeks.
Yes — switch to "From due date" mode. Since the due date is about 266 days after conception, the calculator subtracts 266 days from your estimated due date to get the likely conception date, then shows the fertile window around it. This is handy when an ultrasound has already given you a due date that differs from your LMP-based estimate, because the scan date is usually the more reliable starting point.
Gestational age is how far along a pregnancy is, counted in weeks and days from the first day of the last menstrual period. The calculator shows today's gestational age based on your LMP. Because it's measured from the LMP, it's roughly two weeks more than the time since conception ("fetal age"). Doctors use gestational age to track development, schedule scans, and judge whether a baby is preterm, full-term, or overdue.
Yes. Every date you enter is processed entirely in your browser — nothing is sent to a server, stored, or shared. There's no account, no tracking of the dates, and no record kept of your pregnancy. You can use it as many times as you like, privately, and close the page knowing nothing was saved by RECATOOLS.
No. This tool is for general information and to help you understand the biology of dating a pregnancy. It is not medical advice and should not replace professional care. For accurate dating and for any decisions about a pregnancy, see a doctor or midwife — an early ultrasound is the most reliable way to date a pregnancy, and your care team will use it to guide scans, tests, and timing.

Conception Date Estimator