Conception Calculator — When Did Conception Happen?
Work backwards from a due date — or forwards from a last period — to find the day conception most likely happened, the fertile window around it, and every date that ties the story together.
Your dates
🔒 All calculations happen in your browser — no data leaves your device.
The most likely story
- Pregnancy is dated from the first day of the last period — day 1 — not from conception; a due date sits 280 days (40 weeks) after that day.
- Counting back from a due date: conception = due date − 266 days; counting forward: conception = LMP + (cycle − 14) days.
- Worked example from the guide: a due date of 15 Jun 2027 gives conception on 22 Sep 2026 and an estimated last period of 8 Sep 2026.
Your timeline, day one to due date
The last period, the fertile window, conception and the due date — all 280 days on one line.
Keep exploring
Three companion tools for the months ahead.
How it works
Three steps from one date to the whole story.
Choose your anchor
Start from the date you trust most: a due date from a scan, or the first day of your last period.
Add your cycle length
Only needed in last-period mode. Ovulation is assumed to fall 14 days before your next period would have started.
Read the window
The most likely conception day, the fertile window either side of it, and the period and due dates that frame it.
The math behind the dates
Pregnancy is dated from the first day of the last period — day 1 — not from conception. A due date sits 280 days (40 weeks) after that day. Conception happens around ovulation, which in a textbook cycle falls about 14 days before the next period would have started: that is LMP + (cycle − 14) days. Counting backwards from a due date gives the mirror image: conception = due date − 266 days (280 − 14), and the estimated last period = due date − 280 days. The fertile window is wider than a single day because sperm can survive in the body for up to about 5 days while the egg lasts roughly a day, so it runs from 5 days before conception to 1 day after.
Worked examples (checked with real calendar arithmetic):
| Starting point | Calculation | Result |
|---|---|---|
| Due date 15 Jun 2027 | 15 Jun 2027 − 266 days · − 280 days | Conception 22 Sep 2026 · LMP estimate 8 Sep 2026 |
| LMP 8 Sep 2026, 28-day cycle | 8 Sep 2026 + (28 − 14) days · + 280 days | Conception 22 Sep 2026 · due 15 Jun 2027 |
| LMP 8 Sep 2026, 32-day cycle | 8 Sep 2026 + (32 − 14) days | Conception 26 Sep 2026 (ovulation day 18) · due 15 Jun 2027 |
Notice the first two rows land on exactly the same dates from opposite directions — that is the built-in consistency check of the 266/280-day pair. Longer cycles push ovulation (and conception) later; the due date on this page always counts 280 days from the last period.
Go deeper
Why the window is a week wide — and why no calculator can give you a certain day.
Conception is pinned to ovulation, but the days that can lead to it stretch backwards:
- Sperm wait: in fertile cervical mucus, sperm can survive for up to about 5 days, ready and waiting when the egg arrives.
- The egg does not: once released, an egg can be fertilised for only about 12 to 24 hours — which is why the window closes just 1 day after ovulation.
- So the window is 7 days: 5 days before conception, conception day itself, and 1 day after. Intercourse on any of them could be the one that led to the pregnancy.
- Conception day = ovulation estimate: fertilisation itself happens within about a day of the egg being released, so the estimate centres on ovulation.
The maths is exact; the biology it models is not:
- Cycles vary: even a “regular” 28-day cycle can shift by a few days month to month, and stress, illness, travel or breastfeeding can move ovulation further.
- The 14-day assumption: this calculator assumes ovulation 14 days before the next period. That luteal phase is fairly steady for most people, but it is an average, not a guarantee.
- Scans outrank calendars: a dating scan at about 11 to 14 weeks measures the baby directly and can move a due date by several days. If a scan date and this calculator disagree, clinicians work from the scan.
- It cannot identify a specific encounter: with a fertile window a week wide, dates alone can rarely prove which day — or which occasion — led to a pregnancy.
Conception as a window
Why this calculator returns a window rather than a single day — ovulation timing, and how long sperm and egg each survive.
Conception itself happens on one specific day: the day an egg is released and fertilised. The honest difficulty is that no calendar input can observe that day directly, so to calculate conception date is to estimate ovulation — and ovulation is a moving target. This is why the calculator above gives you a most likely day inside a window, and why any baby conception calculator that prints a single date with total confidence is claiming more than the arithmetic knows.
When ovulation falls. The second half of the cycle — ovulation to the next period — is the steadier half, at roughly 14 days, so ovulation lands about 14 days before the next period is due. In a cycle of N days, that places it on about day N − 14: day 14 of a 28-day cycle, day 18 of a 32-day cycle. That is exactly the rule the tool uses from a last period: September 8, 2026 with a 28-day cycle gives a likely conception on September 22, 2026, and the same period with a 32-day cycle moves it to September 26. Even in a regular cycle, though, ovulation can drift by a day or two between months, and in an irregular cycle it can move by much more — the first source of width in the window.
How long each side survives. The window exists because the two cells live on very different clocks. ACOG’s patient guidance puts it plainly: sperm can live in the reproductive tract for up to about 5 days, while an egg is fertile for only about 24 hours after it is released (often quoted as 12 to 24 hours). Put together, intercourse on any of the five days before ovulation can lead to a pregnancy — the sperm simply wait — and the egg’s short life closes the window about a day after ovulation. That is why the fertile window this calculator prints runs from 5 days before the likely conception day to 1 day after it: September 17 to September 23, 2026 in the example above. The day itself is the peak, not the whole story.
What cycle length does to the answer. Because ovulation tracks the end of the cycle rather than its start, the same last period produces different conception windows at different cycle lengths. Holding the period fixed at September 8, 2026:
| Cycle length | Ovulation (day of cycle) | Likely conception | Fertile window |
|---|---|---|---|
| 24 days | Day 10 | September 18, 2026 | Sep 13 – Sep 19, 2026 |
| 28 days | Day 14 | September 22, 2026 | Sep 17 – Sep 23, 2026 |
| 32 days | Day 18 | September 26, 2026 | Sep 21 – Sep 27, 2026 |
| 35 days | Day 21 | September 29, 2026 | Sep 24 – Sep 30, 2026 |
One last period, four cycle lengths. An 11-day spread in cycle length moves the likely conception day by 11 days — which is why the cycle-length field above matters more than any other input on this page.
Not all anchors are equal. The four routes into this calculator carry different certainty. An IVF transfer date is exact — a day-5 transfer on September 25, 2026 means fertilisation happened in the lab on September 20, 2026, and no window is needed at all. An ultrasound anchor inherits the scan’s accuracy, which is strongest in the first trimester. A due date counts back through an estimate, so its conception window inherits that estimate’s slack as well as its own. And a last period is the softest anchor of all, because it prices in the standard cycle described above. Same arithmetic throughout — very different confidence in what comes out.
Fertilisation is not implantation. One more interval is worth separating, because it explains the wait for a test result. Fertilisation happens in the fallopian tube within the egg’s short fertile span; the embryo then travels and implants in the uterus about 6 to 12 days after ovulation (the range measured in the classic implantation studies). So even a perfectly dated conception cannot produce a positive test on the day — the biology inserts a week or more between the two events, and “when did I conceive” and “when could a test first show it” are questions about different dates.
Read the result the way it is built: the centre day is the single most likely answer, the six-day window around it is the honest one, and a dating ultrasound — which can see the pregnancy rather than infer it — outranks both when the two disagree.
Conception and due date, both directions
Conception to due date is +266 days; due date back to conception is −266. The same line, walked both ways.
One number connects the two dates this family of tools moves between: 266 days. Pregnancy is counted as 280 days from the last period, and conception sits about 14 days into that count, so conception to due date is 280 − 14 = 266 days — 38 weeks. Every route on this page is that single interval added or subtracted.
Forward — from conception to a due date. Add 266 days to the conception date. Conceived on January 15, 2026, the estimated due date is October 8, 2026 — the same date the EDD Calculator reaches from a January 1 last period, because the two anchors are the same pregnancy read from different ends. The common form of the question — “if I conceived today, when am I due?” — is the same addition with today as the start: a conception on October 23, 2026 gives a due date of July 16, 2027.
Backward — from a due date to conception. Subtract 266 days. A due date of June 15, 2027 counts back to a likely conception on September 22, 2026, with the fertile window running September 17 to September 23, 2026, and an implied last period of September 8, 2026 — which is why this page’s two main modes agree with each other: feed the September 8 period forward and you land on the same September 22.
The conception chart. For the month-level version of the question, the chart below counts 266 days from the middle of each conception month. Used as a conceive calendar, it reads in both directions: down the left column to find when a baby conceived in a given month would be due, or down the right column to find the conception month behind a target due date. A dedicated conceive date calculator — the tool at the top of this page — then turns the month into your exact window.
| Conception month (mid-month date) | Conception date used | Estimated due date (+266 days) |
|---|---|---|
| January | Jan 15, 2026 | Oct 8, 2026 |
| February | Feb 15, 2026 | Nov 8, 2026 |
| March | Mar 15, 2026 | Dec 6, 2026 |
| April | Apr 15, 2026 | Jan 6, 2027 |
| May | May 15, 2026 | Feb 5, 2027 |
| June | Jun 15, 2026 | Mar 8, 2027 |
| July | Jul 15, 2026 | Apr 7, 2027 |
| August | Aug 15, 2026 | May 8, 2027 |
| September | Sep 15, 2026 | Jun 8, 2027 |
| October | Oct 15, 2026 | Jul 8, 2027 |
| November | Nov 15, 2026 | Aug 8, 2027 |
| December | Dec 15, 2026 | Sep 7, 2027 |
Each row counts 266 days from the 15th of the month. Conceive a week earlier or later than the 15th and the due date moves by the same week — the chart gives the month shape, the calculator above gives your exact dates.
Conception date vs due date. The two dates answer different questions and are routinely confused. The due date is a count — 280 days from a period — assigned before anyone can know when conception happened; the conception date is an event, usually derived afterwards by counting back. When a provider or an app shows both, the conception figure has almost always been calculated from the due date (or the period), not observed: outside IVF, nobody watches conception happen. That is also why the two can disagree slightly between tools — each has chosen a slightly different anchor or cycle assumption for the same underlying line.
Two honest limits apply in both directions. First, the 266-day line assumes conception on ovulation day in the standard frame; a cycle that runs longer or shorter moves the true day by the same few days discussed in the window section above. Second, the due date at the end of the line is an estimate like every other: only about 5% of babies are born on it, most arriving in the surrounding weeks. The line is exact — what it connects are estimates.
Furthest back: from a birthday. The same line extends one step further, to the question of when a person was conceived given only their birth date. If the baby arrived on the due date, conception was 266 days before the birthday — a birth on October 8, 2026 points to January 15, 2026. Because most babies are not born on the date, the honest version is a window: births between 37 and 42 weeks put conception between 280 and 245 days before the birthday (January 1 to February 5, 2026 in this example). The guide linked below works that reverse problem in full.
Dating backwards from a birth date
The reverse problem: a birthday in hand, a conception window to recover — and why the answer is always a range.
Sometimes the known date is the end of the story, not the beginning: a birth date, and the question of when conception happened. The arithmetic runs the 266-day line backwards. A pregnancy lasts about 280 days from the first day of the last period, and conception falls about 14 days into that span, so the central estimate is the birth date minus 266 days — and the last-period estimate is the birth date minus 280 days. Take a birth date of October 8, 2026: the central conception estimate is January 15, 2026. But a central estimate is not a day, and this page never presents it as one.
Why the answer is a window. Only about 5% of babies arrive on their exact due date, and most arrive within two weeks on either side of it, so the birth date itself already carries that much uncertainty backwards. Working from the example above, allowing the birth to fall anywhere in the usual window around the due date puts the last period between January 1 and February 5, 2026, and conception between about January 15 and February 19, 2026 — a span of roughly five weeks, not a single day. An early ultrasound or an IVF transfer date can narrow the window considerably, because they anchor the pregnancy to a measurement rather than to a remembered date. Nothing in the date arithmetic can prove one exact day, and any tool that prints a single conception day from a birth date alone is claiming more than the numbers know.
Where cycle length fits in. The backwards route from a birth date does not use cycle length at all — it leans on the length of the pregnancy itself. Cycle length matters on the forwards route, from a last period: ovulation lands about 14 days before the next period, so in a cycle of N days it falls on about day N minus 14, and a regularly longer or shorter cycle moves the conception estimate by the same number of days. The two routes answer the same question from opposite ends, and when both are available they should land in the same window; if they do not, the ultrasound or transfer date is the anchor to trust.
Frequently asked questions
Quick answers about conception dates.
How is a due date calculated?
Most due dates are counted from the first day of the last period: add 280 days (40 weeks), which is Naegele’s rule. From a known conception date, add 266 days instead. An early ultrasound can reset the date when it disagrees with the period date by more than the margin allowed for that stage, and IVF pregnancies are dated from the embryo transfer. Every method produces an estimate — only about 5% of babies arrive on the day itself.
What is Naegele’s rule?
Naegele’s rule estimates the due date from the last menstrual period: add 7 days, subtract 3 months and add 1 year — the same as adding 280 days. It assumes a 28-day cycle with ovulation on day 14, so it can run a few days early or late when your cycle is regularly shorter or longer than that.
How many weeks pregnant am I?
Count the whole weeks between the first day of your last period and today; the leftover days are the extra days — 30 days after a last period of January 1, 2026 is 4 weeks and 2 days. If you only know the due date, subtract the weeks remaining from 40. When the last period is unknown or cycles are irregular, an early ultrasound gives the most accurate count.
What is gestational age?
Gestational age is how far a pregnancy has progressed, counted from the first day of the last period and written as weeks plus days — 12 weeks, 3 days, for example. Because the count starts roughly two weeks before conception in a standard cycle, gestational age runs about two weeks ahead of the baby’s fetal age.
When did I conceive?
Most likely on ovulation day — about 14 days after the last period in a 28-day cycle, and later in a longer cycle. Sperm can survive for up to about 5 days, so the intercourse that led to the pregnancy may have happened up to 5 days before ovulation itself. That is why conception is honestly given as a window of a few days, never as a certain single day.
How do I calculate my due date from conception?
Add 266 days (38 weeks) to the conception date. Conceived on January 15, 2026, the estimated due date is October 8, 2026 — the same date Naegele’s rule gives from a January 1 last period, because the two anchors sit exactly 14 days apart.
Can an ultrasound change my due date?
Yes. ACOG’s guidance is that a first-trimester ultrasound is the most accurate way to date a pregnancy, and the due date is revised when the scan disagrees with the period date by more than a set margin — more than 5 days up to 8 weeks and 6 days, more than 7 days from 9 weeks to 13 weeks and 6 days, with wider margins later in pregnancy (10, 14 and then 21 days). Once a date is set by an early scan, it is normally left alone; later scans measure growth, not dates.
How are IVF due dates calculated?
From the transfer date and the embryo’s age: add 266 days, then subtract the embryo age — 261 days for a day-5 blastocyst, 263 days for a day-3 embryo. Because those dates are known exactly, ACOG’s guidance is that the IVF-derived due date is used as assigned and is not revised by later ultrasound measurements.
What does LMP mean in pregnancy?
LMP is the last menstrual period — specifically, its first day. It is the standard starting line for pregnancy dating: the due date falls 280 days after it and gestational age counts from it, even though conception happens about two weeks later in a typical cycle.
How accurate are due dates?
They are estimates by design. Only about 5% of babies are born on the estimated due date itself; most arrive in the surrounding weeks, and a birth anywhere from 37 to 42 weeks sits inside the normal term range. A date set by an early ultrasound is the most accurate version — a date from the last period alone can be off by a week or more when cycles are irregular.
Can the exact day of conception be determined from dates alone?
No. Dates alone can only recover a window: ovulation moves by a day or two even in regular cycles, and the length of a pregnancy varies, so the backwards arithmetic from a birth date spans several weeks. An IVF transfer date, with the embryo age known, pins the timing far more closely than any calendar route, and an early ultrasound narrows it further — but no date calculation can prove one exact day.
How do you work back to a conception date from a birth date?
Subtract 266 days from the birth date for the central estimate, and 280 days for the estimated first day of the last period, then widen both figures into a window to allow for the natural variation in pregnancy length. For a birth date of October 8, 2026, the central conception estimate is January 15, 2026, inside a window of roughly January 15 to February 19, 2026.
Does a longer or shorter cycle change the conception estimate worked back from a birth date?
Not directly: the backwards route leans on the length of the pregnancy, not on the cycle. Cycle length changes the estimate on the forwards route, from a last period, where ovulation falls about 14 days before the next period — day 14 of a 28-day cycle, but day 18 of a 32-day cycle. If the two routes disagree, the ultrasound or transfer date is the anchor to trust.
Related guides
Two guides that take this page’s arithmetic further — the 266-day line worked in full, and the three dating methods compared side by side.
From Conception Date to Due Date: The 266-Day Line and Its Windows
The conception-to-due-date arithmetic worked forwards and backwards — the fertile-window uncertainty stated honestly, and the reverse problem of working back from a birthday.
🧭Due-Date Methods Compared: Naegele’s Rule, Ultrasound Dating and IVF Dates
How each method dates a pregnancy, when an ultrasound actually moves an LMP date under ACOG’s thresholds, and why the IVF date is the most exact of the three.
Related calculators
More tools for cycles and pregnancy dates.
Disclaimer
This calculator is for information only and is not medical advice. Every date it shows is an estimate based on standard cycle maths (280 days from the last period; ovulation 14 days before the next period): real cycles vary, and a dating scan is the more reliable source. It cannot establish the timing of a specific encounter and should not be relied on for medical, legal or paternity decisions — talk to your midwife, GP or another qualified professional about your own dates.