Conception Date Calculator
Work backwards from a due date, a last period or a gestational age to the window in which conception is likely to have happened.
Result
Conception window opens: January 24, 2026Estimate
A window from a calendar, not a determination of when conception happened. It cannot establish paternity or any other fact.
- Conception window closes
- January 29, 2026Estimate
- Most likely date
- January 29, 2026Estimate
- The estimated ovulation date. Conception can follow it by a day or precede it by several.
- Implied last period
- January 15, 2026Estimate
- Implied due date
- October 22, 2026Estimate
https://calcgallery.com/pregnancy/conception-date-calculator/
How to use the Conception Date Calculator
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- Enter that date, plus your average cycle length.
- Read the window, not the single most likely date.
- Note what the page says about what this arithmetic cannot establish.
Why the answer is a window
Conception is not an event that can be pinned to a day from a calendar. Sperm remain viable in the reproductive tract for up to about five days, and an egg for less than one after release. So intercourse on any of six days can result in a pregnancy dated identically.
Reporting a single conception date would be a precision the biology does not have. This page reports the window, and names the most likely day inside it as secondary information rather than as the answer.
ovulation = last period + (cycle length − 14)window = ovulation − 5 days to ovulationThe three bases
From a due date. Counting back 280 days gives the notional last menstrual period, and the window follows from that and your cycle length. This is the inverse of what the due date calculator does, and the two agree exactly — a date fed through one and back through the other returns to where it started, which a test on this page pins.
From a last period. The same arithmetic without the first step.
From a gestational age. Subtracting the weeks and days from the date that age was measured on gives the last period, and the rest follows.
All three reduce to the same calculation once a last menstrual period is established, so they cannot disagree with one another.
What this cannot establish
It is arithmetic on a calendar. It cannot establish paternity. It cannot establish a legal fact. It cannot distinguish between the days inside the window it reports, and it has no way to know whether your cycle behaved the way the average predicts.
An ultrasound date is more reliable than a calculated one, and a first-trimester scan is the most reliable of all. If a conception window matters to you for any consequential reason, the input to improve is the due date, and the way to improve it is a scan.
Practical tips
Use the best due date you have. A scan-based due date makes everything downstream better; one from a half-remembered period makes it worse.
Enter your real cycle length. It moves the window directly, by a day for every day your cycle differs from 28.
Read the window, not the single date. The estimated ovulation date is the last day of a six-day span, and conception on it is not meaningfully more likely than on the days before.
The window is asymmetric for a reason. Five days before and one after reflects how long each cell survives, not a rounding choice.
Two bases must agree. If you get different answers from a due date and from a last period, one of the two dates is wrong rather than the calculator.
With your numbers
- Counting back 280 days gives an implied last period of January 15, 2026
- Ovulation from that cycle lands on January 29, 2026, with the window closing January 29, 2026
- Sperm survive several days, so the window opens January 24, 2026
Quick reference
A window from a calendar, not a determination of when conception happened. It cannot establish paternity or any other fact.
| Example | Conception window opens |
|---|---|
| Due 2026-10-22, 28-day cycle | January 24, 2026 |
| Due 2026-10-22, 35-day cycle | January 31, 2026 |
| Due 2027-03-01, 28-day cycle | June 3, 2026 |
| LMP 2026-01-15, 28-day cycle | January 24, 2026 |
| LMP 2026-01-15, 35-day cycle | January 31, 2026 |
| LMP 2026-06-01, 26-day cycle | June 8, 2026 |
| 19w 3d on 2026-06-01 | January 25, 2026 |
| 8w 0d on 2026-03-01 | January 13, 2026 |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
3 sources
ACOG Committee Opinion No. 700, Methods for Estimating the Due Date, Obstet Gynecol 2017;129:e150-4 (reaffirmed 2025), https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date: "By convention, the EDD is 280 days after the first day of the LMP."
Used for
- Gestational duration added to the last menstrual period.
Thiyagarajan DK, Basit H, Jeanmonod R, Physiology, Menstrual Cycle, StatPearls (NCBI Bookshelf NBK500020), last update 27 September 2024, https://www.ncbi.nlm.nih.gov/books/NBK500020/ (read from the Internet Archive capture of 5 December 2024; the live page serves a CAPTCHA): "the luteal or secretory phase is relatively consistent within an individual, typically lasting 14 days" and "Ovulation typically occurs 14 days before the onset of menses". A clinical convention: real luteal phases vary between women.
Used for
- Conventional luteal phase length, counted back from the next expected period.
Wilcox AJ, Weinberg CR, Baird DD, Timing of sexual intercourse in relation to ovulation, NEJM 1995;333(23):1517-21, PMID 7477165: conception occurred only "during a six-day period that ended on the estimated day of ovulation", so the window opens 5 days before it.
Used for
- Days before ovulation the fertile window opens.
Common questions about the Conception Date Calculator
- How accurate is this?
- As accurate as the date you put in, which is usually not very. If the due date came from a last period it inherits every assumption in Naegele rule, including a 28-day cycle and ovulation on day 14. If it came from a first-trimester ultrasound it is considerably better. Either way the answer is a window of about a week, and the biology does not support narrowing it.
- Can this prove when conception happened?
- No. It is arithmetic on a calendar. It cannot establish paternity, it cannot establish a legal fact, and it cannot distinguish between two dates inside the window it reports. If a precise date matters for a reason of consequence, a calculator is not the tool, and the honest answer is that the biology itself does not supply one.
- Why does it differ from my scan date?
- A dating scan measures the pregnancy; this works backwards from a date. If your scan moved your due date, feed the scan-based due date in here rather than the one from your last period, and the window will move with it. Where the two disagree, the scan is the better input.
- What if I do not know my cycle length?
- The calculator assumes 28 days, which is the convention. If your cycles are longer, ovulation happens later in the cycle and the real window is later than the default shows. Entering an accurate average is the single input that most improves the answer.
- Why is the window six days and not one?
- Because sperm survive several days in the reproductive tract, so intercourse up to five days before ovulation can result in conception, while the egg remains viable for less than a day. The window runs from five days before the estimated ovulation to the day of ovulation, following the Wilcox 1995 finding that conception occurred only in those six days.