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Pregnancy Conception Calculator
Most probable conception dates: Nov 18, 2025 – Nov 22, 2025
Most probable dates of sexual intercourse that led to the pregnancy: Nov 15, 2025 – Nov 22, 2025
Possible conception dates: Nov 17, 2025 – Nov 27, 2025
Possible dates of sexual intercourse that led to the pregnancy: Nov 12, 2025 – Nov 27, 2025
The results of this calculator are estimation only.
Ovulation is estimated as LMP + (cycle length − 14). Ranges allow for biological variability and sperm survival; not a substitute for dating from your clinician.
Pregnancy Conception Calculator: When Did I Conceive?
How to use
- Choose Due date, Last period, or Ultrasound.
- Enter the main date and your average cycle length (22–45 days) for due date or LMP modes.
- For ultrasound, enter the scan date, gestational age in weeks and extra days (0–6), and cycle length.
- Read the four ranges in the emerald result panel; they update as soon as the inputs are valid.
What math we use
LMP from due date: same reverse rule as our pregnancy due-date tool (280 days from LMP adjusted by cycle vs 28 days). Ovulation day = LMP + (cycleLength − 14). Conception bands: narrow window (±2 days) and wider window (−3 to +7 days) around ovulation; intercourse bands extend further backward (−5 / −8 days from ovulation) through the end of each conception window.
Guide
What this tool estimates
A pregnancy conception calculator estimates when fertilization may have occurred and a window of days when sex could have led to pregnancy, using your due date, the first day of your last menstrual period (LMP), or an ultrasound-based gestational age. It follows the same general idea as https://www.calculator.net/pregnancy-conception-calculator.html: separate “most probable” and wider “possible” ranges, plus intercourse windows that account for sperm surviving several days before ovulation.
Biologically, conception (implantation after fertilization) cannot be pinned to a single calendar day from dates alone. Tools like this use rules of thumb—especially linking LMP to ovulation and full-term length—to produce helpful ranges for curiosity or discussion with your care team.
How we calculate (simplified)
We estimate ovulation as the first day of LMP plus (your average cycle length minus 14 days), assuming a roughly 14-day luteal phase. From a due date, we infer LMP using the same 280-day gestation rule used elsewhere on this site (with your cycle length), then apply the same ovulation estimate.
From an ultrasound date and gestational age (weeks + days, LMP-based), we back-calculate LMP and then ovulation. “Most probable” conception is a short band around that ovulation estimate; “possible” is wider. Intercourse ranges extend backward to reflect sperm viability (about 3–5 days in common educational models).
Notes
Irregular cycles
Ovulation timing varies; ultrasound dating in early pregnancy is often more reliable than recall of LMP alone.
Medical dating
Your clinic may use CRL or other criteria; their estimated due date can differ slightly from any online tool.
Limitations
- Educational only; not for diagnosing pregnancy or paternity.
- Does not model IVF transfer dates, contraception failures, or bleeding mistaken for a period.
FAQ
Why two intercourse ranges?
Sperm can survive several days; intercourse before ovulation can still lead to pregnancy, so the window starts earlier than the conception window.
Is the “most probable” conception day exact?
No—it is a short band around an estimated ovulation day. Only your clinician can refine dating.