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Printed on 6/29/2026
For informational purposes only. This is not medical advice.
This conception date calculator estimates when conception most likely occurred using either cycle-based timing (last menstrual period plus average cycle length via [Period Calculator](/tools/period-calculator)) or an estimated due date (from [Pregnancy Due Date Calculator](/tools/pregnancy-due-date)). It provides a likely conception date, a short conception window, estimated ovulation timing (compare with [Ovulation Calculator](/tools/ovulation-calculator)), and estimated due date alignment for planning and timeline review.
Formula: Conception estimate ≈ EDD - 266 days, or LMP + cycle length - 14 days
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Choose your calculation method: enter the first day of your last menstrual period (with cycle length) for forward calculation, or enter a confirmed due date to work backward to the estimated conception date.
From LMP: conception = LMP + (cycle length minus 14), assuming a fixed 14-day luteal phase. From due date: conception = due date minus 266 days (38 weeks of embryonic age). Note: gestational age from LMP is 40 weeks because LMP is counted 2 weeks before conception.
The result is an estimated date within a 5–7 day conception window — the biological range during which fertilization could have occurred around predicted ovulation. First trimester ultrasound provides the most accurate gestational dating when exact dates are uncertain.
Legal and medical professionals
Establish the estimated conception date window for paternity assessment timelines. Note that conception windows span 5–7 days around predicted ovulation, and DNA testing remains the definitive paternity determination method.
Obstetricians
Estimate conception timing when the LMP is uncertain or irregular cycles make LMP-based dating unreliable. Used alongside first-trimester ultrasound for the most accurate gestational dating.
Reproductive endocrinologists
In IVF pregnancies, calculate exact conception date from the embryo transfer date. Day-3 transfers add 17 days; day-5 (blastocyst) transfers add 14 days to reach conception date. IVF conception dates are the most precise available.
Patients
Review your own pregnancy timeline, understand when conception likely occurred, and correlate with personal memories or events. Useful for pregnancy journals and medical history documentation.
Forensic medicine
Provide estimated conception date ranges in forensic medical contexts. The biological conception window (sperm survival up to 5 days, egg viability 12–24 hours) must be considered when establishing date ranges.
Conception date is never a single exact day — it is an estimated date within a biological window of 5–7 days around calculated ovulation. Sperm can survive up to 5 days; the egg survives 12–24 hours. Even in women who track ovulation precisely, the exact fertilization moment is unknown.
First trimester crown-rump length (CRL) ultrasound between 7–13 weeks has a dating error of only ±5–7 days — more accurate than LMP-based calculation. If ultrasound dating differs from LMP by more than 7 days in the first trimester, ACOG recommends using the ultrasound date.
Standard gestational age counts 40 weeks from LMP — but the first 2 weeks include the period and follicular phase, before conception occurs. Embryonic age (from conception) is 38 weeks (266 days). This is why subtracting 266 days from due date gives conception date, not 280.
In IVF, the exact fertilization date is documented. From a day-3 embryo transfer: add 17 days to get due date (or subtract from due date). From a day-5 blastocyst transfer: add 14 days. IVF conception dates are the most accurate available — no calendar estimation needed.
When LMP is uncertain, irregular, or the patient was on hormonal contraception before conception, first trimester CRL ultrasound is the gold standard. ACOG Practice Bulletin 700 (2020) endorses ultrasound as the primary dating method when LMP is uncertain.
For patients with irregular cycles, the conception date window should be widened accordingly. A woman with cycles varying by 10 days has a conception window that could span 2–3 weeks. Calculator estimates for irregular cycles should be interpreted as approximate ranges.
Sperm survive up to 5 days in fertile cervical mucus, and the egg survives only 12–24 hours after ovulation. Conception outside the 6-day fertile window ending on ovulation day is biologically extremely unlikely. This is relevant in legal timeline analysis.
Conception date calculators provide biological timeline estimates, not paternity evidence. DNA paternity testing is the definitive and legally recognized method for paternity determination, regardless of any date calculations.
Gestational age from LMP (Naegele's rule) established in 1812 by Franz Naegele. Ultrasound vs LMP dating accuracy: Whitworth et al. (Cochrane 2015) — ultrasound reduces post-dates induction. First trimester CRL dating accuracy: Hadlock et al. (Radiology 1992) — ±5–7 days. Fertile window conception timing: Wilcox et al. (NEJM 1995). ACOG Practice Bulletin 700 (2020) on gestational age assessment endorses ultrasound as gold standard when LMP uncertain.
The likely conception date is an estimate based on reproductive timing assumptions, not a direct measurement. The conception window reflects the fact that fertilization may occur around, rather than exactly at, predicted ovulation.
If you used due-date mode, the estimate is back-calculated from a typical 266-day fetal interval. If you used cycle-based mode, the estimate depends on your entered cycle length and assumes ovulation occurs about 14 days before the next period. For pregnancy timeline context, compare with the [Pregnancy Due Date Calculator](/tools/pregnancy-due-date) and [Gestational Age Calculator](/tools/gestational-age).
Use this tool when reviewing pregnancy timelines, planning prenatal milestones, or understanding likely conception timing from known dates. It can be helpful for personal records, counseling conversations, and general pregnancy date orientation.
For people actively trying to conceive, the [Ovulation Calculator](/tools/ovulation-calculator) and [Period Calculator](/tools/period-calculator) are better for forward-looking fertility timing, while this tool is best for retrospective estimation.
Conception-date estimation is sensitive to cycle variability and uncertain dates. Irregular cycles, recent hormonal contraception changes, postpartum cycles, and ovulatory disorders (track cycles with [Period Calculator](/tools/period-calculator)) can reduce accuracy. Even in regular cycles, ovulation timing can shift by several days. Optimize pre-conception health with [BMI Calculator](/tools/bmi-calculator) and [ASCVD Risk Calculator](/tools/ascvd-risk).
This tool does not replace clinical dating methods. First-trimester ultrasound remains the most reliable method for establishing pregnancy dating when LMP or cycle data are uncertain.
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
April 21, 2026 · trust-baseline
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