How do I calculate my conception date from my due date?

Subtract 266 days (38 weeks) from your estimated due date to get the approximate conception date. For example: Due date September 1 minus 266 days = approximately December 9 of the prior year as the estimated conception date. This works because pregnancy is 40 weeks from LMP but only 38 weeks from conception.

How is conception date estimated from my last period (LMP)?

From LMP: conception date = LMP + (cycle length minus 14). For a 28-day cycle: LMP + 14 days. For a 32-day cycle: LMP + 18 days. This assumes ovulation occurs approximately 14 days before the next period, which reflects the typical fixed 14-day luteal phase.

How accurate is the conception date calculation?

The conception date is an estimate within a ±5–7 day window. Calendar-based calculation has this inherent uncertainty due to cycle variation and the biological conception window (sperm survival up to 5 days; egg viability 12–24 hours). IVF pregnancies with known transfer dates have the most precise conception dates.

What is the difference between gestational age and embryonic age?

Gestational age counts from LMP (the standard in medical records — approximately 40 weeks at birth). Embryonic age counts from fertilization/conception (approximately 38 weeks at birth). The difference is about 2 weeks. Most pregnancy milestones and due date calculations use gestational age.

How does IVF change conception date calculation?

In IVF, the exact fertilization date is documented. From a day-3 transfer: the embryo is already 3 days old, so conception (fertilization) occurred 3 days before transfer. From a day-5 (blastocyst) transfer: fertilization occurred 5 days before transfer. IVF conception dates are the most accurate — no calendar estimation is needed.

How do I calculate conception date for a legal or paternity case?

Provide the estimated conception date range (the ±5–7 day window around predicted ovulation) rather than a single date. The fertile window spans the 5 days before ovulation through ovulation day. If multiple potential fathers were present during this window, DNA paternity testing is required for definitive determination.

Why is first trimester ultrasound more accurate for dating?

First trimester crown-rump length (CRL) measurement between 7–13 weeks has a dating accuracy of ±5–7 days — more accurate than LMP-based dating. Second trimester ultrasound is accurate to ±2 weeks. Third trimester ultrasound has ±3–4 week uncertainty due to growth variation. ACOG recommends using first trimester ultrasound as the primary dating reference when LMP is uncertain.

Can I use the conception date to figure out who the father is?

No calculator can determine paternity. A conception date window identifies the 5–7 day biological window during which fertilization occurred. If intercourse with multiple partners occurred within this window, paternity cannot be determined by dates alone. DNA paternity testing (>99.9% accuracy) is the only definitive method.

I conceived through IVF — how do I calculate my due date from the transfer date?

Day-3 embryo transfer: add 263 days (37 weeks + 6 days) to the transfer date. Day-5 blastocyst transfer: add 261 days (37 weeks + 4 days) to the transfer date. Alternatively, assign a 'virtual LMP' = transfer date minus 17 days (day-3) or minus 14 days (day-5), then add 40 weeks to the virtual LMP.

My LMP-based conception date and ultrasound dating disagree — which is right?

First trimester ultrasound is more reliable than LMP-based calculation, especially when cycles are irregular or LMP dates are uncertain. ACOG recommends using ultrasound dating when it differs from LMP by more than 7 days in the first trimester (14 days in the second trimester). Your provider's official dating should take precedence over calculator estimates.

Does knowing the conception date tell me my baby's sex earlier?

No. The conception date does not provide information about fetal sex. Sex is determined at fertilization by chromosomal combination (XX or XY). Sex can be identified by: cell-free DNA (cfDNA/NIPT) testing as early as 10 weeks; chorionic villus sampling (CVS) at 10–13 weeks; amniocentesis at 15–20 weeks; anatomy ultrasound at 18–22 weeks.

What if I was on birth control when I conceived?

Hormonal contraceptive failures (pill, patch, ring) can make LMP-based conception date less reliable because ovulation timing may have been suppressed or irregular. If you were on hormonal contraception when you conceived, first trimester ultrasound is especially important for accurate dating. LMP in these cases may not reflect a natural cycle.

Online Medical Tools — Conception Calculator

Printed on 8/14/2026

For informational purposes only. This is not medical advice.

Obstetrics

Conception Calculator

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) or an estimated due date (from Pregnancy Due Date Calculator). It provides a likely conception date, a short conception window, estimated ovulation timing (compare with Ovulation Calculator), and estimated due date alignment for planning and timeline review.

Formula: Conception estimate ≈ EDD - 266 days, or LMP + cycle length - 14 days

Calculation Method

First Day of Last Period

Average Cycle Length

days

Save your results with a free account

Keep a history of calculations, favorite tools, and access your dashboard anytime.

Create free account Sign in

How It Works

Enter either your LMP or confirmed due date

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.

Calculate 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.

Interpret the conception window

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.

Who Uses the Conception Calculator

Paternity Testing Date of Conception Range

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.

Obstetric Dating When LMP is Uncertain

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.

IVF Transfer Date Conception Calculation

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.

Personal Pregnancy History Documentation

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.

Criminal Investigation Timeline of Conception

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.

Pro Tips

Conception date is an estimate within a 5–7 day window

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.

Ultrasound dating is more accurate than LMP calculation

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.

The 40-week pregnancy count includes 2 pre-conception weeks

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.

IVF provides the most precise conception date

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.

First trimester ultrasound is the gold standard for gestational dating

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.

Cycle irregularities widen the conception window

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.

Conception outside the fertile window is biologically implausible

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.

DNA paternity testing is definitive regardless of conception date

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.

Common Questions About Your Results

Evidence-Based Methodology

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.

Clinical Content Trust

Last reviewed:: April 21, 2026
Guideline version:: General evidence framework v2026.04
Source set version:: Primary-source set v1

How to Interpret Your Result

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).

When to Use This Tool

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.

Limitations

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.

Changelog

April 21, 2026 · trust-baseline
Clinical trust metadata enabled for this tool page with structured review/version fields.

Related Tools