What is the 6-day conception window?

The conception window is the range of dates during which unprotected intercourse could have produced a pregnancy. It spans 3 days before the central conception estimate to 3 days after. This reflects sperm survival of 3 to 5 days inside the female reproductive tract and an egg viability window of 12 to 24 hours after ovulation. Any intercourse within this window could theoretically result in the pregnancy.

What if I had an irregular cycle or do not know my LMP?

If your EDD was established by early ultrasound (8 to 12 weeks), it is already more reliable than LMP-based dating regardless of cycle irregularity. Enter that EDD with a 28-day cycle length setting. The resulting conception date and window will be accurate to the ultrasound margin of plus or minus 5 days. Avoid using LMP-based dating if your cycles vary by more than 7 days between months.

Can the reverse due date calculator determine paternity?

No. The calculator shows an estimated 6-day conception window, and paternity requires knowing which specific day conception occurred and comparing that to specific dates of intercourse with different partners. Because sperm can survive 3 to 5 days, the window overlaps multiple possible dates. Paternity can only be confirmed through DNA testing, not through conception date estimation.

How far off can an EDD-based conception estimate be if the EDD was set by a second-trimester scan?

Second-trimester ultrasounds (14 to 26 weeks) are accurate to plus or minus 10 to 14 days, compared to plus or minus 5 days for first-trimester scans. This means a conception date derived from a second-trimester EDD can be off by up to 2 weeks in either direction. The 6-day window shown by this calculator does not account for this additional uncertainty; add 10 to 14 days of buffer when the EDD came from a late scan.

What does it mean if the birth date was preterm or post-term?

A preterm birth (before 37 weeks gestational age) means the baby arrived earlier than 266 days after conception. Using the birth date in this calculator will produce a conception date that is earlier than reality. For a baby born at 34 weeks, the actual conception was about 238 days before birth (34 times 7), not 266 days. Similarly, a post-term birth at 42 weeks means conception was about 280 days before birth. Adjust the formula accordingly, or use the due date mode instead if you know the EDD.

Reverse Due Date Calculator

Q: How do I find my LMP if I know my due date?

Subtract 280 days from your EDD to get the LMP for a standard 28-day cycle. If your cycle is longer or shorter than 28 days, subtract an additional number of days equal to your cycle length minus 28. For example, a 32-day cycle means LMP = EDD minus 284 days. This calculator does the arithmetic instantly when you enter your EDD and cycle length.

Q: Can I find the conception date from the birth date?

Yes. Subtract 266 days from the actual birth date to estimate the conception date, or subtract 280 days to estimate the LMP. This assumes a full-term 38-week pregnancy from conception, so preterm births will produce an earlier-than-actual conception estimate. Use the From Birth Date mode in this calculator to get instant results.

Q: How accurate is the estimated conception date?

The conception date estimate carries a margin of roughly plus or minus 3 to 5 days for regular cycles and plus or minus 1 to 2 weeks for irregular cycles or ultrasound-dated pregnancies. The calculator shows a 6-day window (3 days either side of the central estimate) to account for sperm survival time and egg viability. Only DNA testing can confirm conception timing with certainty.

Q: Why does my cycle length affect the LMP but not the conception date?

Ovulation (and conception) always occurs approximately 14 days before the next expected period, which is at the end of the luteal phase. The luteal phase is fixed at 14 days in most women; only the follicular phase (LMP to ovulation) varies with cycle length. A 32-day cycle means ovulation on day 18 rather than day 14, so LMP must be 4 days earlier to produce the same EDD. The conception date, however, is anchored to the EDD at a fixed 266 days.

Enter your estimated due date or birth date and instantly find when your last period started and when conception most likely occurred.

🔄 What is a Reverse Due Date Calculator?

A reverse due date calculator works backwards from a known reference point (the estimated due date or the actual birth date) to determine when the last menstrual period (LMP) most likely started and when conception most likely occurred. This is the opposite of a standard due date calculator, which takes the LMP as input and projects forward to the EDD. The reverse tool is useful when the end-point of pregnancy is already known and the starting dates need to be reconstructed.

The most common use cases are: (1) a pregnant person who received an EDD from an ultrasound and wants to know what their LMP was, particularly if they had irregular or absent periods at the time of conception; (2) a parent who wants to know when their child was conceived, now that the birth date is known; (3) someone trying to determine a conception window for legal, medical, or personal reasons; and (4) anyone who wants to understand the full pregnancy timeline by filling in dates they do not have on hand.

The calculation uses two core obstetric relationships: pregnancy from LMP lasts 280 days (40 weeks) for a standard 28-day cycle, and pregnancy from conception lasts a fixed 266 days (38 weeks). The cycle-length adjustment only changes the LMP estimate, not the conception date, because the time from conception to delivery is constant regardless of how long the follicular phase was. This means the conception date is always EDD minus 266 days, while the LMP is EDD minus 280 days adjusted for cycle length.

A common misconception is that knowing the conception date can definitively establish paternity. In reality, this calculator provides an estimated 6-day window during which conception could have occurred, based on sperm viability (3 to 5 days) and egg viability (12 to 24 hours). Paternity requires DNA testing, not date estimation. Another misconception is that the LMP date is when pregnancy begins; clinically, pregnancy is counted from LMP even though conception occurs roughly two weeks later.

📐 Formula

LMP = EDD − (280 + (Cycle − 28)) days

LMP = estimated first day of last menstrual period

EDD = estimated due date (the known input)

280 = standard gestation length from LMP in days (40 weeks, 28-day cycle)

Cycle = your actual menstrual cycle length in days

Cycle adjustment = adds (Cycle − 28) to LMP subtraction for non-28-day cycles

Conception date: Conception = EDD − 266 days (always, regardless of cycle length)

Conception window: Conception − 3 days to Conception + 3 days (6-day range)

From birth date: Conception = Birth − 266 days; LMP = Birth − 280 days

Example (28-day cycle): EDD = November 8, 2026. LMP = November 8 − 280 = February 1, 2026. Conception = November 8 − 266 = February 15, 2026.

Example (32-day cycle): EDD = January 17, 2027. LMP = January 17 − 284 = April 8, 2026. Conception = January 17 − 266 = April 26, 2026.

The 266-day conception-to-delivery constant comes from Naegele's rule combined with standard ovulation timing. Naegele's rule adds 280 days to the LMP. For a 28-day cycle, ovulation occurs on day 14, so conception is LMP plus 14, and delivery is LMP plus 280, giving 280 minus 14 = 266 days from conception. For any other cycle length, ovulation shifts (cycle minus 14 days after LMP), but the luteal phase and gestation from conception remain the same, so EDD minus conception always equals 266 days.

📖 How to Use This Calculator

Steps

Choose your calculation mode - Select From Due Date if you know your EDD (from a doctor visit, ultrasound report, or standard due date calculator), or From Birth Date if the baby has been born and you know the actual birth date.

Enter your date - Type or pick your EDD or birth date. For EDD mode, enter the exact date your healthcare provider gave you or that you calculated from your LMP.

Adjust cycle length if needed - In From Due Date mode, drag the Menstrual Cycle Length slider if your cycle is not 28 days. Each day of difference from 28 shifts the estimated LMP by one day. The conception date does not change.

Click Calculate to see your results - The calculator instantly shows your estimated LMP, estimated conception date, and a 6-day conception window. Use the share buttons to copy or send your results.

💡 Example Calculations

Example 1 - Standard 28-Day Cycle from EDD

EDD November 8, 2026, cycle 28 days

Cycle adjustment = 28 - 28 = 0 days. LMP = November 8 minus 280 days = February 1, 2026.

Conception = November 8 minus 266 days = February 15, 2026. Ovulation occurred approximately 14 days after LMP, which is February 1 plus 14 = February 15. This confirms both methods agree.

Conception window = February 12 to February 18, 2026 (3 days on each side). Any unprotected intercourse during this 6-day window could have resulted in this pregnancy.

LMP = February 1, 2026 | Conception = February 15, 2026

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Example 2 - Longer Cycle (32 Days) from EDD

EDD January 17, 2027, cycle 32 days

Cycle adjustment = 32 - 28 = +4 days. LMP = January 17 minus 284 days = April 8, 2026. Without the adjustment, a 28-day assumption would give April 12, 2026, which would be 4 days too late.

Conception = January 17 minus 266 = April 26, 2026. Ovulation in a 32-day cycle occurs on day 18 (32 minus 14 = 18). April 8 plus 18 = April 26. Both methods agree.

Conception window = April 23 to April 29, 2026. Note that the LMP shifted by 4 days compared to a 28-day cycle, but the conception date and window are the same as what a standard conception calculator would show.

LMP = April 8, 2026 | Conception = April 26, 2026

Try this example →

Example 3 - From Birth Date

Baby born June 5, 2026

Conception = June 5 minus 266 days = September 12, 2025. This assumes a full-term 38-week pregnancy from conception. If the birth was preterm at, say, 34 weeks, the actual conception was closer to 34 times 7 = 238 days before birth, or about September 30, 2025.

LMP = June 5 minus 280 days = August 29, 2025 (assuming 28-day cycle). The LMP would be 14 days before conception: September 12 minus 14 = August 29. Both methods agree for a 28-day cycle.

Conception window = September 9 to September 15, 2025. Any intercourse during this window with a viable sperm count could have resulted in conception that led to a June 5, 2026 birth.

Conception = September 12, 2025 | LMP = August 29, 2025

Try this example →

❓ Frequently Asked Questions

How do I find my LMP if I know my due date?+

Subtract 280 days from your EDD for a 28-day cycle. If your cycle is longer, subtract additional days equal to (your cycle length minus 28). For example, a 30-day cycle means subtract 282 days from the EDD. This calculator handles the arithmetic instantly when you enter your EDD and cycle length.

How do I find my conception date from my due date?+

Subtract 266 days from your EDD. This is always the formula regardless of cycle length, because gestation from conception to delivery is a fixed 266 days (38 weeks). A 32-day cycle shifts the LMP estimate but not the conception date. Enter your EDD in From Due Date mode and the calculator computes this instantly.

Can I find the conception date from the birth date?+

Yes. Subtract 266 days from the birth date to estimate conception, or subtract 280 days to estimate the LMP. This assumes a full-term 38-week pregnancy from conception. For a preterm birth (before 37 weeks), the actual conception date will be later than the formula suggests. Use the From Birth Date mode to get instant results, then adjust manually for gestational age at birth if needed.

How accurate is the estimated conception date from an EDD?+

The accuracy depends on how the EDD was set. An EDD from an early ultrasound (8 to 12 weeks) carries a margin of plus or minus 5 days, so the conception date estimate is accurate to within about 5 days. An EDD from LMP alone carries a margin of 1 to 2 weeks for regular cycles, wider for irregular ones. The 6-day window this calculator shows does not fully cover LMP-based uncertainty; add 1 to 2 weeks of buffer when the EDD came from LMP alone.

Why does cycle length affect the LMP but not the conception date?+

The luteal phase (ovulation to next period) is a fixed 14 days in most women. Only the follicular phase (LMP to ovulation) varies with cycle length. Because conception happens at ovulation, and ovulation is always 14 days before delivery minus 266 days, the EDD-to-conception gap is always 266 days. The LMP is simply conception minus the follicular phase length, which equals cycle length minus 14.

What does the conception window represent?+

The conception window is a 6-day range (3 days before the central estimate to 3 days after) representing all dates during which intercourse could have resulted in the pregnancy. Sperm can survive 3 to 5 days inside the reproductive tract, and an egg is viable for 12 to 24 hours after ovulation. The combination means unprotected intercourse anywhere in this window has a biological chance of causing conception.

Can the reverse due date calculator help determine paternity?+

It can provide a conception window that can be compared to known dates of intercourse, but it cannot confirm paternity. Because the window spans 6 days and sperm viability can extend even beyond that, the window often overlaps dates with multiple potential partners. Only DNA paternity testing provides a definitive answer. This calculator is informational only and should not be used for legal paternity disputes.

What if my EDD was changed by a mid-pregnancy ultrasound?+

Use the most recent clinically accepted EDD. If a second-trimester scan (14 to 26 weeks) shifted the EDD by more than 10 to 14 days from the LMP estimate, that scan-based date is likely more accurate. However, second-trimester scans carry a larger margin of error (plus or minus 10 to 14 days) than first-trimester scans (plus or minus 5 days), so the conception window produced by a late scan revision will be wider than this calculator shows.

How does a preterm or post-term birth affect the from-birth-date calculation?+

The From Birth Date mode assumes a full-term 38-week (266-day) gestation from conception. A preterm birth at 34 weeks means actual gestation from conception was 34 times 7 = 238 days, not 266. To correct for this, subtract the actual gestational age in days (weeks times 7) from the birth date to find conception, rather than subtracting 266. Similarly, a post-term birth at 42 weeks (294 days from LMP) means conception was 280 days before birth.

Is this the same as the conception calculator?+

They complement each other but serve different directions. The Conception Calculator takes LMP as input and works forward to show the fertile window and EDD. The Reverse Due Date Calculator takes the EDD or birth date as input and works backwards to show the LMP and conception date. Both use the same underlying math; they differ in which date is the known starting point.

Can I use this calculator if I had an IVF pregnancy?+

Yes, with a caveat. For IVF pregnancies, the EDD is set by adding a fixed number of days to the egg retrieval or transfer date (for example, 266 days for Day-3 transfer or 263 days for Day-5 blastocyst transfer). The EDD is precise. Entering it in From Due Date mode will give you the equivalent LMP and conception date, though for IVF the LMP is a calculated reference rather than an actual period date. The actual fertilization date is known from the retrieval date, so use that directly rather than relying on the conception estimate.

🔗 Related Calculators

📌 Quick Tips

💡The estimated conception date is always EDD minus 266 days, regardless of your cycle length. Only the LMP estimate shifts when you change the cycle length setting.

💡The 6-day conception window reflects biology: sperm can survive 3 to 5 days inside the reproductive tract, and a released egg is viable for 12 to 24 hours. Any unprotected intercourse within this window could result in conception.

💡If your EDD was set by an early ultrasound (before 12 weeks), the conception date estimate will be more accurate than if it was set from LMP alone, because ultrasound dating directly measures fetal size rather than assuming cycle regularity.

💡Use the From Birth Date mode when the pregnancy is complete. Enter the baby's actual birth date to estimate conception, keeping in mind that most births occur between 37 and 42 weeks, so the conception estimate carries a plus or minus 2 to 3 week margin.

💡If your cycle is not exactly 28 days, adjust the cycle length slider. Every day your cycle differs from 28 shifts the LMP estimate by one day. The conception date stays the same.