Reverse Due Date Calculator
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
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
💡 Example Calculations
Example 1 - Standard 28-Day Cycle from EDD
EDD November 8, 2026, cycle 28 days
Example 2 - Longer Cycle (32 Days) from EDD
EDD January 17, 2027, cycle 32 days
Example 3 - From Birth Date
Baby born June 5, 2026
❓ Frequently Asked Questions
🔗 Related Calculators
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.
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 conception to delivery is a fixed 266 days (38 weeks), and the cycle-length adjustment only shifts the LMP, not the conception event. Example: EDD November 8, 2026 minus 266 days gives a conception date of February 15, 2026.
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.
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.
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.
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.
Is the reverse due date calculator the same as the conception calculator?
They overlap but differ in focus. The Conception Calculator works forward from LMP or EDD to show the fertile window and gestational milestones. This reverse due date calculator works backwards from EDD or birth date to reconstruct the LMP and conception date, which is useful when the EDD or birth date is the known starting point rather than the period date.