Article — Reverse Due Date Calculator
Reverse due date calculator
A reverse due date calculator works backwards from an estimated due date (EDD) to find the probable conception date, the last menstrual period (LMP), and the fertile window. Standard obstetric dating uses Naegele's rule: EDD = LMP + 280 days, with conception about 266 days before EDD. Run that subtraction and you have a useful pregnancy timeline.
People reach for a reverse calculator for three reasons: curiosity about their own conception date, working out when a child was conceived from a known due date, or planning a pregnancy toward a target birth month. The math is the same for all three.
What the reverse due date calculator does
The calculator inverts standard obstetric dating. You enter the EDD and the calculator returns five anchor points: the LMP, the ovulation/conception date, the fertile window, the implantation window, and the date a urine pregnancy test would first read positive. Each anchor is a date — and each has its own uncertainty.
Forward calculators move from LMP to EDD because LMP is the date most people can identify. Reverse calculators target the case where the EDD is already known (from an ultrasound or a prenatal record) and the user wants the upstream timeline. The arithmetic is symmetric; the uncertainty is not, because EDD itself was estimated with some error.
Only about 4-5% of babies arrive on their estimated due date. The standard obstetric assumption is that any delivery between 37 and 42 weeks of gestation is term — a five-week window. The EDD is the center of that window, not a prediction.
The math behind the reverse due date
Naegele's rule, named for the 19th-century German obstetrician Franz Karl Naegele, says estimated due date equals the first day of the last menstrual period plus 280 days. That equals 40 weeks, which is the convention used worldwide for gestational age. Ovulation typically falls 14 days before the next period, so in a textbook 28-day cycle, conception is 14 days after LMP — or 266 days before EDD.
- EDD - 280 days = estimated LMP
- EDD - 266 days = estimated conception (28-day cycle)
- LMP + (cycle - 14) = ovulation day for non-28-day cycles
- Conception - 5 to +1 days = fertile window (sperm 5 days, egg 24 h)
- Conception + 6-12 days = implantation window
- Implantation + 5-8 days = first reliably positive urine HCG test
How to use this reverse due date calculator
Type the estimated due date into the date input. If your cycle is not 28 days, adjust the cycle length — the calculator shifts the ovulation offset to match. The result panel shows the conception date in bold, the fertile window as a range, and a timeline with LMP, ovulation, implantation, first missed period, and EDD aligned on a single chart.
A worked example: EDD of August 15, 2026, 28-day cycle. LMP estimate: November 8, 2025. Conception estimate: November 22, 2025. Fertile window: November 17 through November 23, 2025. First missed period: around December 6, 2025. The calculator does this in a single subtraction once the date is in.
EDD - 280d LMPEDD - 266d conceptionLMP + 14d ovulation (28-day)LMP + 28d missed periodConception date vs last menstrual period
The most common source of confusion in pregnancy dating is the gap between conception and LMP. Medical care counts gestational age from the first day of the last menstrual period, not from conception. So a "4-week pregnancy" is only about 2 weeks past actual conception. The convention exists because LMP is remembered far more often than ovulation date.
This creates a quirk: when a positive home pregnancy test arrives, the embryo is roughly 2 weeks old, but the chart reads 4 weeks of gestation. By the first prenatal visit at 8 weeks gestation, the embryo is 6 weeks old. The reverse calculator preserves the standard convention while showing both dates clearly.
The fertile window around conception
Conception does not happen at a single instant. Sperm cells deposited in cervical mucus can survive up to 5 days in healthy mucus; an egg is fertilizable for only about 24 hours after ovulation. So intercourse anywhere from 5 days before ovulation to 1 day after can produce a pregnancy. That 6-day window is what reproductive endocrinologists call the fertile window.
For curiosity-driven calculations, the fertile window is what most people actually want. Single-date conception estimates feel exact but oversell the precision. The 6-day window is honest about the biology.
How accurate is a reverse due date
Accuracy depends on where the EDD came from. A first-trimester ultrasound dates pregnancy to within 3-5 days. Reversing from that EDD gives a fertile window of about a week — usable for legal or curiosity purposes but not for paternity questions. A second-trimester ultrasound is accurate to 7-10 days, and third-trimester ultrasound to 14-21 days. An EDD calculated only from LMP in a cycle of unknown regularity can be off by 2 weeks.
Reverse due date calculations have a built-in 5-10 day error window. They are not appropriate for paternity disputes, legal disputes, or any decision requiring date certainty. DNA testing is the only reliable answer for paternity.
When to use the reverse due date
Common reasons people use the reverse calculator: working out their own conception date from a birthday, planning a pregnancy toward a school-year cutoff or summer birthday, understanding when a partner conceived if the timeline is unclear, or checking the consistency of a clinical due date against a remembered LMP. None of these need clinical-grade precision — a window of about a week is enough.
For clinical use, the American College of Obstetricians and Gynecologists recommends using a first-trimester ultrasound to set the EDD when possible. From that EDD, the reverse calculation is as accurate as the forward one. The calculator on this page mirrors ACOG dating conventions and adjusts for cycle length, which removes a common source of error in shorter or longer menstrual cycles.
For pregnancies conceived through assisted reproduction the math is different — and far more precise. In an IVF cycle the date of egg retrieval and embryo transfer is recorded to the hour. EDD is calculated as embryo transfer date plus 263 days for a day-3 transfer or 261 days for a day-5 transfer. Running a reverse calculation on an IVF EDD therefore gives a conception date that is almost exact, because the underlying timeline never depended on cycle estimation.
One more subtlety worth noting: about 1 in 200 pregnancies ends up with a corrected EDD after the dating ultrasound, when the ultrasound measurement differs from the LMP-based estimate by more than 7 days in the first trimester or more than 10-14 days in the second trimester. ACOG advises using the ultrasound EDD in those cases. A reverse calculator anchored to the corrected EDD will give the better timeline.