To estimate when you got pregnant, subtract 266 days from your due date — or add 14 days to the first day of your last menstrual period (LMP). Both methods give you the approximate conception date, which typically falls within a 5–10 day window rather than a single exact day.
The calculation matters because conception does not happen the moment sperm and egg meet — it happens when fertilization is successful, which depends on ovulation timing, sperm survival, and egg viability. According to the American College of Obstetricians and Gynecologists (ACOG), ovulation typically occurs 14 days before the next expected period in a standard 28-day cycle. That two-week gap is the foundation of every conception date estimate.
Use the free Conception Date Calculator at RoughTools to estimate your conception window instantly — or follow the step-by-step method below.
The Conception Date Formula
There are two ways to estimate your conception date depending on what information you have: your due date, or your LMP date.
Method 1 — From due date:
Conception date ≈ Due date − 266 days
Method 2 — From LMP:
Conception date ≈ LMP date + 14 days
Conception window (full fertile range):
Earliest possible: LMP + 11 days
Latest possible: LMP + 21 days
Where:
- Due date — your estimated delivery date at 40 weeks gestation
- 266 days — the average time from fertilization to delivery (280 days total minus 14 days from LMP to ovulation)
- LMP date — the first day of your last menstrual period
- 14 days — the average time from LMP to ovulation in a 28-day cycle
- Conception window — the range of days during which fertilization could realistically have occurred
Worked example: due date of December 20, 2026
Method 1 — Subtract 266 days from due date:
December 20, 2026 − 266 days = March 29, 2026
Estimated conception date: March 29, 2026
Method 2 — Verify using LMP (March 15, 2026):
LMP: March 15, 2026
+ 14 days = March 29, 2026 ✓
Conception window:
Earliest: March 15 + 11 = March 26, 2026
Latest: March 15 + 21 = April 5, 2026
Conception window: March 26 – April 5, 2026
The result: conception most likely occurred around March 29, 2026, with a realistic window of March 26 – April 5. Both methods agree on the central estimate, which increases confidence in the result. Any intercourse that occurred during this 11-day window could have led to this pregnancy.
In practice, this window is wider than most people expect. Sperm can survive in the female reproductive tract for up to 5 days, and the egg is viable for 12–24 hours after ovulation — which means conception can result from intercourse that occurred several days before ovulation, not just on the day itself.
How to Calculate Your Conception Date Step by Step
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Gather your key date. You need either your estimated due date (from your doctor or a dating ultrasound) or the first day of your last menstrual period. If you have both, use both and compare — they should produce estimates within a few days of each other. If they differ by more than 5 days, the ultrasound-based due date is more reliable.
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Choose your calculation method. If you know your due date, subtract 266 days. If you know your LMP, add 14 days. Both give you the estimated ovulation and fertilization date. If your cycles are irregular or longer than 28 days, the LMP method is less reliable — the due date method is more accurate in that case.
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Calculate the central conception date. Using the due date method: count back 266 days from your due date. For December 20, 2026: subtract 9 months and add 7 days is a quick approximation (December 20 → March 20 → March 27, close to March 29). For precision, use exact day counting or the conception date calculator.
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Calculate your full conception window. Add 11 days to your LMP for the earliest possible date and 21 days for the latest. This 11-day range accounts for sperm survival (up to 5 days before ovulation) and the 24-hour window after ovulation when the egg is viable. For LMP March 15: window runs March 26 – April 5.
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Cross-reference with your calendar. Look back at the conception window dates and consider what you know about your cycle. If you track ovulation (via temperature, LH strips, or an app), the date of your LH surge typically precedes ovulation by 24–36 hours, which narrows the window considerably.
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Verify the estimate is consistent with your pregnancy timeline. If your first ultrasound showed a gestational age of 8 weeks and 3 days, work backward: 8 weeks 3 days before the ultrasound date should align with your LMP. If the numbers are consistent, your conception date estimate is reliable. If there is a discrepancy of more than 7 days, use the ultrasound-derived due date rather than LMP to anchor your calculation.
Pro tip: Ovulation predictor kits (OPKs) that detect the LH surge are the most precise way to identify your actual ovulation date after the fact — if you used them during the cycle in question. A positive OPK result typically means ovulation occurred 24–36 hours later, which is your most probable conception date.
How to Calculate Conception Date from Due Date
To calculate conception date from a due date, subtract 266 days from the due date. This works because the standard pregnancy duration is 280 days (40 weeks) from LMP, and ovulation — when conception occurs — happens approximately 14 days after LMP. Subtracting those 14 days gives 266 days from conception to delivery.
Here is how the calculation works for three different due dates:
| Due date | Conception date estimate | Conception window | |---|---|---| | October 14, 2026 | January 21, 2026 | January 18 – January 28 | | December 20, 2026 | March 29, 2026 | March 26 – April 5 | | February 8, 2027 | May 17, 2026 | May 14 – May 24 |
The 266-day subtraction assumes a 28-day cycle. If your OB adjusted your due date based on an early ultrasound (which is more accurate than LMP), use that adjusted due date — it already accounts for any cycle irregularities.
One thing worth noting: conception date and implantation date are different. Fertilization of the egg typically occurs within 24 hours of ovulation. The fertilized egg then travels to the uterus and implants approximately 6–12 days later. A pregnancy test turns positive after implantation, when hCG begins rising. This means a pregnancy test can turn positive 6–12 days after your estimated conception date — not immediately.
Use the due date calculator if you want to work in the opposite direction — entering an LMP date to confirm a due date.
How Many Days After Ovulation Does Conception Occur?
Conception — the moment of fertilization — occurs within 12–24 hours of ovulation, not days later. The window is narrow because the egg is only viable for approximately 12–24 hours after it is released. If sperm are not already present in the fallopian tube when ovulation occurs, the window closes quickly.
This is why the fertile window extends before ovulation, not primarily after it. Sperm deposited up to 5 days before ovulation can survive in cervical mucus and the reproductive tract, waiting for the egg to arrive. Sperm deposited after ovulation has already occurred have a much lower probability of fertilization.
The practical breakdown of conception probability by day relative to ovulation:
| Day relative to ovulation | Approximate conception probability | |---|---| | 5 days before | ~5% | | 3 days before | ~15–20% | | 2 days before | ~25–30% | | 1 day before | ~25–30% | | Day of ovulation | ~15–20% | | 1 day after | ~5% or less |
Research published in the New England Journal of Medicine by Wilcox et al. found that 92% of pregnancies resulted from intercourse during the six-day window ending on the day of ovulation. The day before ovulation and two days before ovulation had the highest conception rates — not ovulation day itself.
This has a direct implication for conception date estimates: if your calculation points to March 29 as the ovulation date, the actual intercourse that resulted in conception most likely occurred on March 27 or 28.
What Is the Difference Between Conception Date and Implantation Date?
Conception date is when sperm fertilizes the egg — typically within 24 hours of ovulation. Implantation date is when the fertilized egg attaches to the uterine wall — typically 6–12 days after conception. They are separate events, and only implantation triggers the hormonal changes that make a pregnancy test positive.
The timeline looks like this:
- Day 0: Ovulation (egg released)
- Day 0–1: Fertilization — sperm meets egg in the fallopian tube
- Days 1–5: The fertilized egg (now called a blastocyst) travels down the fallopian tube toward the uterus
- Days 6–12: Implantation — the blastocyst burrows into the uterine lining and begins releasing hCG
- Days 10–14: hCG levels rise high enough for a home pregnancy test to detect
This timing explains why pregnancy tests taken immediately after suspected conception are negative — the body has not yet produced detectable hCG. Testing too early is the most common reason for a false negative. A test taken 12–14 days after ovulation (not after intercourse) gives the most reliable result.
If you are trying to pinpoint implantation — because of cramping, spotting, or test timing questions — the ovulation calculator can help you map your cycle and estimate both ovulation and the implantation window.
Common Mistakes to Avoid When Calculating Conception Date
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Assuming conception happened on the day of intercourse. Conception requires both sperm and a viable egg to be present simultaneously. Sperm can survive 3–5 days, so conception often results from intercourse 1–4 days before ovulation — not necessarily on the day it occurred. The conception date estimate points to ovulation, not to the specific day of intercourse.
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Using the wrong starting point for the LMP formula. The LMP must be the first day of bleeding — not the last day, not the heaviest flow day. Off-by-one errors on the LMP date cascade into off-by-one errors in every downstream calculation: gestational age, due date, and conception window.
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Treating the conception date as a single exact day. Every calculation method produces an estimate with inherent uncertainty. Even with a known LMP and confirmed due date, the conception date carries a ±5 day margin of error in most cases. Presenting a single date as definitive is overconfident and medically unsupported.
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Ignoring cycle length when using the LMP + 14 formula. The +14 days assumes a textbook 28-day cycle. If your cycles are consistently 32 days, ovulation occurs around Day 18, not Day 14 — meaning the conception estimate should be LMP + 18. Subtract 10 from your typical cycle length and add that number to LMP for a more personalized estimate.
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Confusing gestational age with time since conception. A pregnancy described as "8 weeks" is 8 weeks since LMP — approximately 6 weeks since fertilization. The two-week offset between gestational age and fetal age trips up many calculations. When someone asks "how far along are you?", the answer in weeks always references gestational age (from LMP), not time since conception.
Frequently Asked Questions
How accurate is a conception date estimate? A conception date calculated from a reliable LMP or ultrasound-confirmed due date is accurate to within approximately ±5 days for most women with regular cycles. For women with irregular cycles, the range widens to ±7–10 days. No formula can pinpoint the exact day of fertilization with certainty — the estimate identifies the most probable ovulation date, which is the closest proxy for when you got pregnant.
What if I had intercourse on multiple days around my fertile window? The calculation still gives you the same estimated conception date — it points to when ovulation most likely occurred, not which specific encounter resulted in pregnancy. If you had intercourse on March 25, 27, and 29, and ovulation was March 29, any of those three dates could have produced the pregnancy (sperm from March 25 and 27 may still have been viable on March 29). There is no way to determine which encounter specifically resulted in fertilization.
What is the difference between conception date and LMP date? The LMP date is the first day of your last period — typically 14 days before conception in a 28-day cycle. Conception date is when fertilization occurred, approximately two weeks after LMP. They are different events. Gestational age counts from LMP; fetal age counts from conception. A pregnancy at "6 weeks gestational age" is approximately 4 weeks since conception.
How many days after my missed period did conception occur? If your period is late by 14 days (two weeks), conception occurred approximately 14 days before your missed period — which is roughly the day your period was due, minus 14 days, minus 14 more days from LMP. In practical terms: if your cycle is 28 days and you are 14 days late, conception occurred approximately 28 days ago (at ovulation, midway through your last cycle). The pregnancy week calculator can map this timeline precisely.
When should I use a conception date calculator vs trusting my doctor's due date? Use the conception date calculator when you want to understand your pregnancy timeline, estimate a conception window for personal clarity, or check whether the calculator's output aligns with your OB's dates. Your doctor's due date — especially if set by an early ultrasound — is the authoritative clinical reference. The calculator is a planning and information tool, not a medical determination. Always confirm your pregnancy timeline with your healthcare provider, as individual cycles and ultrasound measurements affect these estimates.
Use the Free Conception Date Calculator
The Free Conception Date Calculator at RoughTools estimates your conception date and full fertile window from either your due date or your LMP date — whichever you have available. It accounts for standard cycle length, displays the earliest and latest possible conception dates, and shows how the estimate aligns with your gestational age. No account needed, no data stored, completely free.
Free Conception Date Calculator →
You might also need:
- Due Date Calculator — calculate your estimated due date from LMP or conception date
- Pregnancy Week Calculator — find your current gestational age and trimester
- Ovulation Calculator — map your fertile window for future cycle planning
- Period Calculator — track your cycle to identify LMP for future reference