Conception Calculator
Calculate your estimated conception date from a due date, last menstrual period, or current gestational week. Get a realistic conception window that reflects the biology of fertilization timing.
Enter your values above to see the results.
Tips & Notes
- ✓Conception dates estimated from LMP or due date are never exact — they represent the midpoint of a 10–12 day biological window during which fertilization could have occurred.
- ✓The due date is conception + 266 days (38 weeks). Working backward: if your due date is December 15, your most likely conception was approximately March 23.
- ✓First-trimester ultrasound dating (measuring crown-rump length at 8–12 weeks) provides the most accurate gestational age — from this you can calculate conception date as: ultrasound date − (gestational age at ultrasound − 14 days).
- ✓IVF conceptions are the exception: with a Day 5 embryo transfer, conception is defined as the transfer date itself, and fertilization occurred 5 days earlier at egg retrieval.
- ✓Gestational age (used by all healthcare providers) counts from LMP, not conception — a gestational age of 10 weeks means the embryo is approximately 8 weeks old from the day of fertilization.
Common Mistakes
- ✗Treating the estimated conception date as a precise known day — biological variability in ovulation timing and sperm survival means conception occurred within a window, not on a single calculable date.
- ✗Confusing gestational age with fetal age — healthcare providers always use gestational age (from LMP), which is 14 days longer than fetal age (from conception). When you hear "8 weeks pregnant," the embryo is about 6 weeks old.
- ✗Calculating conception from due date without accounting for cycle length — standard calculation assumes a 28-day cycle. For longer cycles, ovulation (and conception) occurred later, shifting the conception estimate forward.
- ✗Using conception date to determine paternity — conception windows of 5–7 days make precise paternity determination impossible from date calculation alone. DNA testing is the only definitive method.
- ✗Expecting the conception date calculation to exactly match first trimester ultrasound dating — ultrasound measures fetal size, which can vary by 5–7 days from gestational age predictions even in normal pregnancies.
Conception Calculator Overview
Conception — the moment a sperm fertilizes an egg — cannot be pinpointed to a single day in most cases. Sperm survive 3–5 days, the egg is viable for 12–24 hours, and ovulation timing varies. The result is a biological window of 5–7 days during which conception could have occurred.
Conception date calculation from LMP:
Conception date from different starting points: From LMP (most common): conception ≈ LMP + (cycle length − 14) days ± 5 days From due date: conception ≈ due date − 266 days (38 weeks) ± 5 days From gestational week: conception ≈ LMP − 14 days + gestational age in days − 14 days Gestational age: measured from LMP (adds ~14 days to fertilization age) Fertilization age = gestational age − 14 days
EX: Due date = December 15 Estimated conception = December 15 − 266 days = March 23 Conception window = March 18–28 (± 5 days around estimated date) LMP equivalent = conception − 14 days = March 9 Standard gestational age at conception: 2 weeks (day 14 of the 40-week count) EX: LMP = April 1, 28-day cycle Estimated ovulation = April 1 + 14 = April 15 Probable conception window = April 10–16 (fertile window) Conception most likely on: April 13–15 (days −2, −1, and 0 relative to ovulation)
Conception date from due date:
Conception window from different cycle lengths: 24-day cycle: ovulation day 10 → conception window days 5–11 28-day cycle: ovulation day 14 → conception window days 9–15 32-day cycle: ovulation day 18 → conception window days 13–19 35-day cycle: ovulation day 21 → conception window days 16–22 Fertilization timing within the fertile window matters: Intercourse on day −5 (5 days before ovulation): ~10% probability per cycle Intercourse on day −1 (day before ovulation): ~31% probability per cycle Intercourse on ovulation day: ~33% probability per cycle
EX: LMP = March 1, 32-day cycle Estimated ovulation = March 1 + (32 − 14) = March 19 Conception window = March 14–20 Most probable single conception date (statistically, intercourse closest to ovulation): If intercourse occurred March 17, 18, and 19, conception most likely on March 19 or 18. Due date from this conception: March 19 + 266 = December 10 Or from LMP equivalent: March 1 + 280 + 4 (cycle adjustment) = December 10
Dating method accuracy comparison:
| Calculation method | Accuracy of conception estimate | Best used when |
|---|---|---|
| From LMP + cycle length | Conception window ± 5–7 days | Regular cycles, known LMP |
| From due date (Naegele reverse) | Conception window ± 5–7 days | Know due date but not LMP |
| From first-trimester ultrasound | Conception window ± 3–5 days | Early CRL measurement available |
| From IVF transfer date | Conception date known exactly | IVF — precise because egg retrieval timing known |
| From positive OPK + intercourse records | Conception day within 1–2 days | Active fertility tracking with LH tests |
Key pregnancy terminology — definitions and relationships:
| Term | Counted from | Relationship to other terms |
|---|---|---|
| Gestational age | First day of LMP | Standard clinical measure — 40 weeks total |
| Fetal age (fertilization age) | Day of conception | Gestational age minus 14 days = fetal age |
| Conception date | Day of fertilization | Approximately LMP + 14 days (in 28-day cycle) |
| Embryonic age | Post-fertilization | Same as fetal age — used in early development |
| Due date (EDD) | LMP + 280 days | Conception + 266 days = due date |
The gap between gestational age and actual age of the embryo is a source of frequent confusion. When a healthcare provider says "you are 8 weeks pregnant," they mean 8 weeks from LMP — but the embryo has only existed for approximately 6 weeks from the day of fertilization. This 2-week difference reflects the convention of measuring from LMP rather than from conception, which is established internationally for consistency. The embryo is not two weeks younger than your gestational week count suggests — it is exactly what that week represents when you subtract 14 days.
Frequently Asked Questions
Subtract 266 days (38 weeks) from your due date to get your estimated conception date. For example, a due date of January 10 gives a conception estimate of January 10 minus 266 days = April 19 of the previous year. However, this is the midpoint of a conception window spanning approximately April 14–24, because ovulation and conception can vary by 5–7 days from the calendar-based prediction. The due date itself has an accuracy of approximately ±2 weeks from Naegele's Rule, so the derived conception date carries the same uncertainty. First-trimester ultrasound dating provides a more precise starting point.
Not precisely, in most naturally conceived pregnancies. The biological window during which conception can occur (the fertile window) spans approximately 6 days — the five days before ovulation and the day of ovulation itself. Sperm can survive up to 5 days in the female reproductive tract, and the egg is viable for 12–24 hours after ovulation. Without knowing the exact timing of ovulation (which requires real-time LH testing or ultrasound monitoring), and without knowing which specific act of intercourse resulted in fertilization, only a window — not a specific date — can be estimated. The only exception is IVF, where egg retrieval and fertilization timing are precisely known.
Gestational age is the official clinical measurement used by all healthcare providers worldwide. It is counted from the first day of the last menstrual period, regardless of when conception actually occurred. In a standard 28-day cycle, conception occurs approximately 14 days after LMP, so gestational age is always approximately 2 weeks longer than the actual age of the embryo from fertilization. When your provider says you are "10 weeks pregnant" (gestational age), the embryo has existed for approximately 8 weeks from conception. This convention was established before ovulation timing was well understood, and it has been maintained globally for consistency in clinical communication.
You can narrow the window significantly. If you know which days intercourse occurred and can estimate ovulation timing from your cycle length or LH test results, the most probable conception date is the intercourse date closest to ovulation — preferably the day of or day before ovulation. Sperm deposited 5 days before ovulation could technically fertilize the egg, but conception probability is much higher from intercourse in the 48 hours preceding and including ovulation day. Combining known intercourse dates with an ovulation estimate gives the most precise conception window possible without laboratory methods.
First-trimester ultrasound measures the actual size of the embryo (crown-rump length), which reflects its biological age independent of any calendar calculation. When ultrasound dating and LMP-based dating agree within 7 days, both methods are considered consistent. Discrepancies beyond 7 days are common and occur for several reasons: irregular cycles where ovulation happened later or earlier than the cycle-length formula predicts, a cycle length that was longer or shorter than typical for that specific month, natural variation in fetal growth rates, or an inaccurate LMP date. When there is a discrepancy, obstetric guidelines generally use the ultrasound date because it reflects actual fetal size — making the derived conception estimate based on ultrasound more accurate than LMP-based calculation.
Conception date calculation cannot definitively determine paternity. The biological conception window (6 days centered around ovulation) overlaps in time, and even with precise knowledge of intercourse dates, if multiple partners were involved during the fertile window, date calculation alone cannot distinguish the father. Genetic (DNA) paternity testing remains the only definitive method — it can be performed prenatally through NIPT-type testing from week 10 onward or with amniocentesis, and postnatally once the child is born. Conception date ranges should never be used as a basis for legal paternity determination — DNA testing should always be sought when paternity is genuinely uncertain.