Conception Calculator
Estimated Date · Fertile Window · Timeline
Calculate From
Last Period (LMP)
Due Date
The first day of your most recent period before conception
Your confirmed or estimated due date (EDD)
Enter your last period date or due date to estimate when conception occurred.
Estimated Conception
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Fertile Window
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Key Dates
Last menstrual period—
Ovulation date—
Implantation window—
hCG detectable from—
Estimated due date—
Gestational age today—
Early Pregnancy Timeline
Conception
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Implantation
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Pregnancy test positive
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First missed period
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Estimated due date
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How Conception Date Is Estimated
Conception most likely happened 266 days (38 weeks) before your due date, which in a 28-day cycle is about 14 days after the first day of your last period. A due date of June 15, 2027 points to conception around September 22, 2026. Enter your last period or your due date above and the calculator adjusts for your cycle length.
Conception happens on or just after ovulation, which usually comes about 14 days before the next period is due. That is roughly 14 days after the first day of the last menstrual period (LMP) in a 28-day cycle, 7 days after in a 21-day cycle and 21 days after in a 35-day cycle. Ovulation moves one day later for every extra day of cycle length.
Working backwards from a due date, conception is estimated by subtracting 266 days (38 weeks from conception to birth). The due date itself is calculated using Naegele's Rule: LMP + 280 days (40 weeks). Since sperm can survive in the fallopian tubes for up to 5 days, the actual conception window is typically 5 to 6 days ending on ovulation day.
Fertile Window Explained
The fertile window is the 6-day period during which intercourse can lead to conception: the 5 days before ovulation plus ovulation day itself. Sperm survive up to 5 days inside the female reproductive tract, so intercourse several days before ovulation can still result in conception. Eggs are viable for only 12-24 hours after release. The 2-3 days just before ovulation are the most fertile.
Implantation Timeline
After fertilization, the embryo travels to the uterus and implants 6-12 days after conception (most commonly days 8-10). Implantation triggers hCG (human chorionic gonadotropin) production: the hormone detected by pregnancy tests. hCG levels become detectable in blood 10-14 days after conception, and in urine 12-15 days after. Home pregnancy tests are most reliable from the day of a missed period.
Cycle Length Impact
Ovulation timing varies with cycle length. The luteal phase (from ovulation to next period) is fairly constant at 12-14 days for most women. The follicular phase (from LMP to ovulation) is what varies. For a 21-day cycle, ovulation is around day 7-8. For a 35-day cycle, around day 21-22. This is why cycle length significantly affects conception date estimates.
Accuracy of Estimates
Conception date estimates have a natural uncertainty of plus or minus 5 days due to cycle variability. Ultrasound dating (especially in the first trimester at 8-12 weeks) is the most accurate method for establishing gestational age. If ultrasound dating differs from LMP dating by more than 5-7 days, the ultrasound date is typically used as the reference. Irregular cycles increase the uncertainty of formula-based estimates.
Conception Date by Due Date
Each row counts back 266 days from the due date. The fertile window is the 6 days when sex could have led to this pregnancy, because sperm can survive up to 5 days before ovulation. The LMP column assumes a 28-day cycle.
| Due date | Likely conception | Fertile window | Last period started |
|---|---|---|---|
| Jan 15, 2027 | Apr 24, 2026 | Apr 19 to Apr 24, 2026 | Apr 10, 2026 |
| Feb 15, 2027 | May 25, 2026 | May 20 to May 25, 2026 | May 11, 2026 |
| Mar 15, 2027 | Jun 22, 2026 | Jun 17 to Jun 22, 2026 | Jun 8, 2026 |
| Apr 15, 2027 | Jul 23, 2026 | Jul 18 to Jul 23, 2026 | Jul 9, 2026 |
| May 15, 2027 | Aug 22, 2026 | Aug 17 to Aug 22, 2026 | Aug 8, 2026 |
| Jun 15, 2027 | Sep 22, 2026 | Sep 17 to Sep 22, 2026 | Sep 8, 2026 |
| Jul 15, 2027 | Oct 22, 2026 | Oct 17 to Oct 22, 2026 | Oct 8, 2026 |
| Aug 15, 2027 | Nov 22, 2026 | Nov 17 to Nov 22, 2026 | Nov 8, 2026 |
| Sep 15, 2027 | Dec 23, 2026 | Dec 18 to Dec 23, 2026 | Dec 9, 2026 |
| Oct 15, 2027 | Jan 22, 2027 | Jan 17 to Jan 22, 2027 | Jan 8, 2027 |
| Nov 15, 2027 | Feb 22, 2027 | Feb 17 to Feb 22, 2027 | Feb 8, 2027 |
| Dec 15, 2027 | Mar 24, 2027 | Mar 19 to Mar 24, 2027 | Mar 10, 2027 |
If your due date was set by an early ultrasound, counting back from it gives a better estimate than counting forward from your period, because the scan has already corrected for late or early ovulation.
How Cycle Length Moves Ovulation
The time from ovulation to the next period stays close to 14 days, so ovulation falls about cycle length minus 14 days after your period starts. Every extra day of cycle moves ovulation, and the likely conception date, one day later.
| Cycle length | Ovulation, days after period starts | Compared with a 28-day cycle |
|---|---|---|
| 21 days | 7 | 1 week earlier |
| 24 days | 10 | 4 days earlier |
| 26 days | 12 | 2 days earlier |
| 28 days | 14 | same |
| 30 days | 16 | 2 days later |
| 32 days | 18 | 4 days later |
| 35 days | 21 | 1 week later |
When you work back from a due date, cycle length does not change the conception date: it is 266 days before the due date either way. It only changes which LMP date that due date implies.
From Conception to a Positive Test
A worked timeline for conception on September 15, 2026 (last period September 1, 2026, 28-day cycle), using the ranges the calculator shows.
| Step | Days after conception | Date |
|---|---|---|
| Ovulation and fertilization | 0 | Sep 15, 2026 |
| Implantation window opens | 6 | Sep 21, 2026 |
| Most common implantation days | 8 to 10 | Sep 23 to Sep 25, 2026 |
| Implantation window closes | 12 | Sep 27, 2026 |
| Missed period, home test reliable | 14 | Sep 29, 2026 |
| Due date | 266 | Jun 8, 2027 |
Once you know the due date, the pregnancy due date calculator shows your week and trimester, and the ovulation calculator maps fertile days if you are still trying.
Method and sources. Ovulation = first day of last period + (cycle length minus 14) days; conception is taken as the ovulation day; fertile window = 5 days before ovulation through ovulation day; due date = LMP + 280 + (cycle length minus 28) days, which is always conception + 266 days. All table dates were computed with the calculator's own day count. Sources: ACOG Committee Opinion 700, Methods for Estimating the Due Date; Wilcox and colleagues, New England Journal of Medicine 1995, on the 6-day fertile window; MedlinePlus. Estimates only; an early ultrasound gives the most reliable dating.
This calculator provides estimates based on average cycle patterns. Individual conception dates vary. For medical purposes, conception and due dates should be confirmed by a healthcare provider using ultrasound. Results are informational only and not a substitute for obstetric care.
Conception Calculator Questions
Conception date is estimated by identifying when ovulation most likely occurred. For a standard 28-day cycle, ovulation happens around day 14 (counting from the first day of the last period). For other cycle lengths, ovulation occurs approximately 14 days before the next expected period. Since sperm can survive 3-5 days and the egg is viable for 12-24 hours after release, the estimated conception date is the ovulation date, with a natural window of plus or minus 5 days. Working backwards from a due date: subtract 266 days to get the estimated conception date, or 280 days to get the LMP date.
Not with certainty through calculation alone. Even with precisely tracked ovulation, fertilization occurs within a 24-hour window after ovulation, but the sperm may have entered the fallopian tube several days earlier. Ultrasound measurements in the first trimester (particularly the crown-rump length at 8-12 weeks) can estimate gestational age within 5-7 days, but cannot identify the exact conception date. For most practical purposes, a 5-7 day window around ovulation is the best estimate of when conception occurred.
Gestational age is counted from the first day of the last menstrual period (LMP) and is the standard used in obstetrics. It includes approximately 2 weeks before conception actually occurred. Fetal age (or fertilization age) is counted from the actual conception date and is about 2 weeks less than gestational age. When doctors say "you are 10 weeks pregnant," they mean 10 weeks of gestational age, which corresponds to approximately 8 weeks of fetal development. This distinction is important when interpreting ultrasound measurements and development milestones.
The timing depends on the type of test. Blood tests (beta-hCG) can detect pregnancy as early as 10-14 days after conception, before a missed period. Standard home urine tests are most reliable from the first day of a missed period, which is approximately 14-16 days after conception for a 28-day cycle. Some "early result" home tests claim to detect pregnancy 4-5 days before a missed period, but sensitivity varies by brand and hCG levels rise differently in each pregnancy. Testing too early produces a higher rate of false negatives. The most reliable window is the day of or after a missed period.
Naegele's Rule, developed in the 19th century, calculates the estimated due date (EDD) by adding 280 days (40 weeks) to the first day of the last menstrual period. A simpler way to apply it: take the LMP date, add one year, subtract 3 months, and add 7 days. For example, LMP of January 1 gives a due date around October 8. The rule assumes a 28-day cycle with ovulation on day 14. For cycles other than 28 days, the due date is adjusted: add days beyond 28 or subtract days below 28 from the Naegele calculation. Most modern obstetric dating uses ultrasound to confirm or replace Naegele's estimate.
LMP-based due dates have meaningful uncertainty. Only about 4-5% of babies are born on their exact due date. The main sources of inaccuracy: uncertain LMP date (especially with irregular cycles), assumption of day-14 ovulation (which varies with cycle length and individual variation), and the natural range in pregnancy duration (37-42 weeks is considered normal). First-trimester ultrasound (before 14 weeks) is more accurate than LMP dating and is used to adjust the EDD when the difference is more than 5-7 days.
After fertilization, the zygote divides as it travels through the fallopian tube toward the uterus. It arrives as a blastocyst 4-5 days after fertilization and implants into the uterine lining 6-12 days after conception, most commonly on days 8-10. Implantation is the moment the embryo attaches to and begins embedding in the endometrium. It triggers the production of hCG (human chorionic gonadotropin), which is what pregnancy tests detect. Some women experience light spotting (implantation bleeding) around this time, which can be confused with an early period. Implantation must occur for a pregnancy to continue: if it fails, the cycle ends normally.
Conception requires fertilization of an egg, which can only happen during the 12-24 hours after ovulation. Since sperm can survive up to 5 days in the reproductive tract, intercourse up to 5 days before ovulation can result in conception. This means the fertile window is approximately 6 days ending on ovulation day. Outside this window, conception is not possible in a given cycle. However, because ovulation timing can vary, even in "regular" cycles, the actual fertile window is not always predictable from the calendar alone. Ovulation tracking tools (LH tests, basal body temperature) provide more precise fertile window identification than calendar methods.
At the moment of actual conception, gestational age is already counted as approximately 2 weeks (the time from LMP to ovulation in a standard cycle). This is why early ultrasounds at "week 6" show a 4-week-old embryo by fetal age. The 2-week offset between gestational age and fetal age is a convention in obstetrics that has been used for over a century, because the LMP is a reliably known date while the exact conception date is not. When comparing fetal development milestones between countries, be aware that some sources use gestational age (LMP-based) while others use fetal age (conception-based), creating an apparent 2-week discrepancy.
Cycle length directly affects when ovulation occurs, which determines the conception date estimate. The formula: ovulation day = cycle length minus 14 (the luteal phase is approximately constant at 14 days). For a 21-day cycle, ovulation is around day 7, meaning conception would be estimated about 2 weeks earlier than for a 35-day cycle. This shifts the estimated conception date by up to 2 weeks depending on cycle length. For the same LMP, a woman with a 35-day cycle would have conceived about 1 week later than assumed by the standard 28-day calculation. Using the correct cycle length significantly improves accuracy.
Usually not. Sperm can live up to 5 days, so sex on any day of the 6-day fertile window can lead to conception on ovulation day, and even a first trimester ultrasound is only accurate to about 5 to 7 days. If the two dates are more than a couple of weeks apart and you had an early scan, the dating may make one of them far more likely, but only a DNA test can settle a question of paternity.
With IVF the fertilization date is known: it is the day of egg retrieval. A day-5 embryo transferred on October 1, 2026 was fertilized on September 26, 2026, and the due date is 261 days after the transfer, June 19, 2027. For a day-3 embryo, fertilization was 3 days before transfer and the due date is 263 days after it. You do not need this calculator for IVF dating; your clinic gives the exact dates.