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 typically occurs 11–21 days after the first day of the last menstrual period (LMP), with ovulation — and the most likely conception moment — happening around day 14 for a standard 28-day cycle. The exact ovulation day shifts proportionally with cycle length: for a 35-day cycle, ovulation occurs around day 21.
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 5–6 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.
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.
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Conception Calculator Questions
How is the conception date calculated?+
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.
Can I determine the exact date of conception?+
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.
What is the difference between gestational age and fetal age?+
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.
When can I take a pregnancy test after conception?+
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.
What is Naegele's Rule for calculating due dates?+
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.
How accurate is the due date calculated from the last period?+
LMP-based due dates have meaningful uncertainty. Only about 4-5% of babies are born on their exact due date. About 70% of babies are born within 10 days of the EDD, and 90% within 2 weeks. 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.
What is implantation and when does it happen?+
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.
Can conception occur outside of the fertile window?+
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.
What does gestational age at conception mean?+
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.
How does cycle length affect the estimated conception date?+
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 3 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 approximately 2 weeks later than assumed by the standard 28-day calculation. Using the correct cycle length significantly improves accuracy.