Conception Date Calculator: When Did I Conceive? Formula & Fertile Window
A conception date calculator helps you estimate the most likely date you became pregnant. Whether you're curious about when your baby was conceived, tracking prenatal milestones, or simply want to understand your pregnancy timeline better, this tool uses medically accepted formulas to pinpoint the approximate date of fertilization based on the information you already have.
Our calculator offers three proven methods to estimate your conception date: working backward from your estimated due date, counting forward from your last menstrual period (LMP), or using your ultrasound dating scan results. Each method provides a slightly different level of accuracy, and you can compare results across all three to get the most reliable estimate possible.
Knowing your conception date matters for more than just curiosity. It helps your healthcare provider establish an accurate gestational timeline, schedule critical prenatal tests at the right time, and track your baby's developmental milestones against expected benchmarks. An accurate conception estimate also helps you understand which trimester you're in and when to expect key moments like the anatomy scan or viability milestone.
The calculations behind this tool are based on Naegele's Rule, the same formula obstetricians have used for over 200 years to estimate due dates and gestational age. While no calculator can pinpoint the exact moment of conception (since sperm can survive up to five days and ovulation timing varies), the results provide a medically sound window that aligns with how your doctor dates your pregnancy.
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Estimated delivery date from your healthcare provider
Enter your information to calculate conception date
How is Conception Date Calculated?
The most widely used method for estimating conception dates is Naegele's Rule, developed by German obstetrician Franz Naegele in the early 1800s. The rule calculates an estimated due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). Since conception typically occurs around day 14 of a standard cycle, the estimated conception date is calculated as LMP + 14 days, or equivalently, Due Date - 266 days.
Ultrasound dating offers a more individualized approach. During a first-trimester ultrasound (7-13 weeks), your provider measures the embryo's crown-rump length and determines a precise gestational age. From that measurement, the calculator works backward to establish your LMP date and then estimates ovulation and conception. First-trimester ultrasounds are accurate to within 3-5 days, making them the gold standard for pregnancy dating.
All of these methods rely on the 28-day cycle assumption, which presumes ovulation occurs on day 14. In reality, cycle lengths vary from 21 to 35 days, and ovulation can happen anywhere from day 10 to day 21. Women with shorter cycles may have conceived earlier than the standard estimate, while those with longer cycles may have conceived later. This is one reason why calculators provide a conception window rather than a single date.
Because sperm can survive for up to five days and the egg remains viable for 12-24 hours, there is always a range of possible conception dates rather than one definitive day. Even with the most accurate ultrasound dating, the true moment of fertilization falls within a window of several days. This biological variability is why our calculator displays both a most likely date and a broader conception range.
Conception Date Formulas
From Due Date
Conception Date = Due Date - 266 days
Counts back 38 weeks (266 days) from the estimated delivery date to find the likely ovulation and conception date.
From Last Menstrual Period (LMP)
Conception Date = LMP + 14 days (±2 days)
Adds 14 days to the first day of your last period, assuming ovulation occurs mid-cycle on a standard 28-day cycle.
From Ultrasound
Conception Date = Ultrasound Date - Gestational Age + 14 days
Uses the ultrasound-determined gestational age to calculate the LMP, then adds 14 days for the estimated ovulation date.
Fertile Window
Fertile Window = Ovulation Date - 5 days to Ovulation Date + 1 day
The six-day window when intercourse is most likely to result in pregnancy, accounting for sperm survival and egg viability.
Worked Example: Meet Jessica
Jessica just learned her due date is November 20, 2026. She wants to figure out when she likely conceived and what her fertile window was. Here's how the calculation works step by step:
Count back 266 days from the due date: November 20, 2026 - 266 days = February 27, 2026. This is Jessica's estimated conception date.
Estimate the LMP: November 20, 2026 - 280 days = February 13, 2026. This is the estimated first day of Jessica's last menstrual period.
Ovulation timing: Ovulation likely occurred around February 27 (day 14 of her cycle), which aligns with the estimated conception date.
Fertile window: Five days before ovulation through one day after gives Jessica a fertile window of February 22 - February 28, 2026.
Conclusion: Intercourse during the February 22-28 window most likely led to conception. The most probable single date is February 27, 2026, but conception could have occurred on any day within that fertile window.
Pregnancy Timeline: Week-by-Week Milestones
Once you know your conception date, you can track your baby's growth through these key milestones. Gestational age is measured from your LMP (about two weeks before conception).
| Week | Milestone | Size Comparison |
|---|---|---|
| 4 | Implantation complete | Poppy seed |
| 8 | Heartbeat detectable | Raspberry |
| 12 | End of 1st trimester | Lime |
| 16 | Gender may be visible | Avocado |
| 20 | Anatomy scan (halfway!) | Banana |
| 24 | Viability milestone | Ear of corn |
| 28 | 3rd trimester begins | Eggplant |
| 32 | Baby is head-down | Squash |
| 36 | Lungs nearly mature | Honeydew |
| 40 | Full term! | Watermelon |
How to Use This Calculator
This calculator offers three methods to estimate your conception date. Choose the one that matches the information you have:
- Due Date method -- If you already know your estimated due date from your healthcare provider, select "Due Date" and enter the date. The calculator counts back 266 days to estimate when conception occurred.
- Last Menstrual Period method -- Select "Last Period" and enter the first day of your most recent period. The calculator adds 14 days to estimate your ovulation and conception date, assuming a standard 28-day cycle.
- Ultrasound Dating method -- Select "Ultrasound Dating" and enter the date of your scan along with the gestational age determined by your provider. This is the most accurate method, especially when performed in the first trimester.
- Review your results -- The calculator displays the most likely conception date, a conception window, your estimated ovulation date, and your current gestational and fetal age.
Understanding Conception
How Conception Date Is Estimated
Conception date estimation relies on the relationship between your last menstrual period, ovulation, and fertilization. In a textbook 28-day cycle, ovulation occurs around day 14. Since the egg survives only 12 to 24 hours after release, fertilization must happen within that narrow window. However, because sperm can live up to five days in the reproductive tract, intercourse from several days before ovulation can still result in conception. This is why calculators provide a range rather than a single definitive date.
Conception vs. Implantation
Conception and implantation are two distinct events. Conception refers to the moment the sperm fertilizes the egg, which typically happens in the fallopian tube within 24 hours of ovulation. After fertilization, the resulting embryo travels down the fallopian tube over the next several days, undergoing rapid cell division. Implantation occurs approximately 6 to 12 days after conception, when the embryo embeds into the uterine lining. Only after successful implantation does the body begin producing human chorionic gonadotropin (hCG), the hormone detected by pregnancy tests.
Accuracy Factors
Several factors influence how accurately a conception date can be estimated. The most significant variable is cycle regularity -- women with irregular cycles may ovulate earlier or later than the assumed day 14. The calculation method matters as well: first-trimester ultrasound dating is accurate to within three to five days, while LMP-based estimates carry a margin of error of about seven days. Stress, travel, illness, and hormonal conditions can all shift ovulation timing. For the most reliable estimate, compare results from multiple methods and discuss the findings with your healthcare provider.
Gestational Age vs. Fetal Age
Gestational age counts from the first day of your last menstrual period, which is roughly two weeks before conception actually occurs. Fetal age (also called embryonic age) counts from the estimated date of fertilization. A pregnancy that is described as "8 weeks gestational age" is about 6 weeks from the actual conception date. Healthcare providers use gestational age as the standard measurement for scheduling prenatal tests and tracking developmental milestones.
Method Accuracy Comparison
| Method | Accuracy | Best Used When |
|---|---|---|
| Due Date | +/- 7 days | You have a confirmed due date from your provider |
| Last Menstrual Period | +/- 7 days | You have regular 28-day cycles and recall your LMP |
| First-Trimester Ultrasound | +/- 3-5 days | Gold standard; most reliable when performed at 7-13 weeks |
Important Disclaimer
This conception date calculator provides estimates for educational purposes only. The true date of conception cannot be determined with absolute certainty because ovulation timing and sperm survival introduce natural variability. These results should not be used for legal purposes, paternity determination, or critical medical decisions. For accurate pregnancy dating, always consult your healthcare provider who can use clinical methods including ultrasound measurements.
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Medical Disclaimer
This conception calculator provides estimates for educational purposes only. Actual conception dates can vary due to individual cycle differences, ovulation timing variations, and other biological factors. These calculations should not be used for legal purposes, paternity determination, or critical medical decisions. For accurate gestational age assessment and prenatal care timing, always consult with your healthcare provider who can use medical dating methods like ultrasound measurements.