# Conception Calculator > Estimate conception timing from the date you know. Page: https://period-tracker.org/conception-calculator/ ## How to Use the Conception Calculator Start with the date you know, then review one consistent set of pregnancy-dating estimates. 1. **Choose the Date Type You Know**: Select estimated due date, last period, ovulation date, conception date, ultrasound date, or IVF transfer. The calculator opens on estimated due date by default. 2. **Enter the Date and Requested Details**: Fill in that date. Add cycle length for the last-period method, gestational weeks and days for ultrasound, or the documented embryo age for IVF. 3. **Read the Dating Estimate**: See estimated fertilization timing, possible intercourse dates when applicable, estimated due date, and current pregnancy week as of the selected date. 4. **Refine LMP Estimates if Needed**: Open Advanced options to adjust the luteal phase or enter up to three recent cycle lengths for a wider, cycle-based range. ## What This Conception Calculator Can Do Six starting points, transparent date math, and one private result experience. - **Starts From Six Date Types**: Calculate from an estimated due date, last period, ovulation date, conception date, ultrasound date with reported gestational age, or IVF transfer. - **Uses One Result Model**: Each method connects estimated fertilization timing, due date, pregnancy dating, and relevant milestones, with possible intercourse dates shown only when applicable. - **Lets You Adjust the Luteal Phase**: For an LMP estimate, Advanced options let you replace the 14-day luteal-phase default with a value that better matches your tracked cycles. - **Builds a Range From Recent Cycles**: Enter up to three recent cycle lengths to use their rounded average for the main LMP estimate and their shortest-to-longest span for date ranges. - **Shows Its Calculation Assumptions**: The result names the dating basis and displays the cycle, luteal-phase, ultrasound, or embryo-age assumptions used for that method. - **Keeps Dates in Your Browser**: The calculation runs on your device. Dates entered here are not uploaded, saved, or sent to a server. ## Quick Tips - **Pregnancy weeks begin before conception**: Gestational age starts from an actual or equivalent last-period date, so the pregnancy count is usually about two weeks ahead of time since fertilization. - **Intercourse and fertilization are different dates**: Sperm can remain viable for several days, while the egg is available for about a day after ovulation. The possible intercourse range is therefore wider than the fertilization estimate. - **Different anchors have different uncertainty**: A due date, LMP, tracked ovulation date, reported ultrasound age, and IVF record provide different dating clues. Use the date documented by your care team for medical decisions. - **Recent cycles can show timing variability**: If cycle lengths change, enter up to three recent lengths under Advanced options. The calculator will show a range instead of hiding that variation. - **Ultrasound mode converts report data**: This calculator converts the gestational age printed on an ultrasound report into dates. It does not analyze an image or decide whether a clinical due date needs to change. - **A conception date is not proof**: Cycle math cannot prove paternity or the exact day sperm met egg. Treat every natural-conception result as a dating estimate. ## Sources - [Methods for Estimating the Due Date](https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date), American College of Obstetricians and Gynecologists (ACOG) - [Fertility Awareness-Based Methods of Family Planning](https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning), American College of Obstetricians and Gynecologists (ACOG) - [Conception](https://my.clevelandclinic.org/health/articles/11585-conception), Cleveland Clinic ## Conception Calculator FAQ ### How do I calculate conception date from my due date? Select the estimated due date method and enter the date from your pregnancy record. The calculator subtracts 266 days to estimate fertilization, shows possible intercourse dates from five days before that estimate through the estimated day, and uses the same due date to calculate pregnancy weeks. Because the original due date is itself an estimate, the back-calculated conception date is also an estimate. If you want to go the other way and estimate a due date from your last period, ovulation, conception, or an ultrasound, use our [due date calculator](https://period-tracker.org/due-date-calculator/). ### Can a conception calculator tell the exact day I got pregnant? No. Natural conception timing cannot usually be reduced to one certain day. Ovulation can shift, sperm may remain viable for up to five days, and fertilization can occur for about a day after ovulation. A due date, LMP, ovulation date, or ultrasound can narrow the timing, but the result remains an estimate. ### Is conception date the same as the date of intercourse? Not necessarily. Intercourse can happen several days before fertilization because sperm may remain viable while waiting for an egg. The calculator therefore separates estimated fertilization timing from possible intercourse dates. IVF results do not show an intercourse window because embryo-transfer dating does not infer one. ### Why am I two weeks pregnant at conception? Pregnancy weeks use gestational age, which starts from the first day of an actual or equivalent last period rather than from fertilization. In standard dating, that starting point is about 14 days before conception. This is why a person may be dated at roughly 2 weeks pregnant when fertilization occurs. ### How is conception estimated from my last period? The LMP method estimates ovulation by adding the cycle length and subtracting the luteal phase from the first day of the last period. With the defaults, that is cycle day 14. It then estimates fertilization from ovulation through the following day and estimates the due date 266 days after ovulation. ### How does cycle length change the estimate? With the luteal phase held constant, a longer cycle moves estimated ovulation, fertilization, and the due date later; a shorter cycle moves them earlier. For example, a 30-day cycle with a 14-day luteal phase places estimated ovulation near cycle day 16. Advanced options let you adjust the luteal phase for the LMP method. ### What if my cycles are irregular? Open Advanced options in the LMP method and enter up to three recent cycle lengths. The calculator uses their rounded average for the main estimate and the shortest and longest entries to create fertilization, intercourse, and due-date ranges. Wider variation means wider uncertainty, so use the result as a timing range. ### Can I use a known ovulation date? Yes. Select the ovulation-date method and enter the known or tracked date. The calculator treats that date through the next day as the likely fertilization window, shows possible intercourse dates from five days before ovulation through the following day, and adds 266 days for the estimated due date. ### How is conception estimated from an ultrasound? Enter the ultrasound date and the gestational weeks and days printed on the report. The calculator counts backward to an equivalent LMP date, places estimated fertilization 14 days later, and calculates the due date from the same dating anchor. It converts report data only and does not analyze the scan or change a clinical due date. ### How is an IVF conception date calculated? Select IVF transfer, enter the transfer date, and choose the documented embryo age: Day 3, Day 5, or Day 6. The calculator subtracts embryo age from the transfer date to estimate fertilization and adds 263, 261, or 260 days to estimate the due date. IVF results do not infer possible intercourse dates; use your clinic's recorded dates if they differ. ### Can a conception date be used to prove paternity? No. A conception estimate cannot identify which intercourse event led to fertilization and cannot prove biological parentage. Overlapping possible dates are common because sperm may remain viable for several days. Only appropriate genetic testing can address paternity; date math cannot. ## Related Fertility and Pregnancy Calculators - [IVF Due Date Calculator](https://period-tracker.org/ivf-due-date-calculator/) - [Ovulation Calculator](https://period-tracker.org/ovulation-calculator/) - [Fertility Calculator](https://period-tracker.org/fertility-calculator/) - [Implantation Calculator](https://period-tracker.org/implantation-calculator/) - [Period Calculator for Irregular Periods](https://period-tracker.org/irregular-period-calculator/) - [Period Calculator](https://period-tracker.org/period-calculator/) This calculator provides estimates only. Not for medical use. Consult your doctor for personal advice.