Conception Date Calculator
If you are asking “When did I conceive?”, use the due date in your pregnancy record, the first day of your last period with your average cycle length, or pregnancy weeks and days from a dated pregnancy record or dating scan. The calculator returns one around-date estimate.
It cannot confirm the exact date of fertilization, intercourse, implantation, or parentage.
Free browser-based pregnancy and preconception calculators with transparent methods, sources, assumptions, and limitations.
Already have a documented conception or fertilization date and want to estimate the due date? Use the Due Date Calculator.
How to use this conception date calculator
- Use the current recorded due date—also called the estimated due date, or EDD—when your pregnancy record already has one.
- Otherwise, use a reliable first day of the last menstrual period (LMP) with an average cycle length in the supported 21–35-day range.
- Use dated pregnancy weeks only when the completed weeks, additional days, and record or dating-scan date appear together.
For example, an LMP of January 1, 2024 with a 35-day average cycle gives an around-date estimate of January 22, 2024 and a related EDD of October 14, 2024. The cycle input shifts the shared calendar model; it does not identify the biological day of ovulation or fertilization.
This calculator does not read ultrasound measurements or re-date an established clinical due date. If you have a documented conception or fertilization date and need an EDD, use the Due Date Calculator.
How accurate is a conception date estimate?
The result is one around-date calendar estimate, not confirmation of an exact biological event. Intercourse, ovulation, fertilization, and implantation are different events, and this result cannot identify a particular intercourse date or determine parentage.
Use the current EDD in the pregnancy record when one has been established. A care team—not this calculator—interprets a dating scan or decides whether a clinical due date should change.
Symptoms, pregnancy health, location, viability, fetal growth, and wellbeing are outside this calculator's scope. Do not use the result to make medical, obstetric, fertility, or treatment decisions.
For pregnancy-week counting, cycle examples, conception boundaries, clinical dating, and the official sources, read How pregnancy dating and conception estimates work. Formula definitions and implementation details are documented in the Methodology.
If you have an embryo transfer date and embryo day, use the IVF Due Date Calculator.
Frequently asked questions
- Can this calculator tell me the exact conception date?
- No. It gives one calendar estimate of when conception may have occurred. It cannot confirm the exact time of fertilization, intercourse, or implantation.
- Which pregnancy date should I use?
- Use the current due date in your pregnancy record when it is available. Otherwise, use a reliable first day of the last period. Use dated pregnancy weeks only when those weeks, days, and the record date appear together.
- How is conception estimated from a due date?
- The calculator counts back 266 days from the recorded due date. This is a transparent calendar estimate, not an official confirmation of fertilization.
- How is conception estimated from my last period?
- The 28-day standard model adds 14 days to the first day of the last period. A supported average cycle length shifts that estimate by the difference from 28 days. If cycles vary substantially, the calendar estimate is less useful for locating ovulation or fertilization; use a current recorded due date when one is available.
- Can I use pregnancy weeks from a dating scan?
- Yes, when the report gives completed pregnancy weeks, additional days, and the scan date together. Enter those recorded values; the calculator does not interpret ultrasound measurements or change a clinical due date.
- Should IVF pregnancies use this calculator?
- If you have an embryo transfer date and embryo day, use the IVF Due Date Calculator. An established IVF-derived due date can be used here, but it cannot recreate transfer details.