Conception Calculator: Estimate When You Conceived
Conception typically occurs 11–21 days after the first day of the last menstrual period (LMP), about day 14 in a 28-day cycle. Alternatively, subtract 266 days from the estimated due date to estimate the conception date. Both yield a window.
Estimate Your Conception Window
Choose your starting point: (A) enter your estimated due date, or (B) enter your LMP and average cycle length. The calculator returns your estimated fertilization date and the intercourse window that could have produced it.
Calculation Methods
- From due date: Conception ≈ EDD − 266 days (280 days of gestation minus the 14 days before ovulation).
- From LMP: Conception ≈ LMP + (cycle length − 14) days. For a 28-day cycle, that is LMP + 14. For a 32-day cycle, it is LMP + 18 days.
Example: EDD is December 15, 2026. Subtracting 266 days gives approximately March 24, 2026. The realistic window is about ±3–5 days around that date.
Intercourse vs. Fertilization
Intercourse and fertilization are different events:
- Sperm survival: Sperm can survive in cervical mucus for up to 5 days.
- Egg viability: The oocyte is fertilizable for only 12–24 hours after ovulation.
- Result: Intercourse on Monday may produce fertilization on Saturday. The only fixed point is ovulation, which is why the intercourse window is the 5 days before ovulation through the day after.
After fertilization, the zygote travels through the fallopian tube for roughly 3–4 days and implants about 6–12 days after ovulation. Conception, in the clinical sense, is not detectable until hCG appears.
Gestational Age vs. Embryonic Age
Gestational age counts from the first day of the LMP. Embryonic (fertilization) age counts from conception. In a standard 28-day cycle, ovulation occurs about 14 days after LMP, so gestational age = embryonic age + 2 weeks. This is why a pregnancy is “2 weeks pregnant” at the moment of fertilization.
| Parameter | Gestational Age (LMP-Based) | Embryonic Age (Conception-Based) | Calculation Method |
|---|---|---|---|
| Week 2 | 2w0d–2w6d (cycle days 14–20) | Ovulation/fertilization window; 0 weeks | Ovulation ≈ LMP + 14 days |
| Week 4 | 4w0d (day 28) | 2w0d. Implantation is complete; the missed period coincides with a positive urine test. | Embryonic age = GA − 14 days |
| Week 8 | 8w0d | 6w0d. Cardiac activity is established; CRL is about 16 mm. | Embryonic age = GA − 14 days |
| Week 12 | 12w0d | 10w0d. The fetal period has begun; CRL is about 54 mm. | Embryonic age = GA − 14 days |
When Conception Dates Matter Clinically
IVF and assisted reproduction. In IVF, conception is known. The due date is calculated from the embryo transfer date and embryo age. For a day-5 blastocyst, the due date is the transfer date plus 261 days, and ultrasound does not override this dating. Use the IVF Due Date Calculator.
Early ultrasound milestones. Conception-based dating helps interpret early findings such as the gestational sac at about 4.5–5 weeks, yolk sac at about 5.5 weeks, and cardiac activity at about 6–6.5 weeks, all LMP-based. If a scan appears “behind” LMP dating, delayed ovulation is a common explanation, not necessarily a problem.
Frequently Asked Questions
Can a doctor tell the exact day you got pregnant?
No. Clinicians estimate gestational age and the due date. A first-trimester ultrasound CRL measurement estimates gestational age within about ±5–7 days. The exact day is known with confidence only in IVF or with documented ovulation monitoring.
Can you conceive on the exact day of intercourse?
Only if intercourse occurs within roughly 12–24 hours of ovulation, or if the egg is released while sperm are already waiting. More commonly, fertilization follows intercourse by one to five days.
How does a conception calculator work if my cycle is 32 days?
Ovulation is calculated as cycle length minus 14: 32 − 14 = day 18, so conception falls about 17 days after LMP. Using the default 28-day assumption would place conception 4 days too early and, if used for due-date calculation, would also place the due date 4 days early. Always enter your actual average cycle length.
Why is my pregnancy dated 2 weeks before I actually had sex?
Clinical dating begins on the first day of the LMP, which precedes ovulation by about 14 days. Your due date and “weeks pregnant” number include those two weeks, during which you were not yet pregnant. It is a convention, not an error.