Pregnancy Conception Calculator

This conception calculator estimates the most likely date of conception using either your estimated due date or the first day of your last menstrual period (LMP). Conception dating helps answer the common question, "When did I get pregnant?" and provides a timeframe for when fertilization most likely occurred.

Conception typically occurs around the time of ovulation, when a mature egg is released from the ovary and can be fertilized by sperm. In a typical 28-day cycle ovulation often happens around day 14, but timing varies between individuals and across cycles. Because sperm can survive up to five days in the reproductive tract and the egg remains fertilizable for about 12–24 hours, the fertile window spans several days. This tool estimates both the likely conception date and the broader fertile window during which intercourse may have resulted in conception.

Calculation methods and interpretation

If you enter a due date, the calculator counts back approximately 38 weeks (266 days) to estimate conception. If you provide your LMP, the tool estimates ovulation timing based on average cycle length. These methods provide reasonable estimates, but they are not precise determinations. Implantation timing, variation in ovulation timing, and individual biological differences mean there is always some uncertainty.

Accuracy and verification

For more precise dating, clinicians rely on early ultrasound measurements, which can more accurately estimate fetal age and conception window. If knowing the exact date is important for medical or personal reasons, discuss ultrasound dating with your healthcare provider.

This tool runs locally in your browser and does not transmit your inputs to our servers. Use the results as guidance and consult a healthcare professional for personalized interpretation and any medically relevant decisions.