Conception Date Calculator

Estimating Conception Date and Fertility Window

A conception calculator estimates the most likely date of conception based on the first day of your last menstrual period (LMP) or a known due date. In a standard 28-day cycle, ovulation typically occurs around day 14, and conception is most likely to happen within the 24-hour window after ovulation when a mature egg is available for fertilization. Since sperm can survive up to 5 days in the reproductive tract, the fertile window extends from about 5 days before ovulation through 1 day after, giving a roughly 6-day window each cycle.

If you know your due date, the calculator works backward using Naegele's rule in reverse: subtracting 280 days (40 weeks) from the due date gives the estimated LMP, and adding 14 days gives the estimated conception date. If you know your LMP, adding 14 days estimates conception for a regular 28-day cycle. For cycles shorter or longer than 28 days, the ovulation day shifts -- a 32-day cycle typically ovulates around day 18, and a 24-day cycle around day 10. This calculator adjusts for cycle length to provide a more accurate estimate.

Implantation of the fertilized egg into the uterine wall occurs 6 to 12 days after conception, which is when pregnancy hormones begin rising and a home pregnancy test may first show positive results. Early ultrasound measurements during weeks 6 to 9 provide the most accurate dating of pregnancy, often within 3 to 5 days of the actual gestational age. Conception date estimation is useful for determining paternity timing, understanding early pregnancy milestones, and coordinating prenatal care schedules with healthcare providers.