Pregnancy Calculator Calculate Based On: Last Period Conception Date Due Date First Day of Last Menstrual Period * Average Length of Cycles From first day of your period to the first day of your next period. Ranges from: 22 to 44. Default = 28 Optional: Leave 28 if unsure. Get The Date! Your Due Date * Calculate! Your Conception Date * Calculate! This is not a diagnosis. The calculations that are provided are estimates based on averages.