How this calculator works
This calculator estimates conception date and last period date by working backward from an expected due date. Enter the expected due date to see the estimated conception date and first day of the last period. Use it for reverse pregnancy dating and understanding timelines. The key input is the expected due date. The output is the estimated conception date and last period date. Real-world applications include reverse dating, pregnancy tracking, and timeline planning. The calculator subtracts 266 days from the due date for conception and 280 days for the first day of the last period. Pregnancy dating commonly starts from the last menstrual period, which is about two weeks before conception in a typical cycle. Ovulation and implantation timing vary, so these are estimates. Ultrasound dating can differ from date-based estimates, especially with irregular cycles.