Reverse Due Date Calculator

Pregnancy dating is based on the standard gestation period, which averages 280 days (or 40 weeks) from the first day of the last menstrual period, or approximately 266 days from actual conception. To find your expected due date from a conception date, the formula is DueDate = Conception + PregnancyLength. Conversely, to find your estimated conception date when you have a target due date, you subtract 266 days. Furthermore, because viable sperm can survive inside the reproductive tract for up to seven days, the fertile intercourse window is estimated as Intercourse = Conception - 7 days to Conception.

Imagine you want to know when conception likely occurred if your scheduled due date is August 1st, 2023. First, we take the target due date of August 1st, 2023. Next, we subtract the standard 266-day gestation period (38 weeks of post-conception development). Counting backward 266 days from August 1st places the estimated conception date right around November 8th, 2022. To find the fertile intercourse window that led to this conception, we look at the week preceding November 8th, meaning unprotected intercourse likely occurred between November 1st and November 8th, 2022.

Keep in mind that biological variation is normal; very few babies arrive precisely on their calculated due date. If your menstrual cycles are irregular or longer than the standard 28 days, your actual ovulation and conception dates may shift by a week or more. Always consult a healthcare professional and use early ultrasound scans for the most clinically accurate dating of a pregnancy.