A conception date calculator works backwards from a known or estimated due date to determine approximately when conception occurred and the likely fertile window.
About this calculator
A conception date calculator works backwards from a known or estimated due date to determine approximately when conception occurred and the likely fertile window. This helps answer the common question of timing when a due date is already established.
Conception typically occurs about 14 days after the start of the last menstrual period for a 28-day cycle, and about 266 days before the due date.
Common uses
- Determine approximate conception date from a known due date
- Identify the fertile window that led to a known pregnancy
- Understand the relationship between cycle length and conception timing
- Resolve timing questions in early prenatal care
Frequently asked questions
How far before the due date does conception occur?
Conception occurs approximately 266 days (38 weeks) before the due date. Since due dates are calculated as 280 days (40 weeks) from LMP, conception typically happens about 14 days after LMP, around the time of ovulation in a regular 28-day cycle.