Implantation bleeding can occur as soon as about 6 to 12 days after conception, typically around 7 to 10 days post-fertilization when the fertilized egg attaches to the uterine lining. This timing often coincides with or occurs just before the expected menstrual period

. More specifically:

• It usually happens between 6 and 12 days after conception, with many sources citing 7 to 9 days as common

• The bleeding results from the embryo embedding into the uterine lining, which disrupts tiny blood vessels

• Implantation bleeding is generally light, lasting from a few hours up to 2 or 3 days, and is much lighter than a normal period

In summary, implantation bleeding can occur as early as about 6 days after fertilization but most commonly happens around 7 to 10 days after conception, aligning with the embryo's implantation into the uterus