Pregnancy apps typically start counting weeks from the first day of pregnancy instead of conception because it is easier to track and calculate. Conception can be difficult to pinpoint accurately, whereas the first day of the last menstrual period (LMP) is usually more easily determined. Additionally, many women may not be aware of their exact date of conception. By using the LMP as a starting point, healthcare providers and apps can provide more accurate estimates for due dates and fetal development milestones. It also allows for consistency in tracking pregnancies across different individuals. However, it's important to note that every woman's body is unique, so individual variations in ovulation and conception may occur.