Pregnancy tests say you can test 4 days before missed period. why does it work for some women and not for others?

Pregnant or not: The difference is only if pregnant or not. 4 days before a missed period is difficult to assess. Ovulation is approximately 14 days after the first day of the last period which in a 28-day cycle is 14 days before the next. 4days before the period would be 10 days after ovulation and conception is around ovulation so if test is positive at 10 days, you are pregnant, if negative, likely not.

Many women wait: Until they have a missed a period before using a standard pregnancy test. Early pregnancy tests may be able to detect pregnancy up to 6 days prior to a missed period.