Your pregnancy is measured from the first day of your last period, rather than the day you conceived. This is because it's easier to know when your last period started than the exact date of conception (as ovulation and conception can happen over several days).
It might seem surprising that the pregnancy timeline includes the first two weeks when conception hasn’t taken place yet. Think of this as part of the preparation for your pregnancy journey—conception can only happen if an egg is ready to be fertilised, so the release of the egg is included in the 40-week timeline.