Your pregnancy is measured from the first day of your last period – rather than from the date that you conceived. This is because you can be more certain of when your last period began than the exact date you conceived (ovulation and conception can occur over several days).
You might find it a bit surprising that the pregnancy timeline includes the initial two weeks when conception hasn’t occurred yet. Think of these as the preparatory steps in your pregnancy journey – conception cannot happen unless there’s an egg to fertilize, so the egg’s release is part of the 40-week timeline.