Confused about how many weeks pregnant I am ?

[bump98]

I don't understand how they work out how far along in pregnancy you are ? They do it from the last day of your period ? But if I had an implantation bleed when I think I did I'd only just be 2 weeks pregnant ? Whereas if they go from my first day of my last menstrual cycle I'm nearly 5 weeks ? I honestly don't get it ? Can anyone help ?

At the time of conception (usually 10-15 days after lmp) you're classed as 2 weeks pregnant , so by the time you miss your next period you're 4 weeks pregnant even though you only conceived two weeks ago .

You go from your last period, the first 2 weeks pregnant are before you conceived. The date will change slightly when you have your 12 week scan